iGCSE Chemistry Multiple Choice (Extended) : 3.3 The mole and the Avogadro constant: Exam Style Questions Paper 2

Question

Calcium carbonate is heated. Calcium oxide and carbon dioxide gas are formed.

The equation for the reaction is shown.

\[ \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \]

225 kg of calcium carbonate is heated until there is no further change in mass.

The yield of calcium oxide is 85 kg.

What is the percentage yield?

A) 37.8%
B) 47.2%
C) 67.5%
D) 85.0%

▶️ Answer/Explanation

Solution

Ans: C

First calculate the theoretical yield:

1. Molar mass of CaCO₃ = 40 + 12 + (16×3) = 100 g/mol

2. Moles of CaCO₃ = 225,000 g / 100 g/mol = 2,250 mol

3. From the equation, 1 mol CaCO₃ produces 1 mol CaO

4. Molar mass of CaO = 40 + 16 = 56 g/mol

5. Theoretical yield = 2,250 mol × 56 g/mol = 126,000 g = 126 kg

Now calculate percentage yield:

Percentage yield = (actual yield / theoretical yield) × 100

= (85 kg / 126 kg) × 100 ≈ 67.5%

Question

The equation for the decomposition of ammonium carbonate, \((\text{NH}_4)_2\text{CO}_3\), is shown.

\[(\text{NH}_4)_2\text{CO}_3(s) \rightarrow 2\text{NH}_3(g) + \text{CO}_2(g) + \text{H}_2\text{O}(l)\]

[\(M: (\text{NH}_4)_2\text{CO}_3, 96\)]

The total volume of gas produced is 360 cm\(^3\) at r.t.p.

Which mass of ammonium carbonate, \((\text{NH}_4)_2\text{CO}_3\), is decomposed?

A) 0.24 g
B) 0.48 g
C) 0.96 g
D) 1.44 g

▶️ Answer/Explanation

Solution

Ans: B

1. First calculate total moles of gas produced: 360 cm³ = 0.36 dm³. At r.t.p., 1 mole occupies 24 dm³, so moles of gas = 0.36/24 = 0.015 mol.

2. From the equation: 1 mole \((\text{NH}_4)_2\text{CO}_3\) produces 3 moles of gas (2 NH₃ + 1 CO₂).

3. So moles of \((\text{NH}_4)_2\text{CO}_3\) decomposed = 0.015/3 = 0.005 mol.

4. Mass = moles × Mr = 0.005 × 96 = 0.48 g.

Note: H₂O is liquid and doesn’t contribute to gas volume at r.t.p.

Question

The structure of ethene is shown.

How many hydrogen atoms and how many carbon atoms are in one mole of ethene?

▶️ Answer/Explanation

Solution

Ans: A

One mole of ethene (C₂H₄) contains 2 moles of carbon atoms and 4 moles of hydrogen atoms. Using Avogadro’s number (6.0 × 10²³ particles per mole):

Carbon atoms: 2 × 6.0 × 10²³ = 1.2 × 10²⁴

Hydrogen atoms: 4 × 6.0 × 10²³ = 2.4 × 10²⁴

Therefore, option A is correct. The other options either miscalculate the number of atoms or don’t account for the molecular formula properly.

Question

The equation for the reaction between aqueous lead(II) nitrate and aqueous sodium chloride is shown.

Pb(NO₃)₂(aq) + 2NaCl(aq) → PbCl₂(s) + 2NaNO₃(aq)

If 100 cm³ of aqueous lead(II) nitrate of concentration 0.1 mol / dm³ is reacted with an excess of aqueous sodium chloride, which mass of lead(II) chloride is obtained?

A 1.16 g B 2.42 g C 2.78 g D 3.31 g

Answer/Explanation

Ans:

iGCSE Chemistry Multiple Choice (Extended) : 3.3 The mole and the Avogadro constant: Exam Style Questions Paper 2

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