[big]

Of NH₄NO₃ and AlCl₃, how do you tell which is the weaker acid? I know you only haev to consider the cation since both HNO₃ and HCl are strong acids, but how do you know?

[DevaDevil]

A bronsted acid donates a proton (I assume you are talking about bronsted acidity here, not lewis acidity)

which one of the two compounds would more easily donate a proton? This is the stronger acid of the 2. (assume no complexation takes place)

[rabolisk]

As the good chemist explained, you can't really compare Bronsted acidity to Lewis acidity in terms of strength. In fact, the whole concept of lewis acid strength is a bit more ambiguous than bronsted strength, which can be simply measured by pKa in water.

[big]

which one of the two compounds would more easily donate a proton? This is the stronger acid of the 2. (assume no complexation takes place)

Well see I figured out the part about the bronsted acid being a proton donor, but how do you tell which will more easily donate the proton? I know that Al forms complexes with water along the lines of [Al(H₂O)₆]³⁺, but how do you know which donates the proton more easily?

[DevaDevil]

pKa Al³⁺ = 5.0
pKa NH₄⁺ = 9.25 <-- weaker

this is the only way to be absolutely sure which is the weaker acid.

[big]

But if you weren't given the pKa's, and you had to make an intelligent guess by looking at the structure of the molecule, how would you determine the weaker acid?

[DevaDevil]

in this case it would be hard. If you do not know that Aluminium complexes then you have no way of knowing if it is an anid or not. The higher pKa of ammonium could be considered something to know, as you form NH₄⁺ when you dissolve ammonia in water.