The hydrocarbon (X) with molar mass 80 g mol^–1 and 90% carbon has ________ degree of unsaturation. 

 Mass of carbon $=\frac{80\times 90}{100}=72\mathrm{gm}$
Number of C-atoms$=\frac{72}{12}=6$
Mass of hydrogen$=\frac{80\times 10}{800}=8\mathrm{gm}$
Number of H-atoms$=\frac{8}{1}=8$
So molecular formula C₆H₈ 
D.U. = 6 + 1 –8/2 = 7 – 4 = 3