Which one of the two salts precipitated?

AgNO3 + K2SO4 = Ag2SO4 + KNO3

I see four salts. Ag2SO4 is the least soluble but I don't consider it insoluble. It is only moderately insoluble.

To determine which salt precipitated, we need to refer to the solubility rules. Solubility rules provide information about the solubility of various compounds in water.

According to the solubility rules:
- Most nitrate (NO3-) salts are soluble.
- Most sulfate (SO4 2-) salts are soluble, except for a few exceptions like barium sulfate (BaSO4), calcium sulfate (CaSO4), and lead(II) sulfate (PbSO4).
- Silver sulfate (Ag2SO4) is sparingly soluble.

In the given reaction, we have the following salts:
- AgNO3 (silver nitrate)
- K2SO4 (potassium sulfate)
- Ag2SO4 (silver sulfate)
- KNO3 (potassium nitrate)

Based on the solubility rules, we can conclude that silver sulfate (Ag2SO4) is the salt that precipitated. This is because silver nitrate (AgNO3) and potassium sulfate (K2SO4) are soluble salts and will remain dissolved in the solution. On the other hand, silver sulfate (Ag2SO4) is sparingly soluble and will precipitate out of the solution.

So, in the given reaction, silver sulfate (Ag2SO4) precipitated.