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    A mass of 0.647g of an unknown diprotic acid is mixed with 100cm3 of deionised water to produce an acid solution. 10cm3 of this solution is transferred into a conical flask and it is titrated against a 0.1M solution of NaOH. The average titre is 8.42cm3. How do I calculate the molecular mass of the unknown diprotic acid? Thanks!

    We need to work out how many moles of acid that is initially, then we can calculate molecular mass.

    I am assuming you realise a diprotic acid has 2 hydrogens that will be neutralised in this reaction. Let's call the unknown acid H2X so that we can write the balanced equation:

    H2X + 2NaOH -> Na2X + 2H2O

    Hence the mole ratio of H2X : NaOH is 1:2

    Now calculate the number of moles of NaOH.
    Halve this to get the number of moles of acid in 10cm3.
    Multiply by 10 to get the number of moles in the whole acid sample (as we only titrated 10 of the 100cm3).

    You now have the number of moles of acid in 0.647g acid sample. The molecular mass in grams is 0.647 / moles.
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