A compound consists of 40% sulfur and 60% oxygen by mass. In a sample of the same compound containing 20 g of sulfur, what mass of oxygen must be present to satisfy the Law of Constant Proportions? [Pg. No. 167]

According to the law of constant proportions, a compound always contains the same elements combined in a fixed ratio by mass. Given composition of the compound:

$$\begin{aligned} \text{Sulphur} &= 40\% \\ \text{Oxygen} &= 60\% \end{aligned}$$

This means the mass ratio of sulphur : oxygen is:

$$40 : 60 = 2 : 3$$

So, for every 2 g of sulphur, 3 g of oxygen must be present.

If 2 g sulphur combines with 3 g oxygen, then 20 g sulphur will combine with:

$$\frac{3}{2} \times 20 = 30 \text{ g}$$

30 g of oxygen must be present with 20 g of sulphur to satisfy the law of constant proportions.