Question

There is an aqueous solution containing 0.0015 M Na2SO4 and 0.0026 M AlCl3. (Ignore the dissociation of water.) (a) Calculate the ionic strength of this solution. (b) Calculate the activity coefficients of all anions in this solution.

Answer 1

(a)

Ionic strength is calculated using the formula:

I = 0.5*(c_i*z_i²)

Here c_i is the concentration of the 'i'th species, and z_i is its oxidation number

Here we have four species Na⁺, SO₄^2-, Al³⁺ and Cl^-, with conc. values 0.003, 0.0015, 0.0026 and 0.0078 M respectively, and z values as +1, -2, +3, -1 respectively.

Putting values and solving we get:

I = 0.020

(b)

At this ionic strength value, the activity coefficients for anions are calculated using the Debye Huckel relation which is as follows:

log f = -0.51*z²*I^0.5/(1+I^0.5)

Here f is the activity coefficient.

Putting values and solving we get:

f_{sulphate anion} = 0.572

f_{chloride anion} = 0.865