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

[student34]

Homework Statement

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).

Homework 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".

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.

 

[Borek]

I don't see how to answer the problem without already knowing strength of these acids (their dissociations constants).

 

[Bystander]

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.

 

[student34]

Thanks, it's probably just bad wording in the textbook.

 

[PWiz]

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.