Write down the balanced reaction first

3NaOH + H_{3}PO_{4} makes Na_{3}PO_{4} + 3H_{2}O

**Given** 0.10M NaOH (0.10mol/lit NaOH)

0.2M H_{3}PO_{4} ( 0.2 mol/lit H_{3}PO_{4})

15 ml of H_{3}PO_{4} or 0.015 lit of H_{3}PO_{4}

**Find** volume of NaOH needed to neutralize the solution

Relate moles of H_{3}PO_{4} with the moles of NaOH with the helps of coefficients in front of NaOH and H_{3}PO_{4} seen in balanced equations.

moles â†” Litre use definition of molarity (moles/litrs)

Start with what is given for H_{3}PO_{4}

0.015 lit of H_{3}PO_{4} x 0.2
moles of H_{3}PO_{4} /
lit of H_{3}PO_{4} x 3 moles of NaOH/1 mole of H_{3}PO_{4} x lit of NaOH/0.1 mol of NaOH = **0.09 lit or 90 ml.**

The same colors in above equations cancel our leaving lit of NaOH as final answer.