Question

A saturated solution of CaSO4 is made by dissolving excess solid CaSO4 in a 1 L solution of 0.077 M Na2S04. When the solid CaSO4 is added to the solution only 43. 3 mg of the solid actually dissolves. Find the Ksp for CaSO4. (A) 2. 66 x 10-5 (B) 2. 58 x 10-5 (C) 2. 88 x 10-5 (D) 2. 46 x 10-5 (E) 2. 73 x 10-5 Submit

Answer 1

Sol.

Mass of CaSO4 = 43.3 mg = 43.3 / 1000 g = 0.0433 g

Molar mass of CaSO4 = 136.14 g/mol

Volume of solution = 1 L

Concentration of CaSO4 = [CaSO4]

= mass of CaSO4 / ( molar mass of CaSO4 × volume of solution )

= 0.0433 g / ( 136.14 g/mol × 1 L )  

= 0.00032 M  

So , Concentration of Ca2+ = [Ca2+]

= [CaSO4] = 0.00032 M   

and Concentration of SO42- = [SO42-] = [CaSO4] = 0.00032 M

Also , from Na2SO4 ,

[SO42-] = [Na2SO4] = 0.077 M

Total [SO42-] = 0.00032 M + 0.077 M = 0.07732 M

Dissociation reaction of CaSO4 :

CaSO4 ----> Ca2+ + SO42-  

Ksp = [Ca2+] × Total [SO42-]  

= 0.00032 × 0.07732

= 2.46 × 10^-5 ( answer )