A question states "rank the following conjugate acids in the order you would predict, from most stable to least stable". The question is referring to the specific molecules, and not the conjugate acids of those molecules, so that part is awkwardly worded.
The two molecules are H2O and NH3. The correct answer from the homework is definitely NH3 is more stable than H2O.
I am a little confused by this question because it seems to be implying "Rank the following acids in stability relative to the stability of their conjugate bases" Given how the question is phrased, does this seem like a valid question or is this just mind reading? I understand that NH3 is a weaker acid than H2O because it's anionic conjugate base is unstable, but does that mean NH3 is "more stable" than H2O.
After all, if we phrased the question: rank the relative stability of the following conjugate bases, wouldn't we then predict the opposite order? H2O would be more stable than NH3 because there is a bigger energy change for H2O to form the conjugate acid.
By the logic of this question, we would be saying that the relative stability of these two molecules changes depending on how a question is phrased. But the relative stability of two molecules can't suddenly change based on how somebody phrases a question, can it?
And as a side note, isn't basicity the better judge of stability? Water's lone pairs are in lower energy orbitals because oxygen is more electronegative. So in terms of raw stability, isn't water more stable than NH3?