How do I find out how many ml of NaOH i need to titrate H3PO4. I used the ice table to knock off the two protons but im using a .2200M NaOH and do not Know how to determine the volume needed before hand. In other words i need some pre-calculations for volume of titrate needed not in mol/L but in ml...thanks

Question

How do I find out how many ml of NaOH i need to titrate H3PO4. I used the ice table to knock off the two protons but im using a .2200M NaOH and do not Know how to determine the volume needed before hand. In other words i need some pre-calculations for volume of titrate needed not in mol/L but in ml...thanks

Answer 1

You need to know what indicator is being used. Methyl red indicator titrates the first H and phenolphthalein titrates the first and second H. The third one cannot be titrated easily. From the equations below,

NaOH + H3PO4 ==> NaH2PO4 + H2O
2NaOH + H3PO4 ==> Na2HPO4 + 2H2O

If you use MR indicator (or an equivalent), moles NaOH = moles H3PO4.
If you use phenolphthalein (or an equivalent), moles H3PO4 = 1/2 mole NaOH.