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A 0.1mol/dm^3 aqueous solution of phosphoric (V) acid, H3PO4, is mixed with a 0.1 mol/dm^3 of aqueous solution of sodium hydroxide.
Which mixture will form the salt Na3PO4?

a) 10cm^3 of H3PO4 with 30cm^3 of NaOH
b) 10cm^3 of H3PO4 with 10cm^3 of NaOH
c) 20cm^3 of H3PO4 with 10cm^3 of NaOH
d) 30cm^3 of H3PO4 with 10cm^3 of NaOH

I've written down the equation, and 1 mol of the acid should mix with 3 mols of the hydroxide to form 1 mol of the salt. But i don't understand, shouldn't all the options form the salt no matter the volume? And, the correct answer should be C, and i don't understand at all!

  • Chemistry -

    I may have missed something on reading it but I agree that all should form at least some Na3PO4. And I don't see how (c) can be the correct answer.

  • Chemistry -

    The correct answer is (a) based on the first chemical equation below.

    The mole ratio of NaOH to H3PO4 determines which salt is produced. Only the first one produces Na3PO4:

    H3PO4 + 3NaOH ---> Na3PO4 + 3H2O

    H3PO4 + 2NaOH ---> Na2HPO4 + 2H2O

    H3PO4 + NaOH ---> NaH2PO4 + H2O

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