0.523 M
The reaction between barium chloride and sodium sulfate is given by
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
We are given,
Volume of  Na₂SO₄ = 15.0 mL
Mass of of solid BaSOâ‚„ = 1.83 g
Required to determine the molarity of Naâ‚‚SOâ‚„ solution
we will use the following steps
Mass of BaSOâ‚„ = 1.83 g
To get the number of moles we divide mass by the molar mass
Molar mass of  BaSO₄ = 233.38 g/mol
Number of moles = 1.83 g ÷ 233.38 g/mol
              = 0.00784 moles
From the balanced equation for every 1 mole of sodium sulfate used 1 mole of BaSOâ‚„ was produced.
Therefore, the mole ratio of Naâ‚‚SOâ‚„ : BaSOâ‚„ is 1 : 1
Hence, moles of Naâ‚‚SOâ‚„ will also be 0.00784 moles
Volume of sodium sulfate = 15.0 mL or 0.015 L
Number of moles = 0.00784 moles
But, molarity = Moles ÷ Volume
           = 0.00784 moles ÷ 0.015 L
           = 0.5227 M
          = 0.523 M
Thus, the molarity of original sodium sulfate is 0.523 M