1
In Activity 3.1 the flask is corked so that —
💡 Hint:
An open mouth can let gas out.
Explanation:
The cork makes a closed system. Only then is the comparison of the two weighings fair.
Class 9 · Science · Chapter 3 · बिहार बोर्ड (BSEB)CBSE · NCERT 2026-27
Atoms and Molecules
Atoms and Molecules
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- 1 Conservation of mass — Activity 3.1
- 2 Constant proportions and Dalton
- 3 Atoms, symbols and u
- 4 Molecules and the atom ratio — Activity 3.2
- 5 Ions and formulae — the group activity
- 6 Molecular mass and formula unit mass
- 7 The mole and the Avogadro number
- Chapter winner — every lesson at mastery ★
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📄 PDF download — notes + answer key
Conservation of mass — Activity 3.1
द्रव्यमान संरक्षण — क्रियाकलाप 3.1 · Bihar 3.1.1 · Activity 3.1
Two laws, two scientistsNotes
By the end of the eighteenth century elements and compounds were told apart. Lavoisier laid the base of the laws of chemical combination. The law of constant proportions is tied to the experiments of Joseph L. Proust.
The first question was whether total mass changes in a reaction. Mass is neither created nor destroyed. The sum of the reactants stays equal to the sum of the products.
Activity 3.1 — the closed flaskActivity
Take one of the three pairs. Make X and Y separately as 5% solutions.
- X copper sulphate, Y sodium carbonate
- X barium chloride, Y sodium sulphate
- X lead nitrate, Y sodium chloride
Put a little of solution Y in a conical flask. Put solution X in an ignition tube. Hang the tube in the flask so the solutions do not mix yet. Cork the mouth. Weigh the flask with its contents. Then tilt and swirl so that X and Y mix. Weigh again.
On mixing, a new substance appears, so a reaction has taken place. The weight does not change, because nothing came in and nothing went out.
Do not weigh with the cork offCaution
Gas can leave an open mouth. The balance then shows a smaller mass and the law looks false. The law is about a closed system, not about an open flask.
Worked exampleExample
Question: 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products are 2.2 g of carbon dioxide, 0.9 g of water and 8.2 g of sodium ethanoate. Check conservation of mass.
Formula: mass of reactants = mass of products.
Substitution: Left side 5.3 g + 6 g = 11.3 g. Right side 2.2 g + 0.9 g + 8.2 g = 11.3 g.
Unit: 11.3 g on both sides. Mass is conserved. Word equation: sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water.
10-second revision
- Total mass does not change in a reaction
- In Activity 3.1 the cork makes a closed system
- 5.3 + 6 = 2.2 + 0.9 + 8.2 = 11.3 g
Board tip · BSEBBoard tip
Write the two sums separately. Only “they are equal” stays incomplete.
Board tip · CBSEBoard tip
The reason for the cork is that gas must not leave. “The flask looks neat” is not the reason.
Check your understandingall correct = mastery ★
2
Mass can be created in a chemical reaction.
Explanation:
False — mass is neither created nor destroyed.
3
The law of conservation of mass is linked with ______.
💡 Hint:
This person laid the base of chemical science.
Explanation:
Lavoisier. The law of constant proportions belongs to Proust.
4
From reactants of 5.3 g and 6 g, the total mass of the products is —
💡 Hint:
The two sums have to match.
Explanation:
5.3 + 6 = 11.3 g. The products are also 2.2 + 0.9 + 8.2 = 11.3 g.
5
Name one pair from Activity 3.1 and say why the second weighing stays equal to the first.
Model answer (board format)
One pair is barium chloride and sodium sulphate. The corked flask is closed. Mass is not destroyed in the reaction, so the weighing does not change.
Check your answer:
Constant proportions and Dalton
स्थिर अनुपात और डाल्टन · Bihar 3.1.2 · Dalton six points
The ratio does not depend on the sourceNotes
According to Proust, the elements in a chemical substance are always present in definite proportions by mass. This is the law of constant proportions, also called the law of definite proportions.
In water the mass ratio of hydrogen to oxygen is always 1:8. From 9 g of water you obtain 1 g of hydrogen and 8 g of oxygen. In ammonia, nitrogen and hydrogen stay in the ratio 14:3. In carbon dioxide, carbon and oxygen are 3:8.
The mass ratio of elements in a compound stays fixed. Dalton explained these laws with atoms. His six points are these. All matter is made of very small particles, atoms. In a chemical reaction atoms are neither created nor destroyed. Atoms of one element are alike in mass and chemical properties. Atoms of different elements differ. Atoms combine in small whole-number ratios to form compounds. In a given compound the relative number and kinds of atoms stay constant.
The second point is the result of conservation of mass. The last point explains the law of definite proportions. Kanad had called the small particles Parmanu. Democritus called them atoms, meaning indivisible. Those ideas were philosophy. Dalton stood them on the laws.
Worked exampleExample
Question: Hydrogen and oxygen form water in the mass ratio 1:8. What mass of oxygen is needed for 3 g of hydrogen?
