[Blah DTMM]

I'm having trouble with some questions regarding hydronium-ion concentration and pH. The question reads as follows:

1.) Each of the following solutions has a concentration of 0.1 mol/L. Compare the hydronium-ion concentration and pH in each pair, and explain why they are different.

a.) H₂SO₄(aq) vs H₂SO₃(aq)

I don't know how to find the hydronium-ion concentration in this question. I know that pH=-log[H₃O⁺(aq)], so I could find the pH if somebody is able to show me how to find the hydronium-ion concentration and explain why they differ between the two solutions.

[Borek]

One is a strong acid, other is a weak acid.

[Blah DTMM]

Yes but how do I find the hydronium-ion concentration and pH? I know the pH of the strong acid will be lower but how do I find an exact value? Or am I just supposed to give an approximation in this situation?

[Borek]

I would go for an approximation. But if you want to try to calculate... just don't hope it will be easy.

http://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-acid-base

http://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-simplified

[rabolisk]

My guess is that this is a qualitative question designed to see if you understand why one would be a strong acid while the other one is weak based on their structures.

[Blah DTMM]

Ahh okay that makes sense. I guess I am just confused about determining strong vs. weak acids and bases. I understand the theory about strong acids reacting completely and weak acids reacting partially, but how do I tell from just looking at the formulas whether a solution is a strong or a weak acid? Like with H₂SO₄ and H₂SO₃, I know from looking it up that the former is a strong acid while the latter is a weak acid, but how would I tell just from looking at the formula?

[rabolisk]

The extra oxygen will act to stabilize the conjugate base by withdrawing electron density. Your textbook should have a more detailed explanation.

[Blah DTMM]

Nahh, I've read through the section several times and it isn't helping me understand. The next question has:

H₂S(aq) vs. NH₃(aq)

So when H₂S is added to water it looks like this:
H₂S(aq) + H₂O(l)  H₃O⁺(aq) + HS^-(aq)
Correct? Now I know from looking it up that hydrogen sulfide is a weak acid. When you look at the above equation, what is it that tells you it is a weak acid and not a strong acid?

And with NH₃(aq):
NH₃(aq) + H₂O(l)  OH^-(aq) + NH₄⁺(aq)
Again, how do I know from writing out the equation that ammonia is a weak base?

[AWK]

From these equations you know that H₂S is  an acid and NH₃ is a base. And that are weak ones you can recognize from pKa. Of course there are some rules that help you to memorize which are strong, which are weak, but these are only rules and sometimes may give you wrong suggestions.

[rabolisk]

Any chemist can tell you that NH₃ is a weak base and H₂S is a weak acid. For one, you should memorize the common strong acids (your book should present this to you). Second, hydrogen sulfide's acidity can be deduced from the fact that it must be a stronger acid than water because sulfur is below oxygen (again, something that they should discuss in the textbook). As you become more knowledgeable about chemistry, you will gain an intuition about things like this.

[Blah DTMM]

Okay, this has cleared things up a good bit for me. Thanks for the help everyone.