Question

Determine the ionic strength, μ, for each of the following solutions. Assume complete dissociation of each salt and ignore any hydrolysis reactions.

a) A solution of 0.00490 M HCl

b) A solution of 0.00213 M CaBr2

c) A solution of 0.000850 M CaBr2 and 0.000489 M NaBr

Answer 1

The relationship between the ionic strength and molar concentration and charge of the ions is-

µ =1/2∑c_iz_i² (I =1 to n), where c_i is the concentration of the particular ion and z_i is the charge of that ion.

So for the HCl solution, HCl = H+ + Cl-, strong electrolyte, equal concentration and 1 unit of charge each.

(a) µ=1/2[0.0049*(1)²*2] = 0.0049 M

For CaBr2 à Ca²⁺ + 2Br^-

(b) µ=1/2[0.00213*(2^2) + 2*0.00213*1^2] = 0.00639 M

For the mixed salts, NaBr = Na+ + Br- and CaBr2 à Ca²⁺ + 2Br^-

(c) µ = ½[0.000489*2 + (0.000850*4 + 2*0.000850)] = 0.003039 M