A250-ml sample of na2so4 is reacted with an excess of bacl2. if 5.28 g baso4 is precipitated, what is the molarity of the na2so4 solution?

To get the molarity you need to follow this equation
                       moles of solute
Molarity (M =
                        Liters of solution

But before you apply that equation you need to find the moles of solute and the liters of solution. Follow this equation

Na2SO4 + BaCl2 = BaSO4 + 2 NaCl

Solution

Moles of BaSO4 = 5.28 g 
                               
                                233.43 g / mol
                             =  0.0226  moles
Moles of NaSO4 = 0.0226 
                  0.0226 mole
Molarity =
                  0.250 L
              =  0.0905 mol / L

So the answer is 0.0905 mol / L