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Posted by on Friday, October 9, 2009 at 11:32pm.

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 - , Friday, October 9, 2009 at 11:52pm

    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 - , Sunday, October 11, 2009 at 8:51pm

    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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