mass of glauber's salt i( Na₂SO₄.10H₂O) = 8.05 x 10^-2kg

molecular mass of glauber's salt = 322 g/mol

we can write it as 0.322 kg/mol

number of moles of Na₂SO₄.10H₂O in 1 dm³ soln = 8.05 x 10^-2 / 0.322 = 0.25

0.25 = Molarity

volume = 1 dm³

density given = 1077.2 kg / m³

total mass = vol. x density = 1x 10^-3 x 1077.2  = 1.077kg

mass of solute (Na₂SO₄.10H₂O) = 8.05 x 10^-2 kg

so masss of solvent = 1.0772 - 8.05 x 10^-2 = 0.9966

molality = 0.25 / 0.9966

no. of moles of solvent = 0.9966/0.018

mole fraction of Na₂SO₄ = 0.25 / [0.25 + no. of moles (what ever they come)]