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Homework Help: How do I know what if an acid is strong or weak?

  1. Dec 23, 2014 #1
    1. The problem statement, all variables and given/known data

    The question asks me to use the modified Arrhenius theory of acids to classify the following as strong or weak acids, HNO3(aq), HI(aq) and HF(aq).

    2. Relevant equations

    But the book only says that the modified Arrhenius theory for acids is, "an Arrhenius acid reacts with water to produce H3O+ (aq) in aqueous solution".

    3. The attempt at a solution

    There just doesn't seem to be enough information for me to know what percent of the solution is H3O+ ions.
  2. jcsd
  3. Dec 23, 2014 #2


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    Staff: Mentor

    I don't see how to answer the problem without already knowing strength of these acids (their dissociations constants).
  4. Dec 23, 2014 #3


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    Science Advisor
    Homework Helper
    Gold Member

    Just googled "Modified Arrhenius Theory" and it looks like "edubabbled" nonsense to rationalize something Arrhenius couldn't explain. Don't give your instructor too much backtalk about it, though --- he might be the idiot who invented it.
  5. Dec 23, 2014 #4
    Thanks, it's probably just bad wording in the textbook.
  6. Dec 31, 2014 #5
    Well if that is what your book says, then stick with it. You can measure the conductivity of the three acids (same number of moles and same volumes) by inserting two inert electrodes in the solution with a particular potential difference across them and connect an ammeter to the circuit, and the acid with the least resistance clearly has the highest H3O+ concentration, since the stronger an acid, the more strongly it ionizes the water molecules(and the more conductive it gets). You can also alternatively use a gas syringe to measure the rate of hydrogen liberation at the cathode, with the acid giving the highest rate being the strongest.
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