Question

A solution contains 5.84×10^-3 M potassium fluoride and 5.84×10^-3 M ammonium carbonate.
Solid magnesium nitrate is added slowly to this mixture.

A. What is the formula of the substance that precipitates first?

formula =

B. What is the concentration of magnesium ion when this precipitation first begins?
[Mg²⁺] =  M

Answer 1

Ksp of MgF2 = 3.7*10^-8

Ksp of MgCO3 = 3.5×10^–8

Ksp = [Mg+2] [ F-2] ^2

Ksp = [Mg+2] [ CO3-2]

KF2 ----> K+ + 2F-

[F-] = 2*5.84*10^-3 M

3.7*10^-8 = [Mg+2] [ 2*5.84*10^-3]^2

[Mg+2] = 2.71*10^-4 M

This much amount of Mg+2 is needed for precipitation

For MgCO3

(NH4)2CO3 ----> 2NH4+ + CO3-2

[CO3-2] = 5.84*10^-3 M

3.5×10^–8 = [Mg+2] [ 5.84*10^-3 M]

[Mg+2] = 6*10^-6 M

From the two case

we have seen less amount Mg+2 is needed for MgCO3 precipitate

A] So the answer is MgCO3 ---> first one

B] [Mg+2] = 6*10^-6 M