Moles and gas volumes At the end of this section you should be able to calculate the amount of substance in moles, using gas volume.

Avogadro’s hypothesis Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules

At room temperature and pressure (RTP): One mole of gas molecules occupies approximately 24.0dm 3 (24000cm 3 ) The volume per mole of gas molecules is 24.0 dm 3 mol -1

To work out amount, in moles, in a solution, use the equations: n = V(in dm 3 ) 24.0 n = V(in dm 3 ) Hence, V= n x 24000cm 3

Worked examples What amount, in mol, of gas molecules are in the following gas volumes at RTP? (i) 36 dm 3 (ii) 250cm 3

(i)n = = 1.5mol (ii) n = = 0.01mol

What is the volume of the following at RTP? (i) 2 mol SO 2 (ii) 0.15 mol H 2

(i) V = 2 x cm 3 = cm 3 (ii) V = 0.15 x cm 3 = 3600 cm 3

What is the mass of the following at RTP? (i) 0.6 dm 3 N 2 (ii) 1950 cm 3 C 2 H 4 NB: mass = mole x molecular mass

(i)Mole = = mol from, mass = mole x M M(N 2 ) = 14 x 2 = 28 mass = x 28 = 0.7g

(ii) Mole = = mol mass = mole x M M(C 2 H 4 ) = (12 x 2) + 1x4 = 28 hence, mass = x 28 = 2.275g

What is the volume of the following at RTP? (i) g SO 2 (ii) 2.5 g CO 2 NB: For this question, you need to calculate the moles and then follow it up with volume

(i)Mole = mass/molecular mass M(SO 2 ) = x2 = 64 mole = / 64 = 0.02 mol from, V = n x cm 3 = 0.02 x = cm 3

Now try question 1 – 3 Page 15 Question 8 (a) and (b) Page 35