Question

Arsenic forms two oxides, one of which contains $65.2$ % and the other, $75.7$ % of the element. Determine the equivalent weights of arsenic in both cases.

Answer 1

Equivalent weight is also known as gram equivalent. Equivalent weight is a mass of a given substance which will combine with a fixed quantity of another substance. We shall take the ratio of both the elements in each of the compounds and use it to determine the equivalent weight of arsenic.
Formula Used: ${\text{equivalent wgt}} = \dfrac{{{\text{molecular wgt}}}}{{{\text{valency}}}}$

Complete step by step solution
To calculate equivalent weight there are various ways. For acids, equivalent weight can be defined as the ratio of the molar mass of the acid divided by the number of ${H^ + }$ ions it releases. For base, equivalent weight can be calculated by taking the ratio of the molar mass of the base and the number of $O{H^ - }$ ions it produces. For salts, equivalent weight is the ratio of molecular weight to the charge of either cation or anion. For elements, equivalent weight is the ratio of the given weight divided by its valency.
Arsenic oxide which contains $65.2$ % arsenic has $34.8$ % oxygen. And arsenic oxide which has $75.7$ % of arsenic has $24.3$ % of oxygen.
The equivalent weight of oxygen can be calculated by taking the ratio of the molecular weight of oxygen and its valency. Molecular weight of oxygen is 16 and its valency is generally 2. Hence, the equivalent weight of oxygen is 8.
In the first arsenic oxide, gram equivalents of arsenic are equal to gram equivalents of oxygen.
Gram equivalent of arsenic = Gram equivalent of oxygen
$\dfrac{{65.2}}{{{\text{Equivalent weight of arsenic}}}} = \dfrac{{100 - 65.2}}{8}$
Equivalent weight of arsenic = 15 g
Therefore, the equivalent weight of arsenic in the first arsenic oxide is 15 g.
In the second arsenic oxide,
Gram equivalent of arsenic = Gram equivalent of oxygen
$\dfrac{{75.7}}{{{\text{Equivalent weight of arsenic}}}} = \dfrac{{100 - 75.7}}{8}$
Equivalent weight of arsenic = $24.9$ g
Therefore, the equivalent weight of arsenic in the second arsenic oxide is $24.9$ g.

Note
The symbol of arsenic is As. The atomic number of arsenic is 33. Arsenic is a metalloid having various allotropes. The main use of arsenic is in alloys of lead. Arsenic is used as wood preservative.