If not all the alum dissolves in the water before adding the Ba(NO3)2 solution, what effect will this have on the sulfate percent error determination?

Answer 1

I don't think the problem is stated very well. You know without any reasoning that the percent error will increase. Surely this isn't the answer you want.
It appears to me that you want an answer that is based on the final result. If all of the alum is not dissolved, then less will be in solution, the amount of BaSO4 will be less, that leads to a smaller mass Alum calculated than it should.