Question

A gaseous hydrocarbon contains 82.76% of carbon. Given that its vapor density is 29, find its molecular formula. [C = 12, H = 11]

Answer 1

Element	Percentage	Molecules	Simple ratio	Ration	Simple whole ratio
Carbon	82.76	`82.76/12` = 6.9	`6.89/6.89` = 1	`6.9/6.9` = 1	2
Hydrogen	17.24	`17.24/1` = 17.24	`17.24/6.89` = 2.5	`17.24/6.9` = 2.5	5

Empirical formula = C₂H₅

Empirical formula mass = (12 × 2) + (1 × 5)

= 24 + 5 

= 29

Vapour density = 29    ...(Given)

Molecular weight = 2 × Vapour density

Molecular weight = 2 × 29

= 58 gm

Molecular formula mass = n × Empirical formula mass

⇒ n = `"Molecular formula mass"/"Empirical formula mass"`

= `58/29`

= 2

Molecular formula = n × Empirical formula

= 2 × C₂H₅

= C₄H₁₀