A) HNO2(aq)
B) H2O(l)
C) H3O+(aq)
D) NO2–(aq)
E) two of these
The right answer is E
Why?
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answers:
pisgahchemist say: Bronsted-Lowry theory and conjugate base....

In the Bronsted-Lowry theory an acid and a base react to produce a conjugate acid and a conjugate base. The acid is the "proton" (H+) donor, and the base is the "proton" acceptor. If you look at the reverse reaction, the conjugate acid is the "proton" donor and the conjugate base is the "proton" acceptor.

HNO2(aq) + H2O(l) <==> H3O^+(aq) + NO2^-(aq)
acid ............ base ........... conj............... conj.
.................... ................... acid............... base

Since HNO2 is a weak acid, and the system reaches an equilibrium, instead of going to completion, then you could consider the reverse reaction where water will be the conjugate base.

H3O^+(aq) + NO2^-(aq) <==> HNO2(aq) + H2O(l)
acid .............. base ................. conj. .......... conj.
...................... ......................... acid ........... base
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david say: HNO2(aq) + H2O(l) H3O+(aq) + NO2–(aq).
CB is the species formed when an acid loses a H+
HNO2 loses a H+ and forms NO2- so NO2- is a CB
the reaction is also reversible .. H3O+ loses a H+ in the reverse reaction to form H2O ... so H2O is another CB.
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Trevor H say: A conjugate base is what is left over after an acid has donated a proton, that is an H+ ion, during a chemical reaction.
In this case HNO2 which is an acid , donates an H+ ( to produce H3O+ in your equation) leaving the conjugate base , NO2-
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