The number of isomers of C 5 H 12 is:

${\mathrm{C}}_5{\mathrm{H}}_{12}$ is an alkane. It can have the following structures:

$\left(\mathrm{a}\right)$ n-pentane

$\left(\mathrm{b}\right)$ $2-$methylbutane

$\left(\mathrm{c}\right)$ $2,2-$dimethylpropane. It is also known as neopropane.

Therefore, there are $3$ isomers of ${\mathrm{C}}_5{\mathrm{H}}_{12}$.