If 3 g of element C combine with 8 g of element D to form compound CD , then 16 g of D are needed to form compound CD₂.

Reasoning step by step

  • In CD , the mass ratio is

C:D=3textg:8textg\text{C}:\text{D}=3\\text{g}:8\\text{g}C:D=3textg:8textg

so 8 g of D combines with 3 g of C.

  • In CD₂ , there are twice as many D atoms per C atom as in CD.
    Therefore, the mass of D that combines with the same 3 g of C doubles:

8textg×2=16textg8\\text{g}\times 2=16\\text{g}8textg×2=16textg

so 16 g of D are needed to form CD₂.

Quick comparison table

Compound| Ratio of atoms (C:D)| Mass of C (g)| Mass of D (g)
---|---|---|---
CD| 1:1| 3| 8
CD₂| 1:2| 3| 16

This follows the law of definite proportions (same elements combine in fixed mass ratios for a given compound).