0.70 g of mixture (NH4)2 SO4 was boiled with 100 mL of 0.2 N NaOH solution till all the NH3(g) evolved and get dissolved in solution itself. The remaining solution was diluted to 250 mL. 25 mL of this solution was neutralized using 10 mL of a 0.1 N H2SO4 solution. The percentage purity of the (NH4)2 SO sample is

(1) 94.3

(2) 50.8

(3) 47.4

(4) 79.8