Answer is: mass of the mixture is 24,47 g. 1) N(Na₂SO₄) = 3,5·10²². n(Na₂SO₄) = 3,5·10²² ÷ 6·10²³ 1/mol. n(Na₂SO₄) = 0,058 mol. m(Na₂SO₄) = 0,058 mol · 142 g/mol. m(Na₂SO₄) = 8,24 g. 2) n(H₂O) = 0,500 mol. m(H₂O) = 0,5 mol · 18 g/mol. m(H₂O) = 9 g. 3) m(total) = 8,24 g + 9 g + 7,23 g. m(total) = 24,47 g. n - amount of substance.