4 grams of Hydrogen reacts with 40 grams of Oxygen to form water. Excess reagent in the reaction is

Alternatively: 4 gm ${\mathrm{H}}_2$ = 4 equivalents which reacts with 4 equivalents of ${\mathrm{O}}_2$, 32 gms. 

Mass of excess ${\mathrm{O}}_2$ = 40  - 32  = 8gm.