Here's how to calculate the molarity of sodium sulfate (Na₂SO₄) in the solution:

1. Calculate the moles of sodium sulfate:

* Molar mass of Na₂SO₄: (2 * 22.99 g/mol) + (32.06 g/mol) + (4 * 16.00 g/mol) = 142.04 g/mol

* Moles of Na₂SO₄: (15.5 g) / (142.04 g/mol) = 0.109 mol

2. Convert the volume to liters:

* Volume in liters: 35 mL * (1 L / 1000 mL) = 0.035 L

3. Calculate the molarity:

* Molarity (M): (0.109 mol) / (0.035 L) = 3.11 M

Therefore, the molarity of sodium sulfate in the solution is 3.11 M.