0.5 gm of fume H2SO4 (Oleum ) is diluted with water, this solution is completely neutralised by 26.7 ml of 0.4 N NaOH. Find the percentage free SO3 in sample solution.?

SOLUTION: Given total wt of Oleum sample is 0.5 gm, let x gm SO3 and (0.5-x) H₂SO₄  

                             (E _wt= SO₃=80/2=40 gm and E _wtH₂SO₄=98/2=49 gm)

At equivalent point     

      No of equivalent of SO₃ + No of equivalent H₂SO₄= no of equivalent of NaOH