Question

8. Determine the ionic strength of the solution and the activity coefficients for each ion in a solution containing 0.01 M NaCl, 0.005 M Ca(OH)2, and 0.01 M MgSO4

Answer 1

I'll help you with the first part, and the second part try to do it yourself.

First step in calculations is calculation of so called ionic strength, using the following formula:

(1)

where Ci is a molar concentration of ith ion present in the solution and zi is its charge. Summation is done for all charged molecules present in the solution.

Second step is calculation of activity coefficients given by the formula:

(2)

An activity coefficient for all ions bearing the same charge is identical (although there exists slightly more precise formula in which activity coefficient is also a function of the ion radius).

And now the most important thing. The activity of any ion (with a charge z) is

(3)

where fz denotes the activity coefficient for z-charged ions.

So, let's calculate the ionic strenght first for every salt, and then, the total.

For NaCl: 1/2 (0.01 * 1 * (1)²) + 1/2 (0.01 * 1 *¨(-1)²) = 0.01

For Ca(OH)₂: 1/2 (0.005 * 1 * 2²) + 1/2 (0.005 * 2 * (-1)²) = 0.015

For MgSO₄: 1/2 (0.01 * 1 * 2²) + 1/2(0.01 * 1 * -2²) = 0.04

I = 0.01 + 0.015 + 0.04 = 0.065

Now for the activity coeeficients, use equation 2 and the previously obtained values of I. If you have problem, tell me in a comment and I'll gladly help you.