Question

If 625 ml of pure water is added to 344 ml of 1.675 m na2so4, what is the new concentration of na2so4?

Answer 1

Molar concentration is the number of Na₂SO₄ moles that are dissolved in 1 L of solution.
when 625 mL is added to 344 mL of Na₂SO₄, then the number of moles that are dissolved in 344 mL remains constant but volume increases. Therefore concentration now decreases. 
Molar concentration of Na₂SO₄ is 1.675 M
the number of Na₂SO₄ moles  in 1000 mL - 1.675 mol
then number of Na₂SO₄ moles in 344 mL - 1.675 mol / 1000 x 344 = 0.5762 mol
the new volume - 344 mL + 625 mL = 969 mL
concentration = number of moles / volume
therefore new concentration = 0.5762 mol / 0.969 L = 0.595 mol/L
new concentration of Na₂SO₄ is 0.595 M