$0.5gm$ of $H_2SO_4$(oleum) is diluted with water.This solution is completely neutralized by 26.4ml of 0.4N NaOH.Find the % of free $SO_3$ in the sample.

Question

$(a)\;21\%\qquad(b)\;22\%\qquad(c)\;20\%\qquad(d)\;21.5\%$