a hydrocarbon contains 82.8% of carbon. Find its molecular formula if its vapour density is 29.

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Solution

1. Vapour Density = $\frac{M o l a r m a s s o f g a s}{M o l a r m a s s o f H_{2}} 29 = \frac{M}{2} M o l a r m a s s o f c o m p o u n d = 58$

2. The gas contains 82.8% of C and 17.2% of H.

Atomic mass of C = 12
Atomic mass of H = 2

So, Number of moles of C = $\frac{82.8}{12} = 6.9$
Number of moles of H = $\frac{17.2}{1} = 17.2$
3. Simplest ratio :
C = $\frac{6.9}{6.9} = 1$
H = $\frac{17.2}{6.9} = 2.4 \approx 2$
So, Molecular mass is C₄H₁₀.

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