Atoms and Molecules - Questions and Answers

Basic Concepts

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 water and 8.2 g of sodium acetate. Show that these observations are in agreement with the law of conservation of mass.

Given: Sodium carbonate + Acetic acid → Sodium acetate + Carbon dioxide + Water

Mass of reactants: Sodium carbonate = 5.3 g, Acetic acid = 6 g

Total mass of reactants = 5.3 g + 6 g = 11.3 g

Mass of products: Sodium acetate = 8.2 g, Carbon dioxide = 2.2 g, Water = 0.9 g

Total mass of products = 8.2 g + 2.2 g + 0.9 g = 11.3 g

Since total mass of reactants (11.3 g) = total mass of products (11.3 g), this observation is in agreement with the law of conservation of mass.

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?

Given ratio of hydrogen to oxygen = 1:8

Mass of hydrogen = 3 g

Let the mass of oxygen required be x g

According to the ratio: 1/8 = 3/x

x = 3 × 8 = 24 g

Therefore, 24 g of oxygen gas would be required to react completely with 3 g of hydrogen gas.

3. Which postulate of Dalton's atomic theory is the result of the law of conservation of mass?

The postulate of Dalton's atomic theory that is the result of the law of conservation of mass is: "Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction."

4. Which postulate of Dalton's atomic theory can explain the law of definite proportions?

The postulate of Dalton's atomic theory that explains the law of definite proportions is: "Atoms of different elements combine in fixed ratios to form compounds."

5. Define the atomic mass unit.

Atomic mass unit (amu or u) is defined as exactly one-twelfth the mass of a carbon-12 atom. 1 u = 1.660539 × 10⁻²⁷ kg.

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

Atoms are extremely small in size, with radii of the order of 10⁻¹⁰ m. They are much smaller than the wavelength of visible light, which makes it impossible to see them with naked eyes or even with ordinary microscopes.

7. What is meant by the term chemical formula?

A chemical formula is a symbolic representation of the composition of a compound. It shows the elements present in the compound and the ratio in which their atoms are present.

8. How many atoms are present in a (i) H₂S molecule and (ii) PO₄³⁻ ion?

(i) H₂S molecule contains 2 hydrogen atoms + 1 sulfur atom = 3 atoms

(ii) PO₄³⁻ ion contains 1 phosphorus atom + 4 oxygen atoms = 5 atoms

Chemical Formulas

1. Write down the formulae of:

(i) Sodium oxide: Na₂O

(ii) Aluminum chloride: AlCl₃

(iii) Sodium sulphide: Na₂S

(iv) Magnesium hydroxide: Mg(OH)₂

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

(i) Al₂(SO₄)₃: Aluminum sulfate

(ii) CaCl₂: Calcium chloride

(iii) K₂SO₄: Potassium sulfate

(iv) KNO₃: Potassium nitrate

(v) CaCO₃: Calcium carbonate

3. What are polyatomic ions? Give examples.

Polyatomic ions are ions that contain more than one atom, which are covalently bonded and carry a net charge. They behave as a single unit in chemical reactions.

Examples:

Ammonium ion: NH₄⁺
Hydroxide ion: OH⁻
Carbonate ion: CO₃²⁻
Sulfate ion: SO₄²⁻
Nitrate ion: NO₃⁻
Phosphate ion: PO₄³⁻

4. Write the chemical formulae of the following:

(a) Magnesium chloride: MgCl₂

(b) Calcium oxide: CaO

(c) Copper nitrate: Cu(NO₃)₂

(d) Aluminium chloride: AlCl₃

(e) Calcium carbonate: CaCO₃

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

(a) Quick lime (Calcium oxide): Calcium and Oxygen

(b) Hydrogen bromide: Hydrogen and Bromine

(c) Baking powder (Sodium bicarbonate): Sodium, Hydrogen, Carbon, and Oxygen

(d) Potassium sulphate: Potassium, Sulfur, and Oxygen

Mass Calculations

1. Calculate the molecular masses of H₂, O₂, Cl₂, CO₂, CH₄, C₂H₆, C₂H₄, NH₃, CH₃OH.

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

H₂ = 2 × 1 = 2 u

O₂ = 2 × 16 = 32 u

Cl₂ = 2 × 35.5 = 71 u

CO₂ = 12 + (2 × 16) = 12 + 32 = 44 u

CH₄ = 12 + (4 × 1) = 12 + 4 = 16 u

C₂H₆ = (2 × 12) + (6 × 1) = 24 + 6 = 30 u

C₂H₄ = (2 × 12) + (4 × 1) = 24 + 4 = 28 u

NH₃ = 14 + (3 × 1) = 14 + 3 = 17 u

CH₃OH = 12 + (3 × 1) + 16 + 1 = 12 + 3 + 16 + 1 = 32 u

2. Calculate the formula unit masses of ZnO, Na₂O, K₂CO₃, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.

ZnO = 65 + 16 = 81 u

Na₂O = (2 × 23) + 16 = 46 + 16 = 62 u

K₂CO₃ = (2 × 39) + 12 + (3 × 16) = 78 + 12 + 48 = 138 u

3. Calculate the molar mass of the following substances:

(a) Ethyne, C₂H₂ = (2 × 12) + (2 × 1) = 24 + 2 = 26 g/mol

(b) Sulphur molecule, S₈ = 8 × 32 = 256 g/mol

(c) Phosphorus molecule, P₄ = 4 × 31 = 124 g/mol

(d) Hydrochloric acid, HCl = 1 + 35.5 = 36.5 g/mol

Exercises

1. A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.

Total mass of compound = 0.24 g

Mass of boron = 0.096 g

Mass of oxygen = 0.144 g

Percentage of boron = (Mass of boron / Total mass) × 100 = (0.096 / 0.24) × 100 = 40%

Percentage of oxygen = (Mass of oxygen / Total mass) × 100 = (0.144 / 0.24) × 100 = 60%

Therefore, the compound contains 40% boron and 60% oxygen by weight.

2. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?

According to the first case:

Carbon + Oxygen → Carbon dioxide

3 g + 8 g → 11 g

This shows that 3 g of carbon requires 8 g of oxygen to form 11 g of carbon dioxide.

In the second case, when 3 g of carbon is burnt in 50 g of oxygen, only 8 g of oxygen will be used (as per the fixed ratio) and the remaining 42 g of oxygen will be left unreacted.

Therefore, the mass of carbon dioxide formed will still be 11 g.

This is governed by the law of constant proportions or law of definite proportions, which states that a chemical compound always contains the same elements combined together in the same proportion by mass.