Were you really given liquid Na
2SO
4?
I think it was a more concentrated solution, do you know its concentration? Anyway, say you had 25 mL of a 0.1 M Pb(NO
3)
2 solution, how many moles of Pb(NO
3)
2 is in solution? Once you have calculated this, you can find the mass of Pb(NO
3)
2 needed