Question

An aqueous solution of na2so4 contains 30% na2so4 by mass.calculate the total number of electrons in 62.5 ml of this solution

Answer 1

well I guess that volume has nothing to do with no of electrons.
lets assume that 30% implies 30 g of Na₂SO₄ in 100g of water.

no of moles of Na₂SO₄ = weight taken in solution / molar mass = 30/142
no of electrons in one molecule of Na₂SO₄ = 11*2  + 26 + 8*4 = 80
in one mole of Na₂SO₄  we'll have 80 moles of electron
in 30/142 moles then we've 30x142x80 =2400/142 =1200/71 moles of electron

total no of electron would be = 6.022 x 10²³ x 1200/71 = 1.017 x 10²⁵

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