Formula: m(O) = 8 × m(H).
Substitution: m(O) = 8 × 3 g = 24 g. Water = 3 g + 24 g = 27 g.
Unit: 24 g of oxygen. This is three times the 1 g and 8 g in 9 g of water.
10-second revision
- In water H:O is 1:8 by mass
- In ammonia N:H = 14:3
- The conservation point: atoms are neither made nor destroyed
Board tip · BSEBBoard tip
Write both the name of the law and the example. Only 1:8, with no water, is incomplete.
Board tip · CBSEBoard tip
If the question asks which point came from which law: the second point is conservation, the last point is definite proportions.
Check your understandingall correct = mastery ★
1
Even if the source of water changes, the mass ratio of hydrogen to oxygen stays 1:8.
Explanation:
True — that is constant proportions.
2
Which point of Dalton explains the law of definite proportions?
💡 Hint:
The last point is the fixed make-up of a compound.
Explanation:
If the relative number and kinds stay fixed, the mass ratio stays fixed too. The conservation point is the second one.
3
Find the mass of oxygen needed to make water completely from 3 g of hydrogen. Write the formula and the unit.
Model answer (board format)
m(O) = 8 × m(H) = 8 × 3 g = 24 g. 24 g of oxygen is needed.
Check your answer:
4
In ammonia the mass ratio of nitrogen to hydrogen is ______.
💡 Hint:
The water ratio is 1:8. This is the other one.
Explanation:
14:3. It stays the same even if the source or the method changes.
Atoms, symbols and u
परमाणु, संकेत और u · Bihar 3.2 · symbols · carbon-12
Name, symbol and the standard of comparisonNotes
An atom is a building block of matter, yet it is far smaller than the thickness of this paper. Stack millions of atoms and the layer is still only about as thick as a sheet. Radius is measured in nanometres. 1 nm = 10−9 m. The radius of a hydrogen atom is about 10−10 m.
An atom is not seen with the naked eye. Modern pictures can show atoms on a surface, such as the surface of silicon. Atoms of most elements do not stay alone. They form molecules or ions.
Berzelius made symbols from one or two letters. The first letter is capital, the second is small. Al is right, AL is wrong. Co is cobalt, and CO is read like carbon monoxide. Some symbols come from Latin: iron Fe (ferrum), sodium Na (natrium), potassium K (kalium). IUPAC accepts the names. You do not need every symbol in one day.
Since 1961 the standard of atomic mass is carbon-12. 1 u = 1/12 of the mass of one carbon-12 atom. Relative atomic mass compares the average mass of the atom with this 1/12. A few table values are these.
| Element | Symbol | Mass (u) |
|---|---|---|
| H | H | 1 |
| C | C | 12 |
| N | N | 14 |
| O | O | 16 |
| Na | Na | 23 |
| S | S | 32 |
| Cl | Cl | 35.5 |
| Ca | Ca | 40 |
Chlorine is 35.5 u. That is not half an atom. The average opens in the next chapter. In calculations here, use the table value.
Worked exampleExample
Question: 3 g of carbon joins 4 g of oxygen to make CO. Take carbon as 12 u. Find the relative mass of oxygen.
Formula: atomic mass (O) = (mass of oxygen / mass of carbon) × atomic mass (C).
Substitution: (4 g / 3 g) × 12 u = (4/3) × 12 u = 16 u.
Unit: 16 u. If carbon were taken as 1 u, oxygen would come out as 4/3 ≈ 1.33 u. The carbon-12 scale is easier for whole numbers.
10-second revision
- 1 u = 1/12 of carbon-12
- Symbols: Al, Co, Fe, Na, K
- An atom is not seen with the naked eye
Board tip · BSEBBoard tip
In the definition of u write the full line “one-twelfth of carbon-12”. Do not write only “a small measure”.
Board tip · CBSEBoard tip
CO and Co are different. The second letter is always small.
Check your understandingall correct = mastery ★
1
1 u is equal to —
💡 Hint:
The standard is the 1961 carbon-12.
Explanation:
1 u is exactly one-twelfth of one carbon-12 atom. 12 g is the mass of one mole, not 1 u.
2
The symbol of iron is ______, because the Latin name is ferrum.
💡 Hint:
Two letters, the second small.
Explanation:
Fe. Sodium Na and potassium K also come from Latin.
3
The symbol of cobalt is CO.
Explanation:
False — cobalt is Co. CO is read as carbon monoxide.
4
From 3 g carbon and 4 g oxygen, taking carbon = 12 u, oxygen comes out as —
💡 Hint:
(4/3) × 12.
Explanation:
(4/3) × 12 u = 16 u. The unit is u, not g.
5
1 nm = ______ m.
💡 Hint:
It is the small part with nine zeros.
Explanation:
10⁻⁹ m. 1 m = 10⁹ nm.
6
Define the atomic mass unit. In one line say why 35.5 u for chlorine is not half an atom.
