How many grams of sodium are in 0.820 moles of na2so4?

the molar mass of a compound is the sum of the products of the atomic masses by the number of atoms of the element.
molar mass of Na₂SO₄ is - 142 g/mol.
1 mol of Na₂SO₄ has a mass of 142 g.
In 1 mol of Na₂SO₄ the mass of Na is 23 g/mol x 2 = 46 g.
                             
Mass of Na in 1 mol of Na₂SO₄ is - 46 g
                           
mass of Na in 0.820 mol of Na₂SO₄ - 46 g /1 mol x 0.820 mol = 37.72 g.
mass of Na is 37.72 g