Question

Apply the law of constant proportions to calculate the mass of oxygen that will be used up for combustion of $5{\text{ }}g$ of ${H_2}$ gas.

Answer 1

The law of constant proportion states that a given compound always contains its component elements in fixed ratio and doesn't depend on its surface and method of preparation. This means that no matter the source, every pure sample of a compound will always consist of the same elements that are present in the same mass ratio.

Complete step by step answer:
Now I am writing combustion reaction for ${H_2}$ which produce steam
$2{H_2} + {O_2} \to 2{H_2}O$
$2 \times 2gram + 32gram \to 2 \times 18gram$(Law of proportion)
Hence the combination of $4gram$ of ${H_2}$ with $32gram$ of ${O_2}$ to give $36gram$ of ${H_2}O$
According to the question we need to find for $5{\text{ }}g$ of ${H_2}$ gas mass how much gas of ${O_2}$ is required.
For $4g$ of ${H_2}$ gas combustion requires $32g$ of oxygen
For $5g$ of ${H_2}$ gas combustion how much of ${O_2}$ we need?
$4gram{H_2} \to 32gram{O_2}$
$5gram{H_2} \to Xgram{O_2}$
$Xgram = \dfrac{{5gram \times 32gram}}{{4gram}}$
$Xgram{O_2} = 40gram$
Hence for $5gram$ of ${H_2}$ we need $40gram$ of ${O_2}$
Answer to this question is $40gram$ of ${O_2}$.

ADDITIONAL INFORMATION:
The law of constant proportions is typically mentioned as Proust’s law or because the law of definite proportions. An illustration describing the mass ratio of elements during a couple of compounds is provided below.
Exceptions:
The ratio of elements in non-stoichiometric compounds varies from point to point. Therefore, these compounds are an exception to the law of constant proportions. Natural polymers also are known to disobey the law of constant proportions.
For all sorts of substances, the law of constant proportion isn't valid. Elements with a stable isotope mixture form a non-stoichiometric product. The role of few elements within the crystal structure is replaced by their isotopes which induce the crystal’s internal composition to vary.

Note: The law of constant proportions is also known as the law of Proust or as the law of definite proportions.
-The law of constant proportions does not hold true for all chemical compounds, even though it is a building block in the development of chemistry.
-Since natural polymers can vary in their compositions, different mass proportions will be exhibited by different samples.