The first reaction (H2S + H2O <=> HS- + H3O+) isn't quantitative.
You will have a remaining part of H2S in solution, you must consider that part.
Talking about the second question, well.. you have to do the proton balance.
You have H2AsO4 into water.
H3AsO4 <= H2AsO4- => HAsO4-- => AsO4---
You can disregard AsO4--- (because of it's very very small concentration).
The last question has the same answer of the first. It is a weak acid, even if the constant is high, the reaction is not quantitative.