A 12 g sample of  $C{H}_4$ and $C_2{H}_4$ yielded 35.2 g of $C{O}_2$ on complete oxidation. What was the mean molar mass of the original sample?

Moles of  ${\text{CO}}_{\text{2}}$ = 35.2/44 = 0.8
(a is moles of  ${\text{CH}}_{\text{4}}$  and b is  ${\text{C}}_{\text{2}}{\text{H}}_{\text{4}}$  moles )
$\begin{array}{l}\text{a+2b = 0.8}\\ \text{a}\left(\text{16}\right)\text{ + b}\left(\text{28}\right)\text{ = 12}\\ \text{a + 1.75b = 0.75}\\ \text{0.25b = 0.05}\\ \text{b = 0.2, a = 0.4}\end{array}$
Mixer molecular mass =  $(0.2/0.6)28+(0.4/0.6)16=9.33+10.66=20$