Question

An organic compound containing oxygen, carbon, hydrogen and nitrogen contains $20\%$ carbon, $6.7\%$ hydrogen and $46.67\%$ nitrogen. Its molecular mass was found to be $60gmo{l}^{-1}$.The total number of atoms in the molecular formula of the compound is_______.
(At. wt. : $C=12;H=1;N=14;O=16$)

Answer 1

Answer: 8

Sum of $\%=20+6.7+46.67=73.37$
This is less than $100\%$.
Hence, compound contains adequate oxygen so that the total percentage of elements is $100\%$.
Hence, $\%$ of oxygen $=100-73.37$
$=26.63\%$
Moles of $C=\frac{\%\text{ of }C}{\text{ Atomic mass of }C}=\frac{20}{12}$
$=1.667mol$
Moles of $H=\frac{\%\text{ of }H}{\text{ Atomic mass of }H}=\frac{6.7}{1.0}$
$=6.700mol$
Moles of $N=\frac{\%\text{ of }N}{\text{ Atomic mass of }N}=\frac{46.67}{14}$
$=3.334mol$
Moles of $O=\frac{\%\text{ of }O}{\text{ Atomic mass of }O}=\frac{26.63}{16}$
$=1.664mol$
$\therefore \frac{1.667}{1.664}\approx 1,\frac{6.700}{1.664}=4,\frac{3.334}{1.664}=2,$
and $\frac{1.664}{1.664}=1$
Hence, the ratio of number of moles of $C:H:N:O$ is $1:4:2:1$.
$\therefore$ Empirical formula $=C{H}_4{N}_{2}O$
Empirical formula mass $=12+4+28+16$
$=60gmo{l}^{-1}$
Hence, Molecular formula $=$ Empirical formula $=C{H}_4{N}_{2}O$
Thus, total number of atoms in its molecular formula is $8$ .