Chemistry, Third edition

132 8 · THE MOLE

Example 8.10

In order to understand the idea of a limiting reagent, imagine

that you are packing biscuits. If each packet can hold ten

biscuits, the ‘reaction’ is

1 empty packet 10 loose biscuits packet of biscuits

You have 25 empty packets and 200 loose biscuits. How many

packets of biscuits can you make?

Answer

20 with 5 empty packets left over. Here, you do not have enough biscuits to fill all

the packets. The number of loose biscuits (the limiting reagent) controls the

number of packets of biscuits you can make.

Comment

If you have 12 empty packets and 150 biscuits, you can make 12 packets of

biscuits and 30 biscuits remain. This time it is the number of empty packets that

is the limiting reagent.

In a chemical reaction, unless the exact amounts of each reagent (as specified

by the balanced equation for the reaction) are reacted together, then one reagent

is always limiting. The limiting reagent can be spotted by comparing the ratio of

the amounts of reactants available with the ratio of the amounts of reactants

obtained from the balanced equation for the reaction.

Example 8.11

Copper reacts with silver nitrate solution according to the

equation

Cu(s)2AgNO 3 (aq)Cu(NO 3 ) 2 (aq)2Ag(s)

If 0.50 mol of copper is added to 1.5 mol of silver nitrate, which is

the limiting reagent and how many moles of silver are formed?

Answer

Decide which is the limiting reagent

According to the equation, 1 mol Cu 2 mol AgNO 3 , so

0.50 mol Cu  2 0.501.0 mol AgNO 3

but there are 0.50 mol Cu and 1.5 mol AgNO 3. Therefore, AgNO 3 is present in

excess and Cu is the limiting reagent.

Calculate how many moles of silver are formed

Use the amount of the limiting reagent to find the amount of product. According to

the equation, 1 mol Cu 2 mol Ag. Therefore,

0.50 mol Cu  2 0.501.0 mol Ag

So 1.0 mol Ag is formed.