Chemistry, Third edition

128 8 · THE MOLE

Example 8.7

What mass of iodine will react completely with 10 g of

aluminium to form aluminium iodide (Al 2 I 6 )?

Answer

Write the balanced equation:

2Al(s)3I 2 (s)Al 2 I 6 (s)

Work out the relationship between the molecules involved:

2 atoms Al 3 molecules I 2 1 molecule Al 2 I 6

You could also write:

4 atoms Al 6 molecules I 2 2 molecules Al 2 I 6

or

200 atoms Al 300 molecules I 2 100 molecules Al 2 I 6

or even

2 6.022 1023 atoms Al  3 6.022 1023 molecules I 2

 1 6.022 1023 molecules Al 2 I 6

The last line can also be written as

2 mol Al 3 mol I 2 1 mol Al 2 I 6

Convert the numbers of moles to grams using molar masses, M(Al)27 g mol^1

andM(I)127 g mol^1 :

2 27 g Al  3 254 g I 2  1 816 g Al 2 I 6

or

54 g Al 762 g I 2 816 g Al 2 I 6

We now have the ratio between the quantities of reactants and products in grams.

In the question, you start with 10 g of aluminium, so what masses of iodine and

aluminium iodide are involved if the above ratio is to apply?

For 1 g of aluminium:

1 g Al 

762

g I 2 

816

g Al 2 I 6

54 54

Therefore, for 10 g aluminium:

10 g Al 

762  10

g I 2 

816  10

g Al 2 I 6

54 54

or

10 g Al 141 g I 2 151 g Al 2 I 6

So, 10 g of aluminium reacts with 141 g I 2.

Comment

The calculation is written in simple steps to help you understand. In practice, you

can leave out some of the more obvious steps and ignore the calculations

involving Al 2 I 6 , since they are not asked for in the question – the question only

asks for the relationship between aluminium and iodine.