The number of n electrons present in $2.6 \mathrm{g}$ of ethyne are: 

Ethyne is ${\mathrm{C}}_2{\mathrm{H}}_2$There are two $\mathrm{\pi }-$bonds in ethyne. One bond contains 2 electrons. So, there are $2 x 2$i.e., $4\mathrm{\pi }$ electrons is one molecule of ethyne.

Mol. wt. of $\mathrm{CH}\equiv \mathrm{CH}=12+1+12+1=26=1 \mathrm{mol}$ of $\mathrm{CH}\equiv \mathrm{CH}$

$\begin{array}{r}\therefore 26\mathrm{g}\cdot \mathrm{CH}\equiv \mathrm{CH}\text{ contain }\mathrm{\pi }\text{ electrons }=4\times {\mathrm{N}}_{\mathrm{A}}\\ 2.6\mathrm{g}\cdot \mathrm{CH}\equiv \mathrm{CH}\text{ contain }\mathrm{\pi }\text{ electrons }\\ =\frac{4{\mathrm{N}}_{\mathrm{A}}\times 2.6}{26}=0.4{\mathrm{N}}_{\mathrm{A}}\end{array}$