Use this reaction: 3naoh + h3po4--> na3po4 + 3h20
how many ml of .225 m naoh will react with 4.568g h3po4?
the answer for this is 621 ml
b) what is the maximum amount of na3po4 that is formed from the reaction of 25.00 ml of .1050 m naoh and 15.00 ml of .08650 m h3po4? answer is .141g na3po4

The balanced reaction would be as follows:

3NaOH + H3PO4 --> Na3PO4 + 3H20

We are given the concentration and the amount of the reactants to be used. We use these values for the calculations as follows:

How many mL of .225 M NaOH will react with 4.568g H3PO4? 

4.568 g (1 mol / 98 g) ( 3 mol NaOH / 1 mol H3PO4 ) ( 1 L / .225 mol NaOH ) = 0.621 L or 621 mL of .225 NaOH