The molarity of H3PO4 solution if 15.0 ml is completely neutralized by 38.5 ml of 0.15m naoh is calculated as follows
find moles of NaOH used = molarity x volume
= 38.5 x 0.15 = 5.775 moles
write the reacting equation
3NaOH + H3PO4 = Na3PO4 + 3H2O
from the equation the reacting ratio between NaOH to H3Po4 which is 3:1 the moles of H3PO4 is therefore = 5.775/3 = 1.925 moles
molarity of H3PO4 is therefore = moles /volume
= 1.925/15 = 0.128 M