In the qualitative analysis scheme, magnesium and nickel precipitate from soluti

Question

In the qualitative analysis scheme, magnesium and nickel precipitate from solution upon the addition of sodium hydroxide. Once separated from the remaining cations by filtration or decanting, the solid mixture is acidified and warmed to dissolve magnesium and nickel cations back into solution. Adding ammonia creates a buffer solution. (Remember, HCl and NH3 makes for NH4+ cation.) The buffered solution should just be basic (say, pH = 8). Adding sodium hydrogen phosphate (Na2HPO4) precipitates magnesium as MgNH4PO4 (Ksp = 3 x 10-13).

a)Write the dissociation equation and corresponding Ksp expression

b)Calculate the molar solubility of the compound

Explanation / Answer

Adding ammonia creates a buffer solution. (Remember, HCl and NH3 makes for NH4+ cation.)
The buffered solution should just be basic (say, pH = 8).

Ammonia equilibria, NH3 = NH4+ ; Kb = 1.8E-5
pKb = -Log(1.8E-5) = 4.74
pOH = 14 -pH = 14-8= 6

Henderson equation for ammonia buffer solution
pOH = pKb + Log([NH4+]/[NH3])
Log([NH4+]/[NH3]) = pOH - pKb = 6-4.74 = 1.26
[NH4+]/[NH3] = 18.2

Adding sodium hydrogen phosphate (Na2HPO4) precipitates magnesium as MgNH4PO4 (Ksp = 3 x 10-13).
Mg2+ + NH3 + HPO4(2-) = MgNH4PO4

a)Write the dissociation equation and corresponding Ksp expression
MgNH4PO4(s) = Mg2+(aq) + NH3(aq) + HPO4(2-)(aq)
Ksp = [Mg2+]*[NH3]*[HPO4(2-)] = 3 x 10-13

b)Calculate the molar solubility of the compound
Let the solubility is S
MgNH4PO4(s) = Mg2+(aq) + NH3(aq) + HPO4(2-)(aq)
[Mg2+] = S
[NH3]=S
[HPO4(2-)]=S
Ksp = [Mg2+]*[NH3]*[HPO4(2-)]= 3 x 10-13
S^3 = 3 x 10-13
S = 0.67 x 10-4 = 0.000067 M

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