Question

Determination of molecular formula of hydrocarbons using combustion data

Answer 1

Example:

10cm3 of a gaseous hydrocarbon required 20 cm3 of oxygen for complete combustion. 10 cm3 of carbon dioxide was produced. Calculate the molecular formula of the hydrocarbon.

and

Suggested Solution:

Using the general equation and applying volume ratio, we have

CxHy (g) + (x + y/4) O2 (g) –> xCO2 (g) + y/2 H2O (l)

10cm3…… 20cm3 ……………….10cm3

1mol             2 mol                              1 mol

Based on the above comparision, we have

x = 1 and (x + y/4) = 2

Solving it gives y = 4

Hence, the molecular formula of the hydrocarbon is CH4.