pH and ionic strength??

Question

If an experiment is performed at pH 3:

(a) I can adjust the ionic strength to 2 x 10-4
(b) I cannot adjust the ionic strength to 2 x 10-4
(c) The ionic strength must be 2 x 10-4 and cannot be changed
(d) The ionic strength must be 1 x 10-3 and cannot be changed

The teacher told us the answer is B, Can someone please explain to me the principle behind it and why the others are wrong. Thanks!

Hmm... ok i understand most of it. Thanks.. I get that at pH = 3 the concentration of Hydrogen ion is 10-3 but where did 5x10-3 come from??

If it is 1/2 the cumulative sum of the concentration of each ionic species in the solution multiplied by the square of its charge... Isnt Hydrogens charge 1. so isnt 1 squared equal to 1 not 5.. Im not understanding where 5 came from.

Answer 1

That's because ionic strenght is calculated as 1/2 the cumulative sum of the concentration of each ionic species in the solution multiplied by the square of its charge. At pH = 3 the concentration of Hydrogen ion is 10-3 Suppose only hydrogen ion exist in solution , its ionic strength will be 5x10-3 (moreover this is Not true as a counterion must exist in solution, DOUBLING the ionic strength ) You could increase the ionic strength from there by adding any ionic substance such as salt, but you could not reduce it.
Depending on the context, one exception perhaps will be to dilute the solution by adding distilled water or extracting the acid by evaporation or something but maybe that is not what your professor means by "adjusting". good luck