Question

Potassium selenite is isomorphous with potassium sulphate and contains $50.0\%$ of Se. Find the atomic weight of Se ( in g).
a.) 142.0
b.) 71.0
c.) 47.33
d.) 248.0

Answer 1

Hint: Here we will first define the term isomorphous then we will solve the few equations with the given conditions and compare the molecular weight to find atomic weight of Se. Isomorphism is the existence of the same crystally structured two or more substances, which form solid solutions.

Complete answer:
It is given that potassium selenate is isomorphous to ${K_2}S{O_4}$ and therefore the molecular formula of potassium selenate is ${K_2}Se{O_4}$
First we will calculate the molecular weight of ${K_2}Se{O_4}$
Let the molecular weight of Se be x.
Now molecular weight of ${K_2}Se{O_4}$
$\Rightarrow 39 \times 2 + x + 4 \times 16 \\\ \Rightarrow 142 + x \\\$
It is given that Potassium selenite is isomorphous with potassium sulphate and contains $50.0\%$ of Se
$\left( {142 + x} \right){\text{g of }}{K_2}Se{O_4}{\text{ contains}} = Se \times xg$
Therefore 100g ${K_2}Se{O_4}$ contains
$100g{K_2}Se{O_4} = \dfrac{{x \times 100}}{{142 + x}}..............\left( 1 \right)$
The percentage of Se = 50 ……………(2)
Now equating equation (1) and (2)
$50 = \dfrac{{100x}}{{142 + x}} \\\ 142 + x = 2x \\\ x = 142g \\\$
Thus, the atomic mass of selenium is 142g and the correct option is A.
Additional Information- Isomorphic substance crystals are nearly the same. The materials sometimes crystallize in a solid solution together. The polarizations and the ratio of the cation and anion radii are usually similar in substances with isomorphic chemical formulas.

Note:
In order to solve these types of questions, you need to have the basic terms related to questions such as in the above question isomorphism. The law of isomorphism has an approximate law that indicates that crystals with the same number of similar elements tend to show isomorphism. Remember these terms and solve the questions step by step.