Calculating pH with and without ionic strength

[Labchem3]

Homework Statement

There are four things to be calculated:

-pH of 0.00196M solution of HCl in water, without taking ionic strength into account,

-The same thing but taking ionic strength into account

-pH of 0.00196M solution of HCl in Na2SO4, without taking ionic strength into account,

-And the same thing but taking ionic strength into account.

Homework Equations

pH = -log([H+])

3. The Attempt at a Solution
So far I've done the first one only. HCl dissociates fully, so pH is - log10(0.00196) = 2.71.

But:
1. How to include "ionic strength" into pH calculation? I have no idea how to do it.
2. How to calculate pH of HCl in Na2SO4? How will it dissociate?

 

[mjc123]

1. Do you know an equation relating activity coefficient to ionic strength?
2. Completely. But the ionic strength is different.