Question

Butyric acid is an organic compound containing only C, H and O and is a component of rancid butter. If 0.44 g of butyric acid is burnt in excess oxygen, it yields 0.88 g of $C{O}_2$ and 0.36 g of water as the only products. The empirical formula of butyric acid is:

• A. $CH{O}_4$
• B. $CHO$
• C. $C_2{H}_{4}O$
• D. $C{H}_{4}O$

Answer 1

The correct option is C $C_2{H}_{4}O$

Butyric acid $\stackrel{Heat}{\to }{H}_{2}O+C{O}_2$

Calculation of mass of C
44g of $C{O}_2$ contains = 12g of C
$\therefore$ 0.88g of $C{O}_2$ contains = $\frac{12}{44}\times 0.88=0.24g$ of C

Calculation of mass of H
18g of $H_{2}O$ contains = 2g of C
$\therefore$ 0.36g of $H_{2}O$ contains = $\frac{2}{18}\times 0.36=0.04g$ of H

Calculation of mass of O
Mass of butyric acid-(mass of C + mass of H) = 0.44-(0.24+0.04) = 0.16g

Calculation of empirical formula:
$\begin{array}{cccc}\text{Element}& \text{Mass of element}& \text{No.of mol}& \text{Relative no. of atoms}\\ C& 0.24g& \frac{0.24}{12}=0.02& \frac{0.02}{0.01}=2\\ H& 0.04g& \frac{0.04}{1}=0.04& \frac{0.04}{0.01}=4\\ O& 0.16g& \frac{0.16}{16}=0.01& \frac{0.01}{0.01}=1\end{array}$

Hence, empirical formula of butyric acid is $C_2{H}_{4}O$.