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Homework Help: Volumetric analysis

  1. Jan 2, 2016 #1
    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.
    Last edited: Jan 2, 2016
  2. jcsd
  3. Jan 2, 2016 #2


    User Avatar

    Staff: Mentor

    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?
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