ICSE Class 10 Chemistry 2016 Solved 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	$\;\;{82 .76}/{12}=6.89$	$\;{6 .89}/{6 .89}=1$	$1 \×2=2$
H	17.21	1	$\;\;{17.24}/{1}=17.24$	$\;{17.24}/{6 .89}=2. 5$	$2.5 \×2=5$

\; ∴ \;\text " The empirical formula is "\; {\C}_2 {\H}_5 \;\text ". "\;

Empirical formula mass of

{\C}_2 {\H}_5=12 × 2+5

\;× 1=29 \;\text ". "\;

\;\;\text " V.D. "\;=29 \;\text " (given) "\;

\;\;\text " Mol. Mass "\;=2 × \;\text " V.D. "\;

\;=2 × 29=58

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

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

Molecular formula

=(\;\text " Empirical formula "\;)_n

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

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

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