Which of the given salt will have maximum pH?

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

1 Answer
Apr 8, 2018

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#!