Model answer (board format)
1 u is 1/12 of the mass of one carbon-12 atom. 35.5 u is the table average, not a half piece of one atom.
Check your answer:
Molecules and the atom ratio — Activity 3.2
अणु और परमाणु-अनुपात — क्रियाकलाप 3.2 · Bihar 3.3 · Activity 3.2
Molecules and atomicityNotes
A molecule is the smallest particle of an element or a compound that can exist on its own and show the properties of the substance. Atoms of the same element may join, or atoms of different elements.
Argon and helium are monoatomic. Oxygen, hydrogen, nitrogen and chlorine are diatomic: O2, H2, N2, Cl2. Phosphorus is tetra-atomic, P4. Sulphur is polyatomic, S8. The number of atoms in a molecule is its atomicity. Ozone has three atoms of oxygen. Metals and carbon do not have a simple small number.
In a compound, atoms of different elements join in a fixed ratio. Water is H and O, mass 1:8. Ammonia is N and H, 14:3. Carbon dioxide is C and O, 3:8.
Activity 3.2 — from mass to numberActivity
Divide the mass ratio by the atomic mass. Turn the ratio you get into small whole numbers. That is the ratio of the number of atoms in the molecule.
For water the book shows these steps. Hydrogen: mass part 1, atomic mass 1 u, ratio 1/1 = 1. Oxygen: mass part 8, atomic mass 16 u, ratio 8/16 = 1/2. Multiply both by 2. H:O = 2:1, so water is H2O.
Worked exampleExample
Question: In carbon dioxide the mass ratio C:O is 3:8. Find the ratio of the number of atoms. C = 12 u, O = 16 u.
Formula: number ratio = mass part / atomic mass.
Substitution: C = 3/12 = 1/4. O = 8/16 = 1/2. Multiply both by 4: C = 1, O = 2.
Unit: the ratio is a count of atoms, 1:2. The formula is CO2.
10-second revision
- Atomicity: Ar = 1, O2 = 2, P4 = 4, S8 many
- Number ratio = mass part / u
- In water atoms are 2:1, mass is 1:8
Board tip · BSEBBoard tip
Do not write the mass ratio and the atom ratio as the same pair. In water 1:8 is mass and 2:1 is number.
Board tip · CBSEBoard tip
After dividing, turn the fractions into whole numbers. Do not build a formula from 1/4 and 1/2 as they stand.
Check your understandingall correct = mastery ★
1
The atomicity of phosphorus is —
💡 Hint:
P4 is tetra-atomic. S8 is different.
Explanation:
4. Argon is 1, oxygen is 2, sulphur is polyatomic.
2
A helium molecule is diatomic.
Explanation:
False — He is monoatomic. Diatomic examples are O2 and H2.
3
The simple ratio of atoms in water, H:O, is ______.
💡 Hint:
Mass is 1:8. For the number, double 8/16.
Explanation:
2:1. So the formula is H2O.
4
In ammonia the N:H mass is 14:3. Using N = 14 u and H = 1 u, find the ratio of atoms.
Model answer (board format)
Number = mass / atomic mass. N = 14/14 = 1. H = 3/1 = 3. The ratio is 1:3, formula NH3.
Check your answer:
5
A correct mark of a molecule is —
💡 Hint:
Both elements and compounds have molecules.
Explanation:
A molecule can exist on its own and show the properties of the substance. Charge is the mark of an ion.
Ions and formulae — the group activity
आयन और सूत्र — समूह क्रियाकलाप · Bihar 3.4 · group activity · figure
Ions, valency, three rulesNotes
Compounds of metals and non-metals contain charged particles. These are ions. A positive ion is a cation and a negative ion is an anion. NaCl has Na+ and Cl−. A group of atoms that carries a charge is a polyatomic ion, such as SO42−, OH− and NH4+.
Valency is the combining capacity. Think of it as hands. In the criss-cross, the valency of one becomes the number of the other. The total charge must stay zero.
There are three rules. Charges or valencies must balance. In a metal and a non-metal, write the metal symbol first, as in CaO and NaCl. If a polyatomic ion comes more than once, use brackets. Once means no brackets, as in NaOH. Do not write charges in the finished formula. If both valencies are equal, cancel: not Ca2O2, but CaO.
Group activity — placards and blistersActivity
This is not a numbered Activity 3.3. In a group, make separate placards for symbols and valencies. Symbol in the right hand, valency in the left. Keep the symbols in place and criss-cross the valencies.
For a cheap model, cut empty medicine blister packs into as many cells as the valency. One kind of ion sits in the other. In sodium sulphate two sodium ions sit on one sulphate, formula Na2SO4. Do this one yourself: sodium phosphate is Na3PO4, because the valency of phosphate is 3.
Worked exampleExample
Question: Write the formula of calcium hydroxide. The valency of Ca is 2 and of OH is 1.
Formula: criss-cross — the number of OH equals the valency of Ca.
Substitution: one Ca, two OH. Brackets are needed because OH comes twice.
