Question

Dissociation of the Cs2CrO4 in aqueous solution to give a concentration of 0.02 M, calculate: the
activity coefficients for Cs' and Croak ' using the two following methods a) the Extended Debye-Huckel Equation b) interpolation. c) Write the generic Kp expression for Cs2CrO4 including your calculated activity coefficients from part a).

Answer 1

For Cs2CrO4

Ci = concentration of ion

zi = charge on ion

ionic strength (u) = 1/2Ci.zi^2

                            = 1/2(0.04 x 1^2 + 0.02 x 2^2) = 0.06 M

activity coefficient Y by

a) extended Debye-Huckel equation,

log[Y] = -0.51 x zi^2 x sq.rt.(u)/(1 + r x sq.rt.(u)/305)

r = ionic radius

Y[Cs] = inv.log[-0.51 x 1^1 x sq.rt.(0.06)/(1 + 250 x sq.rt.(0.06)/305)] = 0.787

Y[CrO4^2-] = inv.log[-0.51 x 1^1 x sq.rt.(0.06)/(1 + 400 x sq.rt.(0.06)/305)] = 0.804

b) interpolation

For Cs, from the table of ion activity coefficients (literature)

activity coefficient for 0.05 M ionic strength = 0.8 and for 0.1 M is 0.75

So by interpolation method activity coeficient for Cs = 0.79

Similarly for CrO4^2-,

activity coefficient for 0.05 M ionic strength = 0.445 and for 0.1 M is 0.355

So by interpolation method activity coeficient for CrO4^2- = 0.427

c) Kp expression for Cs2CrO4

Kp = (0.787)^2.[Cs+]^2.(0.804)^2.[CrO4^2-]