Question

MY and NY₃, two nearly insoluble salts, have the same Kₛₚ
values of 6.2 × 10⁻¹³ at room temperature. Which statement
would be true in regard to MY and NY₃ ? [2016]
(a) The molar solubilities of MY and NY₃ in water are
identica
(b) The molar solubility of MY in water is less than that of
NY₃
(c) The salts MY and NY₃ are more soluble in 0.5 M KY
than in pure water.
(d) The addition of the salt of KY to solution of MY and
NY₃ will have no effect on their solubilities.

Answer 1

the statement  which is true in regard to MY and NY₃ is:

(b) The molar solubility of MY in water is less than that of

NY₃

- The dissociation of MY can be represented as:

MY ⇔ M⁺ + Y⁻

            s       s

let s be the molar solubility of MY

So,         K = s × s

              K = s²

6.2 × 10⁻¹³ = s²

              s =  7.87 × 10⁻⁷

-The dissociation of NY₃ can be represented as:

NY₃ ⇔ N³⁺ + 3Y⁻

            s'       3s'

let s' be the molar solubility of NY₃

 So,         K =  s' × (3s')³

              K = 27s'⁴

6.2 × 10⁻¹³  = 27s'⁴

              s' = 3.89 × 10⁻⁴

- from the above calculations, it is clear that s' > s

⇒The molar solubility of MY in water is less than that of

NY₃

- so the correct option is (b)