The mole and its application to chemical equations
Francine Taylor-Campbell, Contributor

Shanique Coleman speaks during The Gleaner's Students Forum on Agriculture at William Knibb High School in Trelawny on February 7. - Claudine Housen Photo

WE WILL be applying what we have learnt so far to actual examination questions.

QUESTION 1

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

Write a balanced equation for this reaction. (2 mks)

Is the salt formed an acid salt or a normal salt? Give a reason for your answer. (2 mks)

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 mks)

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 sodium react with excess ethanol? (2 mks)

ANSWERS

2. First of all 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 gives 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 mks)

ANSWERS

2NaHCO3 (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 these questions.

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 mks)

* Francine Taylor-Campbell is an independent contributor.