Question

What is the ionic strength of each of the following solutions: a. 0.1 M NaCl b. 0.1 M Na2-oxalate c. 0.1 M CuSO4

Answer 1

The ionic strength of a solution is a measure of the concentration of ions in that solution.

The ionic strength, I, of a solution is a function of the concentration of all ions present in that solution.

where c_i is the molar concentration of ion i (M, mol/L), z_i is the charge number of that ion, and the sum is taken over all ions in the solution.

a) 0.1 NaCl

Charge on Na on dissociation will be +1

Charge on chlorine on dissociation will be -1

I = 1/2 × [(Concentration of NaCl in M × # of Sodium ions × (Charge of Sodium)²) + (Concentration of NaCl in M × # of Cl ions × (Charge of Cl)²)

I = 1/2x[(0.1 x 1 x 1²) +(0.1 x 1 x -1²)]

I = 1/2 x (0.1+0.1)

I = 0.1 M

b) 0.1 M Na2-oxalate

Na2-oxalate 2Na ⁺ + oxalate ^2-

I = 1/2 × [(Concentration of Na2-oxalate in M × # of Sodium ions × (Charge of Sodium)²) + (Concentration of Na2-oxalate in M × # of oxalate ions × (Charge of oxalate)²)

I = 1/2 x [(0.1 x 2 x 1²)+(0.1 x 1 x -2²)]

I = 1/2 x(0.2 + 0.4)

I = 0.3M

c) 0.1 M CuSO₄

CuSO₄ ionizes as

CuSO4 Cu²⁺ + SO₄^2-

I = 1/2 × [(Concentration of CuSO4 in M × # of Copper ions × (Charge of copper)²) + (Concentration of CuSO4 in M × # of sulphate ions × (Charge of sulphate)²)

I = 1/2 x [(0.1 x 1 x 2²) + (0.1 x 1 x -2²)]

I = 1/2 x (0.4+0.4)

I = 0.4 M