Question

Na2SO4 is dissolved in water to make 450 mL of a 0.250 M solution. What is the molar mass of Na2SO4, and how many moles are needed?

Answer 1

Molarity is Given as,

Molarity = Moles / Volume of Solution --- (1)

Data Given:
Molarity = 0.250 mol.L⁻¹

Volume = 450 mL = 0.45 L

Solving equation 1 for Moles,

Moles = Molarity × Volume

Putting values,

Moles = 0.250 mol.L⁻¹ × 0.45 L

Moles = 0.1125 mol

Now, converting moles into grams,
As,
Moles = Mass / M.mass
Or,
Mass = Moles × M.mass

Mass = 0.1125 mol × 142 g.mol⁻¹

Mass = 15.97 g

Result:
Molar Mass of Na₂SO₄ = 142 g.mol⁻¹

Moles needed = 0.1125 mol