Question

Can you identify the following as weak acid, strong acid, weak base, strong base, or neutral? How do you determine this?

`KOH`
`HCl`
`CH_3COOH`
`H_2SO_4`
`H_2CO_3`
`H_3PO_4`
`H_2O`
`H_3O^+`
`HCN`
`NH_4OH`

Answer 1

Simply from the `K_a`, or the `"pKa"`. Or, the `K_b`, or the `"pKb"`, if comparing by dissociation in water.

• The higher the `K_a` or lower the `"pKa"`, the stronger the acid.
• The lower the `K_a` or higher the `"pKa"`, the weaker the acid.
• Converting from `K_a` to `K_b`, `K_b = (10^(-14))/K_a`.
• Converting from `"pKa"` to `"pKb"`, `"pKb" = 14 - "pKa"`.

Weaker acids have stronger conjugate bases and vice versa, so there's no need to think about it much more than that.

These are all relative to water, so clearly, pure water is considered neutral.

• `"KOH"`, an alkali metal hydroxide, easily dissociates to release `"OH"^(-)` into solution, which classifies it as a strong base. No need to check `"pKa"` here.
• `"HCl"`, a binary acid, easily dissociates to release `"H"^(+)` into solution, with `"pKa" ~~ -7`. Strong acid.
• `"CH"_3"COOH"` is a carboxylic acid, whose `"pKa"` is about `5` (actually, this one is `4.74`). Often called acetic acid. Considered to be a weak acid, but still dissociates its proton significantly.
• `"H"_2"SO"_4` has a `"pKa"_1` of `-3` so it dissociates very easily. Strong acid.
• `"H"_2"CO"_3` has a `"pKa"_1` of about `6.4`. Considered to be a weak acid, but still dissociates its first proton significantly.
• `"H"_3"PO"_4` has a `"pKa"_1` of about `2.16`. Considered to be a relatively weak acid, but still dissociates its first proton significantly.
• `"H"_2"O"` has a `"pKa"` of about `15`. Obviously neutral, since it cannot be a solute in its own solution and alter its own pH.
• `"H"_3"O"^(+)` has a `"pKa"` of about `-1.7`, so it's a strong acid.
• `"HCN"` has a `"pKa"` of about `9.2`. Considered to be a weak acid, but still dissociates its first proton significantly.
• `"NH"_4"OH"` forms like this:

`"NH"_3(aq) + "H"_2"O"(l) rightleftharpoons "NH"_4^(+)(aq) + "OH"^(-)(aq)`

`K_b = (["NH"_4^(+)]["OH"^(-)])/(["NH"_3]) = 1.8xx10^(-5) => "pKa" ~~ 9.25`.

Thus, `"NH"_4"OH"` dissociates significantly to form `"NH"_3` in solution, and most consider it a strong acid, even though its `"pKa"` is so close to that of `"HCN"`.

Lastly, make sure you know that acidity and basicity is all relative. You MUST compare the `pKa`s. Strong acids are only considered strong because they dissociate significantly in water.

If water had a lower `"pKa"`, these strong acids would be weaker. Similarly, if you dissolved `"HCl"` in `"H"_2"SO"_4`, `"HCl"` would be considered a weak acid!