Question

A lab assistant needs to prepare solutions of ZnSO4, NaCl, Na2CO3, Fe2(SO4)3 and FeCl3 with identical ionic strengths of I = 1.2 M. What concentration of these salts should he/she use?

Answer 1

Ionic strength of a solution is given by

I = (b₊z₊² + b_-z_-²)/2

Here b₊, b_- are the concentrations of cation and anions, respectively.

b₊ = n₊b, b_- = n_-b, 'b' is the concentration of solution

n₊, n_- are the no. of cations and anions, respectively.

ZnSO₄

I = (1b*2² + 1b*2²)/2

1.2 = 4b, i.e. b = 0.3 M

Therefore, the concentration of ZnSO₄ solution = 0.3 M

NaCl

I = (1b*1² + 1b*1²)/2

1.2 = b

Therefore, the concentration of NaCl solution = 1.2 M

Na₂CO₃

I = (2b*1² + 1b*2²)/2

1.2 = 3b, i.e. b = 0.4 M

Therefore, the strength of Na₂CO₃ solution = 0.4 M

Fe₂(SO₄)₃

I = (2b*3² + 3b*2²)/2​

1.2 = 15b, i.e. b = 0.08 M

Therefore, the strength of Fe₂(SO₄)₃ solution = 0.08 M

FeCl₃

I = (1b*3² + 3b*1²)/2​​

1.2 = 6b, i.e. b = 0.2 M

Therefore, the strength of FeCl₃ solution = 0.2 M