The value of K_sp for Na(OH)_2 (aq) is 6.0 times 10^-16. What is the molar conce

Question

The value of K_sp for Na(OH)_2 (aq) is 6.0 times 10^-16. What is the molar concentration of nickel ions in a saturated pH = 7.0 buffer solution? Assume the buffer is sufficiently strong that the pH does not change during equilibration. Consider the reaction: NiCO_3 (s) + 6 NH_3 (aq) [Ni(NH_3)CO_3 (aq)] Use the information below to determine the equilibrium constant for the reaction above. I) NH_3 (aq) + H^+ (aq) NH_4^+ (aq) K_I = 1.8 times 10^-5 II) Ni^2+ (aq) + 6 NH_3 (aq) [Ni(NH_3)_6]^2^+ (aq) K_II = 5.6 times 10^8 III) NiCO_3 (s) NiCO_3 (aq) K_III = 6.6 times 10^-9

Explanation / Answer

NiCO3(s) + 7NH3(aq) <-> [Ni(NH3)6]+2 + CO3-2

Kf = [ [Ni(NH3)6]+2][CO3-2] / (NH3]^6

so...

K2 = ([Ni(NH3)6]+2) / ([Ni+2][NH3]^6)

K3 = [Ni+2][CO3-2]

so..

we need CO3-2 in products,, so multiply K2 and K3

([Ni(NH3)6]+2) / ([Ni+2][NH3]^6) * [Ni+2][CO3-2]

Ni+2 cancels...

Knew = ([Ni(NH3)6]+2)* [CO3-2] / ([[NH3]^6)

which is what we wanted

so

Knew = K2*K3 = (5.6*10^8)(6.6*10^-9) = 3.696

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