Common Ion Effect

[DrRoyal Pains]

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Can someone elaborate on this: Mixing 500ml of 2x10-5M AgNO3 with 500ml of 1x10-5M NaCl dilutes both concentrations in half (You can use M1V1 = M2V2 to figure this out). These final concentrations after mixing are therefore 1x10-5M AgNO3 and 0.5x10-5M NaCl or 5.0x10-6M NaCl. I just copy and pasted this which is an explanation from one of Chad's quizzes. I understand the question, but why are the concentration's halved? (I also know mixing the two gives you AgCl (s) and NaNO3 (s)). I am obv. missing something.

 

[Dusk]

If you plug in MV=MV for lets say AgNO3,
you get (2E-5)(500)=M(1000)

The 1000 comes from the total of the 2 different solutions. So if you solve for M, you get half.