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 $60\, g\, mol ^{-1}$.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

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 $C =\frac{\% \text { of } C }{\text { Atomic mass of } C }=\frac{20}{12}$
$=1.667\, mol$
Moles of $H =\frac{\% \text { of } H }{\text { Atomic mass of } H }=\frac{6.7}{1.0}$
$=6.700\, mol$
Moles of $N =\frac{\% \text { of } N }{\text { Atomic mass of } N }=\frac{46.67}{14}$
$=3.334\, mol$
Moles of $O =\frac{\% \text { of } O }{\text { Atomic mass of } O }=\frac{26.63}{16}$
$=1.664\, mol$
$\therefore \frac{1.667}{1.664} \approx 1, \frac{6.700}{1.664}=4, \frac{3.334}{1.664}=2,$
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 $= CH _{4} N _{2} O$
Empirical formula mass $=12+4+28+16$
$=60\, g\, mol ^{-1}$
Hence, Molecular formula $=$ Empirical formula $= CH _{4} N _{2} O$
Thus, total number of atoms in its molecular formula is $8$ .