A gaseous hydrocarbon contains 82.76 \% of ...

ICSE Class 10 Chemistry 2016 Paper

Question : 63 of 85

Marks: +1, -0

A gaseous hydrocarbon contains

82.76 \%

of carbon. Given that its vapour density is 29 , find its molecular formula.

[C=12, H=1]

Solution:

Element / Compound	$\%$ Mass	Atomic or Mol. Mass	No. of Atoms	Simplest Ratio	Rounding off Ratio
C	82.76	12	$\;\; \frac{82.76}{12}=6.89$	$\; \frac{6.89}{6.89}=1$	$1 \times 2=2$
H	17.21	1	$\;\; \frac{17.24}{1}=17.24$	$\; \frac{17.24}{6.89}=2. 5$	$2.5 \times 2=5$

\; \therefore \; \text{ The empirical formula is }\; \mathrm{C}_2 \mathrm{H}_5 \; \text{. }\;

Empirical formula mass of

\mathrm{C}_2 \mathrm{H}_5 = 12 \times 2 + 5

\; \times 1 = 29 \; \text{. }\;

\;\; \text{ V.D. }\; = 29 \; \text{ (given) }\;

\;\; \text{ Mol. Mass }\; = 2 \times \; \text{ V.D. }\;

\; = 2 \times 29 = 58

\; n = \frac{\; \text{ Molecular Mass }\;}{\; \text{ Emp. formula Mass }\;}

\; = \frac{58}{29} = 2

Molecular formula

= (\; \text{ Empirical formula }\;)_n

\; = ( \mathrm{C}_2 \mathrm{H}_5)_2

\; = \mathrm{C}_4 \mathrm{H}_{10}

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