[MrHappy0]

Simple problem and I know the answer. I am looking for a detailed explanation so I understand how to solve it.

Find [Hg2(2+)] in saturated Hg2Br2 in 0.001M KBr?

[MrHappy0]

What is used to find the ionic strength? Ionic strength is 0.001 M. How is this calculated without the concentration of Hg₂²⁺? 

[MrHappy0]

I know we are trying to get the activity coefficients in order to plug them into the Ksp equation and to solve for [Hg₂²⁺] but how would you find the ionic strength? 

Oh, is ionic strength found from KBr  ==> u=1/2([K](1¹)
(-1²)) ?

[MrHappy0]

So... apparently that is correct. I got 7.0x10^-17.

I just don't understand why 0.001M is the concentration of Br^- in the Ksp formula. Shouldn't Br^- be unknown. Thus, it would 2s².

or

[Br^-]² is [Br_(Hg2Br2)^-+Br_KBr^-] and the Br ion concentration from Hg2Br2 is considered insignificant so only the concentration from the KBr is considered.

[Borek]

[Br^-]² is [Br_(Hg2Br2)^-+Br_KBr^-] and the Br ion concentration from Hg2Br2 is considered insignificant so only the concentration from the KBr is considered.

That's exactly the case. You already calculated concentration of mercury as 7.0x10^-17, that means concentration of Br^- from dissolution is 1.4x10^-16. 0.001+1.4x10^-16 is - for all practical purposes - just 0.001.