$3 \mathrm{g}$ of a hydrocarbon on combustion in excess of oxygen produces $8.8 \mathrm{g}$ of ${\mathrm{CO}}_2$ and 5.4 g of ${\mathrm{H}}_2\mathrm{O}$. The data illustrates the law of:

Here, 
Weight of hydrogen $= 3\mathrm{g}$ (given)
Weight of C in $8.8\mathrm{g}$ of ${\mathrm{CO}}_2=\frac{8.8}{44}+12=2.4$
Weight of H in $5.4g$of ${\mathrm{H}}_2\mathrm{O}=\frac{5.4}{18}\times 2=0.6$
From equation 1 and 2,

$,2.4+0.62=3\mathrm{g}$
This shows the conservation of mass