Calculating moles of NaOH reacting with H3PO4 from OH⁻ concentration

[wei1006]

Homework Statement

A solution of sodium hydroxide(NaOH) contains 1.70g of hydroxide (OH-) ions per dm^3. 20.0 cm^3 of the solution reacts with 25.00cm^3 of 0.0400 mol/dm^3 phosphoric acid(H3PO4). How many moles of sodium hydroxide reacts with 1 mol of phosphoric acid? What will be the chemical equation for the reaction?

Homework Equations

I have came up with an equation for the acid base reaction:
3NaOH + H3PO4 -----> Na3PO4 + 3H2O

The Attempt at a Solution

Actually I have no idea where to start for this question so I resorted to converting things I know into moles first but realized that I still have no idea how to attempt this question.

Moles of H3PO4
= (25.00/1000) x 0.0400
= 0.00100mol

Mole ratio: 3NaOH = H3PO4

Moles of NaOH
= 3 x 0.00100
= 0.00300mol

Concentration of NaOH in mol/dm^3
= 0.00300/ (20÷1000)
= 0.150 mol/dm^3

I am actually wondering what is the use of the given concentration of OH-

I have no idea how to attempt this kind of question. Please guide me along! Thank you.

 

[Borek]

This is a poorly written question. The reaction equation you wrote is in general a correct one or the reaction between NaOH and H₃PO₄, but it is not what would happen in a typical titration. I feel like question should mention it, otherwise it is confusing.

How many moles of OH^- per 1 L? What is their concentration then? How many moles of NaOH reacted with how many moles of the acid?