100 ml of a mixture of NaOH and Na2SO4 is neutralized by 10 ml of 0.5 M H2SO4. Hence NaOH in 100 mL solution is

As  Na2SO4 is already neutral so only NaOH will be neutralized by H2SO4∴   Eq. of NaOH = Eq. of H2SO4            WENaOH  =  NV(ml)1000H2SO4            W40=0.5×2×101000 W = 40 × 10-2 = 0.4 g