Why is potassium nitrite acidic?

Answer 1

It should be basic, not acidic. The is an example of a salt with a weaker conjugate acid than base. Think of it this way: KOH is a strong base so it dissociates totally. This means that the potassium in potassium nitrite, when dissociated in water has no potential to sequester any of the hydroxide ions (from dissociated water molecules). In short, it is a spectator (ion) in the system. Yet another way of thinking of this is that potassium is a WEAK CONJUGATE ACID. HNO2 (Hydrogen nitrite, or nitrous acid when dissolved in water) on the other hand is a weak acid so the hydrogen and nitrite do NOT completely dissociate - the nitrite has the tendency to hold on to some of the available protons (it is a stronger conjugate base than potassium is a conjugate acid). This means that the nitrite in a potassium nitrite solution sequesters some of the protons from dissociated water away leaving unmatched hydroxide ions. The solution will be basic because of these unpartnered hydroxides.

Answer 2

Potassium nitrate is a salt; the solution is neutral.