Question

Potassium selenate contains 35 75% selenium and is isomorphous with potassium sulphate. Find out the atomic weight of selenium (S = 32, K = 39).

Answer 1

Since potassium selenate is isomorphous with potassium sulphate, K₂SO₄, the former should have the formula as K₂SCO₄. 

Let the atomic weight of Se = x

Then the molecular wt. of K₂SO₄ = 78 + x + 64 = x + 142 

Determination of % of Se in K₂SeO₄

Thus (x + 142) g of K₂SO₄ contain x g of se

100 g of K₂SeO₄ contain \(\frac x{x + 142} \times 100\)

Hence % of Se in K₂SeO₄ = \(\frac{x \times 100}{(x + 142)}\)

or \(35.75 =\frac{x \times 100}{(x + 142)}\)

On usual calculations, x = 79.0116

Hence the atomic weight of Se = 79.0116