Unit: the count is of atoms. The formula is Ca(OH)2. In it Ca = 1, O = 2, H = 2. CaOH2 is wrong.
10-second revision
- Cation first, anion after
- MgCl2 and Al2O3 come from the criss-cross
- Brackets in Ca(OH)2 and (NH4)2SO4
Board tip · BSEBBoard tip
Write one example with brackets and one without. This pair is what the paper asks.
Board tip · CBSEBoard tip
In Cu(NO3)2 nitrate comes twice, so there are brackets. In NaNO3 it comes once, so no brackets.
Check your understandingall correct = mastery ★
1
The formula of magnesium chloride is —
💡 Hint:
Valency of Mg is 2, of Cl is 1.
Explanation:
MgCl2. 2 and 1 are not equal, so nothing is cancelled. Charges are not written in the formula.
2
The formula of aluminium oxide is ______.
💡 Hint:
Criss-cross 3 of Al and 2 of O.
Explanation:
Al2O3. Two aluminium ions and three oxide ions cancel +6 and −6.
3
The correct formula of calcium hydroxide is CaOH2.
Explanation:
False — because of two OH groups we write Ca(OH)2.
4
The formula of sodium phosphate is —
💡 Hint:
The do-it-yourself of the group activity. Phosphate has valency 3.
Explanation:
Na3PO4. Three sodium ions sit on one phosphate.
5
Write separately the number of atoms in an H2S molecule and in a PO4^3− ion.
Model answer (board format)
H2S has 3 atoms, two hydrogen and one sulphur. PO4^3− has 5 atoms, one phosphorus and four oxygen.
Check your answer:
6
Brackets around hydroxide are required in NaOH.
Explanation:
False — OH comes only once, so NaOH. The brackets are in Ca(OH)2.
Molecular mass and formula unit mass
आणविक द्रव्यमान और सूत्र इकाई · Bihar 3.5.1 and 3.5.2
Same addition, different nameNotes
The molecular mass of a molecule is the sum of the atomic masses of all the atoms in it. The unit is u.
A substance made of ions is not one free molecule in the same way. For it we say formula unit. The addition is the same. The formula unit mass of NaCl is 23 + 35.5 = 58.5 u. For an ionic compound write formula unit mass, not molecular mass.
Worked exampleExample
Question: Find the molecular mass of HNO3 and the formula unit mass of CaCl2. H = 1, N = 14, O = 16, Ca = 40, Cl = 35.5.
Formula: sum = mass of each atom × its number.
Substitution: HNO3 = 1 + 14 + 3 × 16 = 1 + 14 + 48 = 63 u. CaCl2 = 40 + 2 × 35.5 = 40 + 71 = 111 u.
Unit: both answers are in u. HNO3 is a molecular mass. CaCl2 is a formula unit mass.
10-second revision
- Water = 2×1 + 16 = 18 u
- HNO3 = 63 u
- CaCl2 = 111 u, NaCl = 58.5 u
Board tip · BSEBBoard tip
Whether the question says molecular or formula unit, show the sum and write the right name.
Board tip · CBSEBoard tip
If Cl = 35.5, write 2×35.5 = 71. 2×35 = 70 is a mistake.
Check your understandingall correct = mastery ★
1
The molecular mass of water is ______ u.
💡 Hint:
Two hydrogen and one oxygen.
Explanation:
18 u. 2×1 + 16 = 18.
2
The 111 u of CaCl2 is called —
💡 Hint:
CaCl2 is made of ions.
Explanation:
Formula unit mass. The sum is 40 + 2×35.5 = 111 u.
3
Find the formula unit mass of ZnO and of Na2O. Zn = 65 u, Na = 23 u, O = 16 u.
Model answer (board format)
ZnO = 65 + 16 = 81 u. Na2O = 2×23 + 16 = 46 + 16 = 62 u.
Check your answer:
4
Calling 58.5 u of NaCl a molecular mass is correct in the language of this chapter.
Explanation:
False — for NaCl we say formula unit mass. 23 + 35.5 = 58.5 u.
The mole and the Avogadro number
मोल और आवोगाद्रो संख्या · Bihar 3.5.3 · 6.022 × 10^23
A number and grams togetherNotes
The reaction that makes water tells both a count of molecules and a mass.
2H2 + O2 → 2H2O
Two hydrogen molecules and one oxygen molecule make two water molecules. In the language of mass, 4 u of hydrogen and 32 u of oxygen make 36 u of water. The count sits in the equation, so chemists wanted a unit of the number of particles before the mass.
1 mole of any particles is the amount whose mass in grams equals the number of the atomic or molecular mass. 1 mole = 6.022 × 1023 particles. This is the Avogadro number. Exactly 12 g of carbon-12 contains this many atoms. Molar mass is the same number, with g in place of u. 1 mole of hydrogen atoms is 1 g. 1 mole of water is 18 g and holds 6.022 × 1023 molecules.
