1. The problem statement, all variables and given/known data 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? 2. Relevant equations I have came up with an equation for the acid base reaction: 3NaOH + H3PO4 -----> Na3PO4 + 3H2O 3. 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 realised 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.