A 4.00\text{ g} sample of a copper oxide is completely reduced by dry hydrogen gas to yiel… — JAMB UTME

A $4.00\text{ g}$ sample of a copper oxide is completely reduced by dry hydrogen gas to yield $3.20\text{ g}$ of metallic copper. According to the Law of Definite Proportions, what mass of this same copper oxide, in grams, will be produced when $5.00\text{ g}$ of pure copper is completely oxidized?

Answer: 6.25 g

Answer

6.25 g

According to the Law of Definite Proportions (or Constant Composition), a chemical compound always contains its component elements in fixed mass ratios. In the first sample, copper makes up

3.20\text{ g} / 4.00\text{ g} = 0.80

80\%

of the total mass. Therefore, in any sample of this oxide,

5.00\text{ g}

of copper represents

80\%

of the total mass. Dividing

5.00\text{ g}

0.80

gives

6.25\text{ g}

of copper oxide.

Step-by-Step Solution

Determine the mass percentage (or mass fraction) of copper in the compound from the first experiment

Mass fraction of Cu = 3.20 / 4.00 = 0.80 (80%)

The first experiment provides quantitative data regarding the mass of copper contained in a known mass of oxide.

Apply the Law of Definite Proportions to calculate the required mass of copper oxide for 5.00 g of copper

Mass of Copper Oxide = 5.00 / 0.80 = 6.25 g

The Law of Definite Proportions dictates that the mass composition ratio remains constant regardless of the sample source or size.

Key Concept

Law of Definite Proportions

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