Complete combustion of 35 cm3 of a straight-chain alkane vapour gave 105 cm3 of carbon dioxide gas. Both gas volumes were measured at the same temperature and pressure. Which of the following is the molecular formula of the alkane?

Answer:

22) D

Options A and C are wrong because they are alkenes.
Let molecular formula of alkane be CxHy

Ratio of volumes of alkane and CO2 = 35 : 105 = 1 : 3

This means that number of carbons (x) = 3

Only options C and D have hydrocarbons where number of carbons are 3. We have already established that option C is wrong because it is an alkene so option D is correct.