A solution contains 1.0410 -2 M silver acetate and 1.2110 -2 M barium nitrate . Solid ammonium sulfate is added slowly to this mixture.

A solution contains 1.04×10-2 M silver acetate and 1.21×10-2 M barium nitrate. 

Solid ammonium sulfate is added slowly to this mixture.

A. What is the formula of the substance that precipitates first?

formula =_____

B. What is the concentration of sulfate ion when this precipitation first begins? 

[SO42-] =  ______M