1 mol of a gaseous aliphatic compound ${\mathrm{C}}_n{\mathrm{H}}_{3n}{\mathrm{O}}_m$ is completely burnt in an excess of oxygen. The contraction n volume is (assume water gets condensed out):

${\mathrm{C}}_n{\mathrm{H}}_{3n}{\mathrm{O}}_m + \left[n+\frac{3n}{4}-\frac{m}{2}\right]{\mathrm{O}}_2 \to n{\mathrm{CO}}_2\mathit{ }+ \frac{3n}{2}{\mathrm{H}}_2\mathrm{O}$

$1 \left(\frac{7n}{4}-\frac{m}{2}\right) - -$

$- - \mathrm{n} -$

Vol. contraction $=1+\frac{7n}{4}-\frac{m}{2}-n=1+\frac{3n}{4}-\frac{m}{2}$