Question

Including activity coefficients, find the [Hg₂²⁺] in saturated Hg₂Br₂ in (a) water, (b) 0.00100 M KNO₃, and (c) 0.00100 M KBr

Answer 1

Hg₂Br₂ <====> Hg₂²⁺_(aq) + 2Br ^-_(aq)

	Hg2Br2(s) <====>	Hg22+(aq)	2Br -(aq)
equilibrium	-	x	2x

Let us suppose solubility of [Hg₂Br₂] = x moles/L

a)

In water (aqueous)

Water ionic strength is very low

So

   and     are close to 1

Therefore ,

But Ksp of Hg₂Cl₂ = 5.6*10^-23

So,

Molarity of Hg₂²⁺ = 2.410*10^-8 M and [Br^-] = 4.82*10^-8 M

b) in 0.00100 M KNO₃

Here KNO_3(aq)   -------------> K⁺_(aq) + NO₃^-_(aq)

[K⁺] = [NO₃^-] = 0.00100 M

Ionic strength =

,

where C = concentration of ions, Z is the valency of ions

Using the above fromula we can find ionic strength

(at 0.00100) = 0.867

(at 0.00100) = 0.964

x= 2.6 *10^-8M = [Hg₂²⁺]

c) 0.00100 M KBr

Let us calculate the ionic strength of KBr =

NO change in the activity coefficient values because ionic strength is same

Hg₂Br₂ <====> Hg₂²⁺_(aq)   + 2Br ^-_(aq)

In the first problem we have sessn that concentration of Br- ions in the solution is ver less

Externally we have added KBr

So the [Br^-] = 0.00100 M