What volume of 0.200 m h3po4 will completely react with 50.00 ml of 0.200 m naoh?

The complete balanced reaction for this would be:H3PO4  +  3NaOH  --->  Na3PO4  + 3H2OSo we can see that 3 moles of NaOH reacts with 1 mole of H3PO4.
moles NaOH = 0.2 M * 0.05 L = 0.01 molesmoles H3PO4 = 0.01 moles NaOH * (1 mole H3PO4 / 3 moles NaOH) = 0.00333 moles
So the volume required would be:V = 0.00333 moles / (0.200 moles / L)V = 0.0167 L = 16.7 mL