Question

Which of the given salt will have maximum pH?

1. `"KNO"_3`
2. `"NaCN"`
3. `"HCOONH"_4`
4. `"NaOH"`

Answer 1

2.

Explanation:

As people pointed out, it is good to first know the definition of a salt:
An ionic compound formed by the neutralization of an acid and a base
- That eliminates 4. As NaOH is not achieved through neutralization. (But it is often used as a reagent in a neutralization reaction)

Compound 1,however, is formed in the neutralization reaction between Potassium hydroxide and Nitric acid:
`KOH (aq) + HNO_3 (aq) -> KNO_3(aq) + H_2O(l)`
But it is not particularly exciting to dissolve- it simply dissociates into ions, `K^+` and `NO_3^-`, neither of which affects the `pH`.

Compound 3 on the other hand, is formed by neutralizing methanoic acid with ammonia, and can dissociate into the ammonium ion `(NH_4^+)` and methanoate ion `(HCOO^-)`. Now, the ammonium ion is a weak acid and will react with Water to form Oxonium ions,
`NH_4^(+)(aq) + H_2O(l) -> NH_3(aq) + H_3O^(+) (aq)`
which will decrease the `pH`!- This was the opposite of what we were looking for.

Finally, we have compound 2.
This is formed from neutralizing Hydrogen cyanide, `HCN`, which is a weak acid with Sodium hydroxide `NaOH`, a strong base.

The cyanide ion, `CN^-`, is, therefore, the conjugate base of `HCN` in the (HCN) / (CN^-) acid/base pair.
So if we put `NaCN` in water, it will dissociate to form `Na^+` and `CN^-`, and `CN^-` will act as a Bronstedt-Lowry base and accept a proton from water:

`NaCN(s) + H_2O(l) -> Na^(+)(aq) + OH^(-)(aq) + HCN(aq)`
Which forms hydroxide ions! These will increase the `pH` of the solution above 7, so this salt will give the maximum `pH`!