Tag: Chapter 3 – Atoms and Molecules

Atoms and Molecules

Exercises – Answers and Solutions

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Question 1

A 0.24 g sample of a compound of oxygen and boron contains 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition by weight.

Solution:

Total mass of compound = 0.24 g

Percentage of boron:

$$\frac{0.096}{0.24}\times 100=40\mathrm{\%}$$

Percentage of oxygen:

$$\frac{0.144}{0.24}\times 100=60\mathrm{\%}$$

Answer:

• Boron = 40%

• Oxygen = 60%

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Question 2

When 3.0 g of carbon is burnt in 8.0 g of oxygen, 11.0 g of carbon dioxide is formed.
What mass of carbon dioxide will be formed when 3.0 g of carbon is burnt in 50.0 g of oxygen?
Which law governs this?

Solution:

Carbon reacts with oxygen in a fixed ratio.

Given:

• 3 g carbon + 8 g oxygen → 11 g CO₂

Even though oxygen is present in excess (50 g), only 8 g oxygen will react with 3 g carbon.

So,

$$\text{Mass of CO₂ formed}=11\text{ g}$$

Law governing this:

Law of Constant (Definite) Proportions

Answer:

• Mass of CO₂ formed = 11 g

• Law: Law of Constant Proportions

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Question 3

What are polyatomic ions? Give examples.

Answer:

Polyatomic ions are groups of two or more atoms that carry a net charge and act as a single ion.

Examples:

• Ammonium (NH₄⁺)

• Hydroxide (OH⁻)

• Carbonate (CO₃²⁻)

• Sulphate (SO₄²⁻)

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Question 4

Write the chemical formulae of the following:

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Question 5

Give the names of the elements present in the following compounds:

(a) Quick lime (CaO)

• Calcium

• Oxygen

(b) Hydrogen bromide (HBr)

• Hydrogen

• Bromine

(c) Baking powder

• Sodium

• Hydrogen

• Carbon

• Oxygen

(d) Potassium sulphate (K₂SO₄)

• Potassium

• Sulphur

• Oxygen

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Question 6

Question:
Calculate the molar mass of the following substances.

(Atomic masses used: H = 1, C = 12, N = 14, O = 16, S = 32, P = 31, Cl = 35.5)

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(a) Ethyne (C₂H₂)

$$(2\times 12)+(2\times 1)=24+2=\boxed{{\displaystyle 26{\text{g mol}}^{-1}}}$$

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(b) Sulphur molecule (S₈)

$$8\times 32=\boxed{{\displaystyle 256{\text{g mol}}^{-1}}}$$

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(c) Phosphorus molecule (P₄)

$$4\times 31=\boxed{{\displaystyle 124{\text{g mol}}^{-1}}}$$

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(d) Hydrochloric acid (HCl)

$$1+35.5=\boxed{{\displaystyle 36.5{\text{g mol}}^{-1}}}$$

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(e) Nitric acid (HNO₃)

$$1+14+(3\times 16)=1+14+48=\boxed{{\displaystyle 63{\text{g mol}}^{-1}}}$$

Atoms and Molecules

In-Text Question Answers

Page 27 – Questions – Answers

Question 1

In a reaction, 5.3 g of sodium carbonate reacted with 6 g of acetic acid. The products were 2.2 g of carbon dioxide, 0.9 g of water and 8.2 g of sodium acetate. Show that these observations are in agreement with the law of conservation of mass.

Answer:

Mass of reactants
= Mass of sodium carbonate + Mass of acetic acid
= 5.3 g + 6.0 g
= 11.3 g

Mass of products
= Mass of carbon dioxide + Mass of water + Mass of sodium acetate
= 2.2 g + 0.9 g + 8.2 g
= 11.3 g

Since the total mass of reactants = total mass of products, the reaction obeys the Law of Conservation of Mass.

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Page 28 – Questions

Question 2

Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

Answer:

Given ratio (H : O) = 1 : 8

If 1 g hydrogen reacts with 8 g oxygen
Then 3 g hydrogen will react with:

$$3\times 8=24\text{ g}$$

Required mass of oxygen = 24 g

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Question 3

Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

Answer:

The postulate stating that:

“Atoms can neither be created nor destroyed in a chemical reaction.”

This explains the law of conservation of mass.

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Question 4

Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

Answer:

The postulate stating that:

“The relative number and kinds of atoms are constant in a given compound.”

This explains the law of definite (constant) proportions.

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Page 30 – Questions

Question 1

Define the atomic mass unit.

Answer:
One atomic mass unit (u) is defined as one-twelfth (1/12th) of the mass of one atom of carbon-12.

Question 2

Why is it not possible to see an atom with naked eyes?

Answer:
An atom is extremely small in size (of the order of ${10}^{-10}$), which is much smaller than the resolving power of the human eye. Therefore, atoms cannot be seen with naked eyes.

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Page 34 – Questions

Question 1

Write down the formulae of the following:

(i) Sodium oxide → Na₂O
(ii) Aluminium chloride → AlCl₃
(iii) Sodium sulphide → Na₂S
(iv) Magnesium hydroxide → Mg(OH)₂

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Question 2

Write down the names of compounds represented by the following formulae:

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Question 3

What is meant by the term chemical formula?

Answer:
A chemical formula is a symbolic representation of a compound which shows the elements present in it and the number of atoms of each element.

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Question 4

How many atoms are present in:

(i) H₂S molecule

• Hydrogen atoms = 2

• Sulphur atoms = 1

Total atoms = 3

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(ii) PO₄³⁻ ion

• Phosphorus atoms = 1

• Oxygen atoms = 4

Total atoms = 5

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Page 35 – Questions

Question 1

Calculate the molecular masses of the following:

(Atomic masses used: H = 1 u, C = 12 u, N = 14 u, O = 16 u, Cl = 35.5 u)

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(i) H₂

$$2\times 1=\boxed{{\displaystyle 2\text{ u}}}$$

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(ii) O₂

$$2\times 16=\boxed{{\displaystyle 32\text{ u}}}$$

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(iii) Cl₂

$$2\times 35.5=\boxed{{\displaystyle 71\text{ u}}}$$

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(iv) CO₂

$$12+(2\times 16)=12+32=\boxed{{\displaystyle 44\text{ u}}}$$

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(v) CH₄

$$12+(4\times 1)=12+4=\boxed{{\displaystyle 16\text{ u}}}$$

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(vi) C₂H₆

$$(2\times 12)+(6\times 1)=24+6=\boxed{{\displaystyle 30\text{ u}}}$$

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(vii) C₂H₄

$$(2\times 12)+(4\times 1)=24+4=\boxed{{\displaystyle 28\text{ u}}}$$

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(viii) NH₃

$$14+(3\times 1)=14+3=\boxed{{\displaystyle 17\text{ u}}}$$

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(ix) CH₃OH

$$12+(4\times 1)+16=12+4+16=\boxed{{\displaystyle 32\text{ u}}}$$

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Question 2

Calculate the formula unit masses of the following:

(Given: Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, O = 16 u)

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(i) ZnO

$$65+16=\boxed{{\displaystyle 81\text{ u}}}$$

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(ii) Na₂O

$$(2\times 23)+16=46+16=\boxed{{\displaystyle 62\text{ u}}}$$

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(iii) K₂CO₃

$$(2\times 39)+12+(3\times 16)=78+12+48=\boxed{{\displaystyle 138\text{ u}}}$$