Question

A well-known antibiotic penicillin contains 57.49% of carbon, 5.39% of hydrogen, 8.39% of nitrogen, 9.58% of sulphur and the remainder being oxygen. Calculate the empirical formula of the antibiotic.

• A. $C_{16}{H}_{18}{N}_{2}S{O}_4$
• B. $C_{18}{H}_{18}{N}_8{O}_7{S}_3$
• C. $C_{16}{H}_{16}{N}_{2}S{O}_4$
• D. $C_{16}{H}_{18}{N}_{2}S{O}_3$

Answer 1

The correct option is A $C_{16}{H}_{18}{N}_{2}S{O}_4$

% of $C$ = 57.49%
% of $H$ = 5.39%
% of $N$ = 8.39%
% of $S$ = 9.58%
% of $O$ = $100–(57.49+5.39+8.39+9.58)=19.15\%$

Calculating the empirical formula:
$\begin{array}{cccc}\text{Element}& \%& \text{Relative no. of atoms}& \text{Simplest Ratio}\\ C& 57.49& 57.49/12=4.79& 16\\ H& 5.39& 5.39/1=5.39& 18\\ N& 8.39& 8.39/14=0.599& 2\\ S& 9.58& 9.58/32=0.299& 1\\ O& 19.15& 19.15/16=1.196& 4\end{array}$
Empirical formula is $C_{16}{H}_{18}{N}_{2}S{O}_4$