help!!! please :)

[swedishfish22]

Which one of the following is the weakest acid?
HF 6.8x10^-4
HNO₂ 4.5x10^-4
HOBr 2.5x10^-9
HCN 4.9x10^-10

Which of the pairs can't be used to make a buffer solution?
NH₃ & NaC₂H₃O₂
H₂S & NaHS
NH₃ & NH₄Cl
HC₂H₃O₂ & NaC₂H₃O₂

Which solution would have the lowest [H₃O+]?
0.10 M Na₂SO₄
0.10 M NaCN
0.10 M NaNO₃
0.10 M NH₄C₂H₃O₂

What's the net ionic equation for the reaction that occurs when HCl is added to the NaHCOO/HCOOH buffer?
H+ + OH- ---> H₂O
HCl + OH- ---> H₂O + Cl-
HCl + HCOO- ---> HCOOH + Cl-
H+ + HCOO- ---> HCOOH