Calculations on the mole concept
Francine Taylor-Campbell, Contributor

It is all about fun and activities for grade six students of the Siloah Primary School with their Gleaner's Children's Own in Siloah, St. Elizabeth, on Tuesday, February 21. - Monique Hepburn Photo

THIS WEEK we will continue to look at questions on the mole concept. You can never have too much practice in this area.

Let us try this question:

1. (a) A compound with relative molecular mass of 180 was found to contain 40 per cent carbon, 6.7 per cent hydrogen and 53.3 per cent oxygen. Determine the molecular of this compound.

(b) 1.89g of zinc nitrate was heated according to the equation

2Zn(NO3)² (s) == 2ZnO (s) + 4NO2 (g) + O2 (g). What is the percentage by mass of oxygen in zinc nitrate? What volume of oxygen at r.t.p. is given off when 1.89g of zinc nitrate are heated?

ANSWERS

Empirical formula of compound = CH2O

Therefore, the molecular formula = (CH2O)n

If the molecular mass of the compound = 180

Then (CH2O)n = 180

(12 + (1*2) + 16)n = 180 thus 30n = 180 ; n=6

Molecular formula = (CH2O)6 = C6H12O6

(b) 2Zn(NO3)2 == 2ZnO + 4NO2 + O2

Molar mass of Zn(NO3)2 = 65 + 14*2 + 16*6 = 189g

% O = mass of oxygen in zinc nitrate = (96/189)*100 = 50.8%

Total mass of zinc nitrate

(ii) 1.89g of Zn(NO3)2 represents 1.89/189 = 0.01 mol

According to the equation, 2 mol of Zn(NO3)2 give off 1 mol O2. Therefore 0.01 mol Zn(NO3)2 will produce 0.01/2 = 0.005 mol O2. At r.t.p. 1 mole of any gas occupies a volume of 24dm3 0.005 mol O2 has a volume of 0.005*24 = 0.12dm³

QUESTION 2

Ammonia and carbon dioxide react to form water and a solid, urea, CON2H4. In the reaction, 72dm² of carbon dioxide at r.t.p. are converted to urea. Write the equation for the formation of urea.

Calculate the volume of ammonia at r.t.p. which reacted. Calculate the mass of urea formed.

ANSWERS

(i) 2NH3 (g) + CO2 (g) == H2O (l) + CON2H4 (s)

(ii) Based on the equation, 2 mol NH² react with 1 mol CO2 (2:1 ratio). Hence if 72 dm3 of CO2 is used then 2 x 72 dm² of NH3 would react.

Volume of ammonia = 144dm³

(iii) 1 mol of gas at r.t.p has a volume of 24dm³

# mol CO2 used = 72/24 = 3 mol

Using the equation again 1 mol CO2 produces 1 mol CON2H4 (urea)

Thus, 3 mol CO2 produce 3 mol urea

Molar mass of urea = (12 + 16 + 14*2 + 1*4) = 60g

3 mol urea has a mass of 60 x 3 = 180g.

Now attempt this question:

3. Give the equation for the reaction between methane and steam. Calculate the maximum volume of hydrogen, measured at s.t.p., which can be obtained from 16g of methane.

Remember that when working problems with moles, it is always best to start by finding the number of moles of the known substance whether by using its mass or volume. Also, in most cases, an equation is essential as it helps to determine in what mole ratio the reactants combine or the products form.

* Francine Taylor-Campbell is an independent contributor.