200mL HNO2 0.04M
+
40mL NaOH 0.2M
=
240mL di:
HNO2 0.04M*200mL / 240mL = 0.033M
+ NaOH 0.2M * 40mL / 240mL = 0.033M

HNO2 + NaOH -> Na+ NO2- + H2O
0.033__0.033_______0_____/__inizio
_0_______0_______0.033___/__fien

NO2- + H2O <--> HNO2 + OH-
0.033___/________0______0_
0.033-X_________X_______X_

Kidrolisi = Kw/Ka = X^2 / (0.033-X)

da cui X = [OH-] = 8.1*10^-7 => pOH = 6.1 => pH = 14 - pOH = 7.9