Three formulae are enough. moles = given mass / molar mass. moles = given number / Avogadro number. number of particles = moles × Avogadro number.
Worked exampleExample
Question: (i) How many moles are in 52 g of helium? He = 4 u. (ii) What is the mass of 0.5 mole of N2? N2 = 28 u.
Formula: n = m / M and m = M × n.
Substitution: (i) n = 52 g / 4 g = 13 mol. (ii) m = 28 g × 0.5 = 14 g.
Unit: the first answer is 13 mol, the second is 14 g. 0.5 mole of N atoms would be 14 × 0.5 = 7 g, because the molar mass of N is 14 g.
10-second revision
- 1 mole = 6.022 × 10^23
- n = m / M
- Molar mass of water is 18 g
Board tip · BSEBBoard tip
Keep the molar mass of the atom and of the molecule apart. 0.5 mole of N and 0.5 mole of N2 have masses 7 g and 14 g.
Board tip · CBSEBoard tip
When the question asks for a number, do not forget to multiply by the Avogadro number. Writing only the moles is incomplete.
Check your understandingall correct = mastery ★
1
The moles in 52 g of helium are —
💡 Hint:
The molar mass of He is 4 g.
Explanation:
n = 52/4 = 13 mol. 6.022 × 10^23 is the number in one mole.
2
The number of atoms in exactly 12 g of carbon-12 is ______.
💡 Hint:
This is called the Avogadro number.
Explanation:
6.022 × 10^23. That is 1 mole.
3
Find the number of molecules in 8 g of oxygen gas, O2. Take O = 16 u and the Avogadro number as 6.022 × 10^23.
Model answer (board format)
The molar mass of O2 is 32 g. n = 8/32 = 0.25 mol. Number = 0.25 × 6.022 × 10^23 = 1.5055 × 10^23 molecules.
Check your answer:
4
The mass of 1 mole of water is 18 g.
Explanation:
True — the molecular mass is 18 u, so the molar mass is 18 g.
5
The mass of 0.5 mole of N atoms is —
💡 Hint:
The molar mass of an N atom is 14 g, of N2 is 28 g.
Explanation:
m = 14 × 0.5 = 7 g. 14 g would be the mass of 0.5 mole of N2.
❓ Full question bank — with answers and explanations — 69 questions
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BSEB modelpractice · not annual60 tried · 0 correctMultiple choiceMCQ140 tried · 0 correctTrue / falseTrue / false80 tried · 0 correctFill in the blanksFill80 tried · 0 correctMatchMatch20 tried · 0 correctAssertion–reasonAssertion–reason50 tried · 0 correctBalancingBalancing60 tried · 0 correctIdentifyIdentify20 tried · 0 correctShort and longVSA, SA, LA120 tried · 0 correctCBSE-stylenot a PYQ60 tried · 0 correct
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Multiple choice
0/14Pick one option. A wrong try brings a hint.
1
According to conservation of mass, in a reaction —
💡 Hint:
It is about a closed system.
Explanation:
The total mass stays the same.
2
Who stated the law of constant proportions?
💡 Hint:
Definite proportions is the same law.
Explanation:
Proust. Lavoisier is linked with conservation of mass.
3
From 9 g of water the oxygen obtained is —
💡 Hint:
H:O = 1:8.
Explanation:
8 g of oxygen and 1 g of hydrogen.
4
The Dalton point that is the result of conservation of mass is —
💡 Hint:
The second point.
Explanation:
Atoms are neither created nor destroyed.
5
The symbol of sodium is —
💡 Hint:
Latin natrium.
Explanation:
Na. S is sulphur and N is nitrogen.
6
Relative atomic mass is compared with —
💡 Hint:
The 1961 standard.
Explanation:
With 1/12 of carbon-12.
7
The diatomic molecule among these is —
💡 Hint:
Cl2.
Explanation:
Chlorine. Argon and helium are monoatomic. P4 has four atoms.
8
The formula of copper nitrate is —
💡 Hint:
Copper(II) has valency 2, nitrate has 1.
Explanation:
Cu(NO3)2. Nitrate comes twice, so there are brackets.
9
The elements in quick lime are —
💡 Hint:
Quick lime is CaO.
Explanation:
Calcium and oxygen.
10
The molecular mass of HNO3 is —
💡 Hint:
1 + 14 + 48.
Explanation:
63 u.
11
Particles in 1 mole are —
💡 Hint:
The Avogadro number.
Explanation:
6.022 × 10^23. 18 g is only the mass of 1 mole of water.
12
A polyatomic ion is —
💡 Hint:
More than one atom and a charge.
Explanation:
SO4^2−. Na+ and Cl− are monoatomic ions.
13
If 3.00 g of carbon burns in 50.00 g of oxygen, the carbon dioxide formed is —
💡 Hint:
3 g of carbon and 8 g of oxygen make 11 g of CO2.
Explanation:
11.00 g. The extra oxygen does not join. This is constant proportions.
14
Which element is not in baking powder, NaHCO3?
💡 Hint:
Count the four symbols.
