What is the pH of a 0.210 M aqueous solution of potassium nitrite, KNO2? pH =?

Refer to: https://depts.washington.edu/eooptic/links/acidstrength.html
Ka for HNO₂ = 7.2 × 10⁻⁴
Hence, Kb for NO₂⁻ = Kw/Ka(HNO₂) = (1.0 × 10⁻¹⁴) / (7.2 × 10⁻⁴) = 1.39 × 10⁻¹¹
(Ka values for different sources may be different.)

Consider the dissociation of NO₂⁻ ions:
_______________ NO₂⁻(aq) ___ + ___ H₂O(l) ___ ⇌ ___ HNO₂(aq) ___ + ___ OH⁻(aq) ______ Kb = 1.39 × 10⁻¹¹
Initial : _________ 0.210 M _________________________ 0 M _____________ 0 M
Change : _________ -y M __________________________ + y M ____________ +y M
Equilibrium : __ (0.210 - y) M _______________________ y M _____________ y M

As Kb is very small, the degree of dissociation is to a very small extent.
It can be assumed that 0.210 ≫ y
[NO₂⁻] at equilibrium = (0.210 - y) M ≈ 0.210 M

At equilibrium :
Kb = [HNO₂] [OH⁻] / [NO₂⁻]
1.39 × 10⁻¹¹ = y² / 0.210
y = √{(1.39 × 10⁻¹¹) × 0.210} = 1.71 × 10⁻⁶

[OH⁻] = y M = 1.71 × 10⁻⁶ M
pOH = -log[OH⁻] = -log(1.71 × 10⁻⁶) = 5.8
pH = pKw - pOH = 14.0 - 5.8 = 8.2

pH of potassium nitrite.....

KNO2 ionizes to give 0.210M NO2^-. NO2^- hydrolyzes (reacts with water) ....
NO2^- + HOH(l) <==> HNO2(aq) + OH- ..............Kb = Kw / Ka = 1.00x10^-14 / 4.0x10^-4 = 2.5x10^-11..(*)
0.210 ............................ 0 ................0 ................. initial
-x ................................ +x .............. +x ................ change
0.210-x ....................... x ...................x ................ equilibrium

Kb = [HNO2][OH-] / [NO2^-]
2.5x10^-11 = x² / (0.210-x)
-- algebra goes here --
x = 2.29x10^-6 .......................... to one extra "guard digit"
[OH-] = 2.29x10^-6

Kw = [H+][OH-]
[H+] = Kw / [OH-]
pH = -log[H+] = -log(Kw / [OH-] = -log(1.00x10^-14 / 2.29x10^-6) = 8.36

The pH is given to two significant digits. When taking the log of a number, only the digits to the right of the decimal reflect the precision in the original number.

* ..... Ka values: http://clas.sa.ucsb.edu/staff/Resource%20folder/Chem109ABC/Acid,%20Base%20Strength/Table%20of%20Acids%20w%20Kas%20and%20pKas.pdf