Question

# In an experiment, 2.4g of iron oxide on reduction with hydrogen gave 1.68g of iron. In another experiment, 2.9g of iron oxide gave 2.09g of iron on reduction. Which law is illustrated from the above data?[A] Law of constant proportions.[B] Law of multiple proportions.[C] Law of reciprocal proportions.[D] Law of conservation of mass.

Hint: To solve this question, we have to find out the mass of iron and oxygen in the starting material in both the cases. Comparing the ratio of mass of iron and oxygen from both the experiments will help us understand which option is correct.

In the question, it is given to us that 2.4g of iron oxide on reduction with hydrogen gave 1.68g of iron in the first experiment. We can write the reaction as-
$FeO\to Fe{{O}_{2}}$
Here, the starting product was 2.4g and in the product there was 1.68g of iron.
Therefore, the percentage of iron in iron oxide is $\dfrac{1.68}{2.4}\times 100=70%$
As 70% of iron oxide is iron therefore, the remaining 30% will be oxygen.
Therefore we can write the ratio of mass of iron and the mass of oxygen in iron oxide in this experiment will be = 7:3
In the second experiment, 2.9g of iron oxide gave us 2.09g of iron on reduction.
Therefore, the percentage of iron in iron oxide in this experiment will be $\dfrac{2.09}{2.9}\times 100=72%$
Here, the percentage of iron is 72% therefore the remaining 28% will be oxygen.
Therefore, the ratio of mass of iron and oxygen in iron oxide in the second experiment will be = 72:28 $\simeq$7:3.
As we can see from the above calculation that the ratio of iron and oxygen in iron oxide is almost similar in both the experiments and the law of constant proportion states that a given chemical compound will always contain its components in fixed ratios irrespective of the method of it was prepared by or the source.
So, the correct answer is “Option A”.

Note: The law of constant proportion does not hold true if we take two isotopes of the same element to make the chemical compound. It is also inapplicable if the elements combine in the same ratio but form different compounds. This law is also inapplicable for when the elements exist in the form of different isotopes in the obtained chemical compound.