# How to determine which salt will precipitate from a solution containing multiple ions?

I have a solution containing $0.2\:\mathrm{mol}$ of $\ce{Br^-}$ ions and $\ce{SO_4^2-}$ ions each.
Now if I add excess $\ce{AgNO3}$ in this solution,
what will precipitate?
$\ce{Ag2SO4}$ or $\ce{AgBr}$ or both?

And if I add just $0.2\:\mathrm{mol}$ of $\ce{AgNO3}$, what will precipitate?

(I don't want to calculate anything. Just wanted to know qualitatively that which compound precipitates.)

## 1 Answer

You need to know that $K_\mathrm{sp}$ is used to determine the solubilities of sparingly soluble salts usually. You can get the values of $\ce{AgSO4,AgBr}$ and as $K_\mathrm{sp}=[\ce{Ag+}]^2[\ce{SO4^2-}]$ or $[\ce{Ag+}][\ce{Br-}]$. Now concentration of silver ion is constant so try calculating the other two anions and try comparing them.

• So in excess both will precipitate eventually while in limited quantity, the one with lower Ksp will precipitate? Commented Apr 19, 2015 at 6:31
• @pikachu i think you got it right. Commented Apr 19, 2015 at 9:50