[jacobk55]

Ive had trouble understanding to determine the strength of an acid given just it's molecular formula. The problem in reference was which one of these acids were the strongest: SiH4 SH2 H3COH TeH2 H2O

I already know the answer is TeH2 but how is that?

I was under the impression that the strength of an acid is determined by the electronegative of the conjugate base. I based my answer off this and chose SH2 because it is the most electronegative of the acids given but apparently it was TeH2.

[bjams12]

Look at the bases you would have when you dissociated a H off of your answer choices.
SiH₃^-
SH^-
H₃CO^-
TeH^-
OH^-
You have the right idea with electronegativity, it increases from left to right across the periodic table. The elements to the right of the table want electrons more, so they're more okay with having a negative charge. If they're more stable with a negative charge, it means that the hydrogen will be more willing to dissociate off of it, which means it's a better acid. Now, there will still be an equilibrium
HA <--> H⁺ + A^- (generic acid-dissociation reaction)
but if A^- is more okay with existing (like we said because they're more electronegative, think Cl^- or Br^- existing in solution), then equilibrium will be shifted to the right, increasing [H⁺] concentration. That was just to give you a few ways to think about it.

Now, let's look at your answer choices. You should have it narrowed down to the three molecules with the elements furthest right on the periodic table, so you have OH^- SH^- and TeH^-. I know you're still probably thinking "okay, electronegativity" but there's a more important factor at play when all of your elements are in the same column - the size of the atom that carries the negative charge. Imagine you have a tennis ball and a beach ball - your OH^- is your tennis ball (8 protons?!) and your TeH^- is your beach ball (55 protons!!), and you have to smear an entire jar of peanut butter over the surface of the ball. It'll be a lot easier to smear that peanut butter over the beach ball than the tennis ball, so the larger molecule is better at stabilizing the negative charge, therefore TeH^- is the most stable anion, meaning TeH₂ is more willing to dissociate, making a higher [H⁺] concentration, making a stronger acid.