Explanation:
Nitrogen is absent. Na, H, C and O are present.
True or false
0/81
If gas leaves an open flask, the check of conservation of mass is still complete.
Explanation:
False — the system must be closed. That is why there is a cork.
2
According to Dalton, atoms of one element are alike in mass.
Explanation:
True — that is their third point. Isotopes come in the next chapter.
3
The symbol of potassium is P.
Explanation:
False — potassium is K. P is phosphorus.
4
The sulphur molecule is polyatomic.
Explanation:
True — S8. Phosphorus P4 is tetra-atomic.
5
When writing MgCl2 we also leave the charges on the formula.
Explanation:
False — the formula is MgCl2, not Mg^2+Cl−2.
6
Formula unit mass is used for an ionic compound.
Explanation:
True — NaCl and CaCl2 are in this class.
7
The mass of 1 mole of hydrogen atoms is 1 g.
Explanation:
True — the atomic mass is 1 u, so the gram atomic mass is 1 g.
8
Argon is diatomic.
Explanation:
False — Ar is monoatomic.
Fill in the blanks
0/81
The other name of the law of definite proportions is the law of ______ proportions.
💡 Hint:
Water at 1:8 comes under this.
Explanation:
The law of Proust.
2
In Activity 3.2 the atom ratio in water, H:O, is ______.
💡 Hint:
2:1.
Explanation:
Turn 1/1 and 8/16 into whole numbers.
3
The Latin name of sodium is ______.
💡 Hint:
natrium.
Explanation:
The symbol Na comes from this.
4
The simple formula of calcium oxide is ______, not Ca2O2.
💡 Hint:
CaO.
Explanation:
Both valencies are 2, so cancel.
5
The class name for the atomicity of S8 is ______.
💡 Hint:
Polyatomic.
Explanation:
P4 is tetra-atomic.
6
The value of the Avogadro number is ______.
💡 Hint:
6.022 × 10^23.
Explanation:
This many atoms are in 12 g of carbon-12.
7
The formula unit mass of K2CO3 is ______ u. K = 39, C = 12, O = 16.
💡 Hint:
78 + 12 + 48 = 138 u.
Explanation:
2×39 + 12 + 3×16.
8
The formula of aluminium chloride is ______.
💡 Hint:
AlCl3.
Explanation:
Valency of Al is 3, of Cl is 1.
Match
0/21
Match the name with the right symbol.
Column B: A. K · B. Ag · C. Au · D. Fe
1. Iron
2. Potassium
3. Silver
4. Gold
Explanation:
Iron Fe, potassium K, silver Ag, gold Au.
2
Match the compound with its mass ratio.
Column B: A. N:H = 14:3 · B. C:O = 3:8 · C. Na:Cl = 23:35.5 · D. H:O = 1:8
1. Water
2. Ammonia
3. Carbon dioxide
4. Sodium chloride
Explanation:
Water 1:8, ammonia 14:3, carbon dioxide 3:8, salt 23:35.5.
Assertion–reason
0/5Check both statements, then see whether the reason explains the assertion.
1
Assertion (A): The two weighings of a closed flask stay equal.
Reason (R): In a chemical reaction mass is neither created nor destroyed.
Explanation:
Both are true and R explains A.
2
Assertion (A): In water from every source, hydrogen and oxygen are in the mass ratio 1:8.
Reason (R): In a pure compound the elements stay in definite proportions by mass.
Explanation:
Both are true and R is the correct reason.
3
Assertion (A): The symbol of cobalt is Co.
Reason (R): CO and Co are two ways of writing the same substance.
Explanation:
A is true. R is false. CO is carbon monoxide, Co is cobalt.
4
Assertion (A): Ca(OH)2 and CaOH2 show the same count.
Reason (R): A polyatomic group gets brackets when it comes more than once.
Explanation:
A is false. In CaOH2 the hydrogen count is read differently. R is true.
5
Assertion (A): The mass of 1 mole of water is 18 g.
Reason (R): The Avogadro number is 6.022 × 10^23.
Explanation:
Both are true, but R does not explain the 18 g. The 18 g comes from the molecular mass.
Balance the equation
0/6These are real equations from the chapter. Put coefficients in the blanks. Water takes 2, 1, 2.
1
This is the water reaction of the chapter. Coefficients are not written in the equation. A blank means 1. Balance H2 + O2 → H2O.
H2 + O2 → H2O
Explanation: 2H2 + O2 → 2H2O
2
In the exercise, carbon burns in oxygen to form carbon dioxide. Balance by filling coefficients (blank = 1).
C + O2 → CO2
Explanation: C + O2 → CO2
3
The Activity 3.1 pair is barium chloride and sodium sulphate. Put coefficients only in the blanks (blank = 1).
BaCl2 + Na2SO4 → BaSO4 + NaCl
Explanation: BaCl2 + Na2SO4 → BaSO4 + 2NaCl
4
The third Activity 3.1 pair is lead nitrate and sodium chloride. Balance by filling coefficients (blank = 1).
