How to find an unknown amount (titration)

[hhs15]

Homework Statement

I'm given an unknown amount (around 20mL) of NaH_2PO_4 and I dilute it in distilled water in a 250mL flask. Then I take 25mL of this solution and add 4 drops of thymolphthalein in another flask. Then I titrate it with 0.1M of NaOH until it becoms light blue. I used 22mL of NaOH.

I'm suppose to find out the amount of NaH_2PO_4 I was given.
I'm lost. Any help would be appreciated.

 

[Borek]

Your first step is to find out what reaction took place and write reaction equation.

General approach to calculations is described here:

http://www.titrations.info/titration-calculation

 

[hhs15]

Oh of course. I knew it was eazy.
Thank you!