How many millimoles of solid NaOH must be added to 50.0 mL. of 0.20 M HOCl to obtain a buffer solution that has a pH of 7.49?  Assume that the addition of the solid NaOH results in a negligible change in volume.  Ka for HOCl is 3.2 x 10^-8

Use Henderson's equation :
PH= pKa +log(mmolOCl-/mmol HOCl))
we know
mmol = M*V(mL)
Now from above equation work out the mMol of OCl-
write the balanced equation
NaOH + HOCl ---> NaOCl + H2O
mmol of OCl- = mmol of NaOH (from the balanced equation)

mMol NaOH can be converted into mg ..and then to grams