Pb(NO3)2 + NaCl → PbCl2 + NaNO3
Explanation: Pb(NO3)2 + 2NaCl → PbCl2 + 2NaNO3
5
The first Activity 3.1 pair is copper sulphate and sodium carbonate. Balance by filling coefficients (blank = 1).
CuSO4 + Na2CO3 → CuCO3 + Na2SO4
Explanation: CuSO4 + Na2CO3 → CuCO3 + Na2SO4
6
The word equation of the text: sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water. The formulae carry no coefficients. Blank = 1.
Na2CO3 + CH3COOH → CH3COONa + CO2 + H2O
Explanation: Na2CO3 + 2CH3COOH → 2CH3COONa + CO2 + H2O
Classify
0/21
Place each molecule in its atomicity.
Argon
Nitrogen
Phosphorus
Chlorine
Explanation:
Ar is one, N2 and Cl2 are two, P4 is four.
2
Place each particle in its type.
Na+
Cl−
NH4+
CO3^2−
Explanation:
Na+ is a cation, Cl− an anion. NH4+ and CO3^2− are polyatomic.
Very short answer
0/51
Write the law of conservation of mass in one line.
Model answer (board format)
In a chemical reaction mass is neither created nor destroyed.
Check your answer:
2
What is a polyatomic ion? Give one example.
Model answer (board format)
A group of atoms that carries a fixed charge is a polyatomic ion. An example is SO4^2− or OH−.
Check your answer:
3
Define the atomic mass unit.
Model answer (board format)
1 u is exactly 1/12 of the mass of one carbon-12 atom.
Check your answer:
4
Name the elements present in hydrogen bromide and in potassium sulphate.
Model answer (board format)
Hydrogen bromide has hydrogen and bromine. Potassium sulphate has potassium, sulphur and oxygen.
Check your answer:
5
What is meant by a chemical formula?
Model answer (board format)
A chemical formula is a symbol of the composition of a compound. It shows the elements and the number of atoms of each element.
Check your answer:
Short answer
0/41
A 0.24 g compound of boron and oxygen has 0.096 g of boron and 0.144 g of oxygen. Find the percentage composition.
Model answer (board format)
Boron % = (0.096 / 0.24) × 100 = 40%. Oxygen % = (0.144 / 0.24) × 100 = 60%.
Check your answer:
2
Find these formula unit masses: ZnO, Na2O, K2CO3. Zn = 65, Na = 23, K = 39, C = 12, O = 16.
Model answer (board format)
ZnO = 65 + 16 = 81 u. Na2O = 46 + 16 = 62 u. K2CO3 = 78 + 12 + 48 = 138 u.
Check your answer:
3
Convert into moles: (a) 12 g of oxygen gas (b) 20 g of water (c) 22 g of carbon dioxide.
Model answer (board format)
(a) O2 = 32 g, n = 12/32 = 0.375 mol. (b) H2O = 18 g, n = 20/18 = 10/9 mol. (c) CO2 = 44 g, n = 22/44 = 0.5 mol.
Check your answer:
4
Write the molar masses of C2H2, S8, P4, HCl and HNO3. P = 31, Cl = 35.5.
Model answer (board format)
C2H2 = 26 g. S8 = 8×32 = 256 g. P4 = 4×31 = 124 g. HCl = 36.5 g. HNO3 = 63 g.
Check your answer:
Long answer
0/31
Write the method of Activity 3.1. From the data of 5.3 g sodium carbonate and 6 g ethanoic acid, show conservation of mass. Why is the cork needed?
Model answer (board format)
Make 5% solutions of X and Y separately. Y goes in the flask, X in the ignition tube. Weigh with the cork on, mix, and weigh again. 5.3 + 6 = 11.3 g and 2.2 + 0.9 + 8.2 = 11.3 g. The cork stops gas from leaving, otherwise the mass would look smaller.
Check your answer:
2
Write the formulae of magnesium chloride, calcium oxide, copper nitrate, aluminium chloride and calcium carbonate. For each, give one reason from the criss-cross or from cancelling.
Model answer (board format)
MgCl2 because Mg is 2 and Cl is 1. CaO because 2 and 2 cancel to 1 and 1. Cu(NO3)2 because nitrate comes twice. AlCl3 because Al is 3 and Cl is 1. CaCO3 because 2 of Ca and 2 of CO3 cancel and carbonate comes once.
Check your answer:
3
(a) Find the number of molecules in 16 g of solid sulphur, S8. (b) Find the number of aluminium ions in 0.051 g of aluminium oxide. Al = 27 u, S = 32 u, Avogadro number 6.022 × 10^23.
Model answer (board format)
(a) S8 = 256 g. n = 16/256 = 1/16 mol. Number = (1/16) × 6.022 × 10^23 = 3.76 × 10^22 molecules. (b) Al2O3 = 102 g. n = 0.051/102 = 0.0005 mol. Ions = 2 × 0.0005 × 6.022 × 10^23 = 6.022 × 10^20.
