Chemistry, Third edition

CALCULATING GAS VOLUMES

Calculating gas volumes

When gases are involved in reactions, it is easier to measure their volumes rather

than weigh them and find their masses. Although we can calculate the masses of

gases used up or produced in chemical reactions using the methods described in the

previous section, it is often more convenient to calculate the volumes involved.

At this point, it is worth remembering that

One mole of ANY gas occupies a volume of 24 dm^3 at room temperature and

pressure.

‘Room temperature’ is taken as 20C or 293 K. ‘Normal’ atmospheric pressure is

1 atm, i.e. 101 325 Pa. (The pascal (Pa) is the SI unit of pressure – see page 157.)

8.6

129

Calculations based on equations

(i) What mass of magnesium oxide is formed by burning 1.2 g of magnesium metal in oxygen?

2Mg(s)O 2 (g)2MgO(s)

(ii)What mass of oxygen is formed when 4.34 g of mercury(II) oxide is completely

decomposed by heating?

2HgO(s)2Hg(I)O 2 (g)

(iii)What mass of copper(II) nitrate is needed to produce 16 g of copper(II) oxide upon

decomposition?

2Cu(NO 3 ) 2 (s)2CuO(s)4NO 2 (g)O 2 (g)

(iv)How much graphite (carbon, C) must be burned in oxygen gas to produce 2.2 g of carbon

dioxide gas? (You provide the balanced equation.)

(v)Solid copper(II) carbonate decomposes on heating to solid copper(II) oxide and carbon

dioxide gas. How much copper(II) oxide is obtained when 1.28 g of the carbonate is

heated? (Again, first provide the balanced equation.)

Exercise 8I

Example 8.8

What volume of carbon dioxide can be obtained by decomposing

50 g of calcium carbonate at room temperature and pressure?

Answer

Write the balanced equation:

CaCO 3 (s)CaO(s)CO 2 (g)

Work out the mole relationship between the substances you are considering:

1 molecule CaCO 3 1 molecule CO 2

Therefore,

1 mol CaCO 3 1 mol CO 2

Note that ‘’ can be replaced by ‘’ which can be taken to mean ‘reacts with’ or

‘reacts to form’ according to the amounts shown by the equation.

Work out the mole relationship between the substances you are considering: