Answer is: the molarity of the solution is 0.0936 M.
Balanced chemical reaction: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq).
Net ionic reaction: Ba²⁺ + SO₄²⁻ → BaSO₄(s).
V(BaCl₂) = 55.8 mL; volume of barium chloride solution.
V(BaCl₂) = 55.8 mL ÷ 1000 mL/L = 0.0558 L.
m(Na₂SO₄) = 742 mg ÷ 1000 mg/g.
m(Na₂SO₄) = 0.742 g; mass of sodium sulfate.
n(Na₂SO₄) = m(Na₂SO₄) ÷ M(Na₂SO₄).
n(Na₂SO₄) = 0.742 g ÷ 142.04 g/mol.
n(Na₂SO₄) = 0.00522 mol; amount of substance.
From chemical reaction: n(Na₂SO₄) : n(BaCl₂) = 1 : 1.
n(BaCl₂) = 0.00522 mol.
c(BaCl₂) = n(BaCl₂) ÷ V(BaCl₂).
c(BaCl₂) = 0.00522 mol ÷ 0.0558 L.
c(BaCl₂) = 0.0936 M.