Check your answer:
BSEB model paper · practice
0/6This model set is for practice. It is not a question from any year’s annual examination. Annual questions will be added only when a source page is available.
1
The symbol from Latin among these is —
💡 Hint:
ferrum.
Explanation:
Fe. H, O and N are the first letters of the English names.
2
According to Dalton, atoms join in a compound —
💡 Hint:
The fifth point.
Explanation:
In a ratio of small whole numbers.
3
The mass of 4 moles of aluminium atoms is ______ g. Al = 27.
💡 Hint:
108 g.
Explanation:
4 × 27.
4
In Activity 3.2, what is done next with 8/16, and what formula of water comes out?
Model answer (board format)
8/16 = 1/2. Hydrogen gives 1/1 = 1. Multiplying both by 2 gives 2:1. The formula is H2O.
Check your answer:
5
Balance the water reaction from the mole section. Do not write coefficients on the equation line. Blank = 1.
H2 + O2 → H2O
Explanation: 2H2 + O2 → 2H2O
6
Write the two Dalton points that explain conservation of mass and definite proportions. Also add the oxygen calculation for 3 g of hydrogen.
Model answer (board format)
Atoms are neither created nor destroyed in a chemical reaction. That is conservation. The relative number and kinds of atoms in a compound stay constant. That is definite proportions. m(O) = 8 × 3 g = 24 g.
Check your answer:
CBSE-style questions
0/6These are competency-based practice questions. They are not copies of a CBSE paper.
1
A student writes the symbol of cobalt as CO and says it is not different from carbon monoxide. The right correction is —
💡 Hint:
First capital, second small.
Explanation:
Cobalt is Co. CO is carbon monoxide.
2
In a sample, 3.00 g of carbon was burnt and the oxygen was 50.00 g. A student wrote CO2 = 53 g. Where is the mistake?
💡 Hint:
Constant proportions. 8 g of oxygen with 3 g of carbon.
Explanation:
The extra oxygen does not join. CO2 is 11.00 g. Oxygen left is 42.00 g.
3
Assertion (A): Ca(OH)2 has two atoms of oxygen.
Reason (R): The 2 outside the brackets applies to the whole OH group.
Explanation:
Both are true and R explains the count in A.
4
Assertion (A): The number of aluminium ions in 0.051 g of Al2O3 is 6.022 × 10^20.
Reason (R): The molar mass of Al2O3 is 54 g.
Explanation:
A is true: on 102 g, 0.0005 mol, ions doubled, 6.022 × 10^20. R is false. The molar mass is 2×27 + 48 = 102 g.
5
A student finds 58.5 u for NaCl and calls it a molecular mass, and writes Ca(OH)2 as CaOH2. Correct both mistakes.
Model answer (board format)
NaCl is ionic, so 23 + 35.5 = 58.5 u is the formula unit mass. CaOH2 is wrong because OH comes twice. The correct formula is Ca(OH)2.
Check your answer:
6
Which has more atoms, 100 g of sodium or 100 g of iron? Na = 23 u, Fe = 56 u. Compare with the formula. You need not find the full number; moles are enough.
Model answer (board format)
n = m/M. Sodium n = 100/23 ≈ 4.35 mol. Iron n = 100/56 ≈ 1.79 mol. Sodium has more moles, so it has more atoms.
Check your answer:
🏛️ Board exam corner — Bihar Board (BSEB)— CBSE
Switch board with BSEB | CBSE above. The lessons follow the same NCERT chapter.
BSEB model · practiceBoard tip
This page has no verified annual-exam question, because no source page has been added. The model set below is practice in the board pattern.
🏛️ Model questions on one page →
The verified label will be used only when a source page for the question is available.
CBSE-style · competency-based
These are case and assertion-reason practice items. Do not treat them as past CBSE questions.
🔁 Spaced review — today’s questions
Wrong questions return soon; correct ones return after a few days.
🧠 What you learned + equation sheet
What you learned
- Total mass does not change in a reaction. That is conservation of mass.
- In a pure compound the elements stay in a fixed ratio by mass. In water H:O = 1:8.
- In Dalton, atoms are neither created nor destroyed, and their number in a compound stays fixed.
- 1 u is 1/12 of one carbon-12 atom. In a symbol the first letter is capital and the second is small.
- A formula comes from a valency criss-cross. Do not write charges. A polyatomic ion gets brackets when needed.
- Molecular mass is the sum of the atomic masses. 1 mole = 6.022 × 10^23 particles, and the molar mass is in grams.
| What | Keep this |
|---|---|
| Water by mass | H:O = 1:8 |
| Water by atoms | H:O = 2:1 |
| 1 u | 1/12 of one carbon-12 atom |
| Avogadro number | 6.022 × 10^23 |
| Molecular mass of water | 18 u |
| Formula unit mass of CaCl2 | 111 u |
The notes are original writing. The textbook was used only for activity order and numbers. “Verified” will be used only on a question that has a source page.