5. Calculate the concentration of free Cr3+ ion when 0.o10 mol of Cr(NO3)3 is dissolved in a litre of solution buffered at pH = 10.0, and the Kf for Cr(OH)4- = 8.0 x 1029.

Equilibrium equation:

Cr3+ (aq) + 4 OH- (aq) Cr(OH)4- (aq)

[ OH- ] = {1.0 x 10-14} / {10-pH} = 1.0 x 10-4 M

I.C.E. grid:

Cr3+ (aq) 4 OH- (aq) Cr(OH)4- (aq)
INITIAL
0M
1.0 x 10-4 M
0.010M
CHANGE
+xM
buffered (no change)
-xM
EQUILIBRIUM
1.0 x 10-4 M

What are the equilibrium concentrations of each species?

a) xM, xM
b) (0.010+x)M, (0.010-x)M
c) xM, (0.010-x)M
d) (0.010-x)M, xM