Question

Complete combustion of $0.858g$ of compound $X$ gives $2.63g$ of $C{O}_2$ and $1.28g$ of $H_{2}O$. The lowest molecular mass $X$ can have

• A. 43 g
• B. 86 g
• C. 129 g
• D. 172 g

Answer 1

Answer: (A)

$\underset{\text{0.858 g}}{[CH]}+O_2\to \underset{\text{2.63}}{C{O}_2}+\underset{\text{1.28}}{H_{2}O}$
% age $C=\frac{12}{44}\times \frac{2.63}{0.858}\times 100$
$=83.59$
% age $H=\frac{2}{18}\times \frac{2.63}{0.858}\times 100$
$=16.57$

Element	$\%$	At. mass	$\%$ At. mass	Simplest ratio	Simplest whole no. ratio
C	83.59	12	$\frac{83.59}{12}=6.97$	$\frac{6.97}{6.97}=1$	3
H	16.57	1	$=\frac{16.57}{1}=16.57$	$\frac{16.57}{6.97}=2.37$	7

$\therefore E.F.=C_3{H}_7$
$\therefore E.F.$ mass (lowest mol mass)
$=12\times 3+7\times 1=43$