Nitrous acid (HNO2) is a weak acid, and when it donates a proton (H+), it forms its conjugate base. The conjugate base of HNO2 is nitrite ion (NO2–).
Here’s the reaction that shows the formation of the conjugate base:
HNO2 ⇌ H+ + NO2–
In this reaction, HNO2 loses a proton and becomes NO2–, which is the conjugate base. The nitrite ion (NO2–) can accept a proton to reform HNO2, making it the conjugate base of nitrous acid.