The molar concentration of H3PO4 solution is calculated as below
find the moles of NaOH reacted =molarity x volume
= 35.5 x0.150 = 5.325 moles
write a balanced reacting equation
=NaOH +3 H3PO4 = Na3PO4 + 3H2O
by use of reacting ratio between NaOH to H3PO4 which is 1:3 the moles of H3Po4 is therefore
= 5.325moles/3= 1.775 moles
molar concentration is therefore of H3PO4= moles /volume
=1.775/55.3 = 0.0320 M of H3PO4