Chemical equations and the mole
Francine Taylor-Campbell, Contributor

REVIEW OF FACTS

To perform calculations based on chemical reactions, an equation must first be written and then balanced, so that the mole concept can be applied.

Let us examine a few questions which deal with calculations involving equations.

1. Two moles of sulphuric acid react with one mole of sodium carbonate to form a salt, water and carbon dioxide.

(i) Write a balanced equation for this reaction. (2 marks)

(ii) Is the salt formed an acid salt or a normal salt? Give a reason for your answer.

(2 marks)

(iii) Calculate the mass of the salt formed when 10.6g of sodium carbonate are used in the reaction.

(Relative atomic mass: Na = 23, C = 12, O = 16, S = 32, H = 1)

(2 marks)

ANSWERS

1. (i) 2H₂SO₄ + Na₂CO₃ == NaHSO₄ + H₂O + CO₂

(ii) An acid salt is formed since it contains hydrogen ions (using the mole ratio given, only an acid salt could be formed).

(iii) Molar mass of Na₂CO₃ = (23*2) + 12 + (16*3) = 106 g

10.6g of Na₂CO₃ = 0.1 mol (10.6/106)

Based on the equation, 1 mol Na2CO3 produce 1 mol of salt

Therefore, 0.1 mol Na₂CO₃ produce 0.1 mol NaHSO₄

Molar mass of NaHSO₄ = 23 + 1 + 32 + 16*4 = 120g

Mass of 0.1 mol of NaHSO₄ = 0.1 x 120 = 12g

2. Sodium reacts with ethanol at room temperature and pressure according to the following equation:

2 Na(s) + 2 C₂H₅OH (l) == 2 C₂H₅ONa (s) + H₂ (g)

What volume of hydrogen at r.t.p. can be produced when 11.5g of sodium react with excess ethanol? (2 marks)

ANSWERS

2. First, we must determine how many moles of sodium are present.

Molar mass of Na = 23g, thus 11.5g = 11.5/23 = 0.5 mol Na

Based on the equation, 2 mol Na give off 1 mol H₂ gas (2:1 ratio)

Thus, 0.5 mol Na will produce 0.25 mol H₂

1 mol H₂ at r.t.p. == 24 dm³

0.25 mol H₂ = 0.25 x 24 = 6 dm³.

3. Sodium carbonate, carbon dioxide and water are produced when sodium hydrogen carbonate is heated.

Write an equation for the reaction.

What is the maximum volume of carbon dioxide that could be produced at room temperature and pressure on heating 21g of sodium hydrogen carbonate? (4 marks)

ANSWERS

2NaHCO₂ (s) ==== Na₂CO₃(s) + CO₂ (g) + H₂O-(l)

Molar mass of NaHCO₃ = 23 + 1 + 12 + 16*3 = 84g

21g of NaHCO₃ = 0.25 mol = 21/84

Based on the equation, 2 mol of NaHCO₃ produce 1 mol CO₂

Thus, 0.25 mol NaHCO₃ will produce 0.125 mol CO₂

At r.t.p. 1 mol CO2 has a volume of 24 dm³

0.125 mol CO2 = 0.125 x 24 = 3 dm³

NOW ATTEMPT THIS QUESTION

4. A sample of calcium carbonate is heated to constant mass.

Write an equation to represent this reaction.

Calculate the mass of solid residue that would be obtained if 10g of calcium carbonate were heated. (5 marks)

Francine Taylor-Campbell is an independent contributor. Send questions and comments to kerry-ann.hepburn@gleanerjm.com