Chemistry, Third edition

REVISION QUESTIONS 133

Limiting reagents

(i) What is the limiting reagent when 0.5 mol of solid phosphorus and 0.5 mol of oxygen react

according to the equation

P 4 (s)5O 2 (g)P 4 O 10 (s)

and how many moles of P 4 O 10 are formed?

(ii)What is the limiting reagent when 1.5 g of magnesium and 1.5 g of nitrogen combine

according to the equation

3Mg(s)N 2 (g)Mg 3 N 2 (s)

and how many grams of Mg 3 N 2 are formed?

(iii)Zinc reacts with heated copper(II) oxide to form zinc oxide and copper metal. If 3.0 g of zinc

are reacted with 3.0 g copper(II) oxide, which is the limiting reagent? What is the mass of

copper metal formed?

Exercise 8L

BOX 8.1

Avogadro

Lorenzo Romano Amedeo Carlo

Avogadro de Quaregnae di Cerreto

(1776–1856) had a very impressive

name – and an equally impressive

intellect. He graduated in law at 16

and received a doctorate at 20. He

practised law for a while, but became

interested in science and mathematics

and eventually gave up law to teach

these subjects.

He had read that one volume of oxygen

gas always reacted with twice the

volume of hydrogen gas to form water. If

the temperature was high enough, then

the water was formed as steam and

exactly two volumes of steam were

produced. He concluded that two

particlesof hydrogen gas must react

with one particleof oxygen gas to

produce two particlesof water. After

looking at the results of the experiments

of another famous chemist, Gay-Lussac,

in which reacting gas volumes had been

measured, Avogadro came up with the

following law (see page 159):

Equal volumes of different gases

(at the same temperature and

pressure) contain the same

number of particles

He called the gas particles molecules. If

Avogadro’s law is applied to volumes of

reacting gases, it is possible to deduce

their formulae. The law was discounted

by the scientific community until very

late in his lifetime.

8.1.Calculate the molecular mass of:

(i) nitrogen gas (N 2 )

(ii)copper(II) chloride

(iii)phosphoric acid (H 3 PO 4 )

(iv)calcium nitrate

(v)sodium sulfate decahydrate.

8.2.What is the mass of one mole of:

(i) chlorine atoms (Cl)

(ii)chlorine molecules (Cl 2 )

(iii)phosphorus molecules (P 4 )

(iv)iodide ions (I)

(v)buckminsterfullerene molecules (C 60 ).

8.3.Calculate the number of moles in:

(i) 30 g of oxygen molecules (O 2 )

(ii)31 g of phosphorus molecules (P 4 )

(iii)25 g of calcium carbonate

(iv)3.28 g of sulfur dioxide

(v)980 g of sulfuric acid.

8.4.Calculate the mass of each of the following:

(i) 0.500 mol of NaCl

(ii)0.250 mol of CO 2

(iii)2 mol of H 2

(iv)2.50 mol of CuSO 4 · 5H 2 O

(v)100 mol of HNO 3.

8.5.A sample of ammonia weighs 2.00 g. What mass of sulfur

dioxide contains the same number of molecules as are in

2.00 g of ammonia?

8.6.What mass of magnesium metal would react completely

with 8 g of sulfur?

Mg(s)S(s)MgS(s)

Revision questions