Nickel ion ( Ni 2+ ) and OH - are inequilibrium with solid Ni(OH) 2 in a 1-L sol

Question

Nickel ion ( Ni2+ ) and OH- are inequilibrium with solid Ni(OH)2 in a 1-L solution formed by dissolving pure Ni(OH)2 in water. What is the equilibrium concentration of Ni2+ ions ( in mole/L andmg/L)? What is the final concentration of Ni2+ ions insolution ( in mg/L) after the pH is adjusted to 7 using HCl (ignore the small increase in volume)? The KSP of Ni(OH)2 is 5.5 x 10-16 at 25oC.

The solution is needed for the second part of the question that is asking for the final concentration of Ni2+ after pH is adjusted.

The final answer is 3,230 mg/L

Explanation / Answer

a)
Ni(OH)2   ----->    Ni2+    +   2OH-
                                       X              2X

Ksp=[Ni2+] [OH-]
5.5*10^-16 = X *(2X)^2
5.5*10^-16 = 4 *X^3
X = 5.2*10^-6 M

Concentration of Ni2+ is 5.2*10^-6 mol/L
molar mass if Ni is 58.7 gm
In mg /L it is 5.2*10^-6*58.7*1000 = 0.3mg/L

b)
pH= 7
[H+]=10^-7
or, [OH-] = 10^-7 M
Ni(OH)2   ----->    Ni2+    +   2OH-
                                       X             10^-7+ 2X

Ksp=[Ni2+] [OH-]
5.5*10^-16 = X *(10^-7+2X)^2
5.5*10^-16 = X *(10^-14 +   4X^2 +   4*10^-7X)
5.5*10^-16 = 10^-14X +   4X^3 +   4*10^-7X^2
Solving we get, (please solve it, its math and no chemistry here)
x= 0.055 mol/L

Concentration of Ni2+ is 0.055 mol/L
molar mass if Ni is 58.7 gm
In mg /L it is 0.055*58.7*1000 = 3230 mg/L

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