Question

if 100. mL of 0.800 M Na2SO4 is added to 200. mL of 1.20 M NaCl, what is the concentration of Na+ ions in the final solution? Assume fhat the volumes are additive

Answer 1

Compounds Na₂SO₄ and NaCl are mixed together are we are asked to find the concentration of Na⁺ in the mixture 
Na₂SO₄ ---> 2 Na⁺ + SO₄³⁻

1 mol of Na₂SO₄ gives out 2 mol of Na⁺ ions 

the number of Na₂SO₄ moles added - 0.800 M/1000 * 100 ml
                                                         = 0.08 mol
therefore number of Na⁺ ions from Na₂SO₄ = 0.08 * 2 = 0.16 mol

NaCl ----> Na⁺ + Cl⁻ 
1 mol of NaCl gives 1 mol of Na⁺ ions
number of NaCl moles added = 1.20 M/1000 * 200 ml
                                               = 0.24 mol
number of Na⁺ ions from NaCl = 0.24 mol

total number of Na⁺ ions in the mixture = 0.16 mol + 0.24 mol = 0.4 mol
as stated the volumes are additive, 
therefore total volume  = 100 ml + 200 ml = 300 ml 
the concentration of Na⁺ ions = number of moles / volume 
                                              = 0.4 mol/ 0.3 dm³
concentration of Na⁺ = 1.33 mol/dm³