As per the law of definite proportion,carbon and oxygen combine in the ratio of $3:8$. Compute the mass of oxygen gas that would be required to react completely with $15\,g$ of carbon.

(D) According to the law of definite proportion,carbon and oxygen combine in a fixed mass ratio of $3:8$.
This means that $3\,g$ of carbon reacts completely with $8\,g$ of oxygen.
Therefore,$1\,g$ of carbon reacts with $\frac{8}{3}\,g$ of oxygen.
For $15\,g$ of carbon,the mass of oxygen required is $\frac{8}{3} \times 15\,g = 8 \times 5\,g = 40\,g$.
Thus,$40\,g$ of oxygen gas is required to react completely with $15\,g$ of carbon.