Question

Determine the minimum concentration of the precipitating agent on the right to cause precipitation of the cation from the solution on the left. 4.0×10−2 M Ba(NO3)2;NaF 8.5×10−2 M CaI2;K2SO4

Answer 1

(a). Given, [Ba²⁺] = 4.0 x 10^-2 M

Ba(NO₃)₂ (aq) + 2 NaF (aq) -----------------> BaF₂ (s) + 2 NaNO₃

Partial dissociation of BaF₂ is :

BaF₂ (s) <--------------------> Ba²⁺ (aq) + 2 F^- (aq)

Expression of K_sp is :

K_sp = [Ba²⁺].[F^-]²

[F^-]² = K_sp / [Ba²⁺]

[F^-] = (2.45 x 10^-5 / 4.0 x 10^-2)^1/2

[F^-] = 2.5 x 10^-2 M

Hence, minimum concentration = 2.5 x 10^-2 M

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(b). Given, [Ca²⁺] = 8.5 x 10^-2 M

CaI₂ (aq) + K₂SO₄ (aq) -----------------> CaSO₄(s) + 2 KI (aq)

Partial dissociation of CaSO₄(s) is :

CaSO₄(s) <----------------> Ca²⁺(aq) + SO₄^2-(aq)

Expression of K_sp is :

K_sp = [Ca²⁺].[SO₄^2-]

[SO₄^2-] = K_sp / [Ca²⁺]

[SO₄^2-] = (7.10 x 10^-5 / 8.5 x 10^-2)

[SO₄^2-] = 8.35 x 10^-4 M

Hence, minimum concentration = 8.35 x 10^-4 M