What is the pH of a solution that is 0.10 M KNO2 and 0.20 M HNO2 (nitrous acid)?

What is the pH of a solution that is 0.10 M KNO2 and 0.20 M HNO2 (nitrous acid)?

thanks

Answer:
A solution of a weak acid and on of its salts forms a buffer solution. Its pH is given by the Henderson-Hasselbach equation:

pH = pKa + log( [A⁻] / [HA] )
in general
pH = pKa - log( [NO₂⁻] / [HNO₂] )
for this system

Due to the weak dissociation of the nitrous acid and the complete dissociation of the potassium nitrite ion, you can take the nitrite ion concentration equal to the apparent salt concentration and actual nitrous acid concentration equal to its apparent concentration.

pH = 3.34 + log( 0.1M / 0.2M ) = 3.04
The pKa value for HNO2 is 3.224.
KNO2 is a basic salt made up of a weak acid, HNO2, and a strong base KOH.

So the system constitutes a buffer.

Using the Henderson-Hasslebach equation for buffers,

pH = pKa + log [Salt]/[Acid]
= 3.224 + log 0.1/0.2
= 3.224 - 0.3010
= 2.922

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