Compare the conjugate bases of these three acids.
Acid 1: nitrous acid, HNO2
Acid 2: carbonic acid, H2CO3
Acid 3: hydrogen sulfide, HS-
What is the formula for the strongest conjugate base?
The Correct Answer and Explanation is:
Correct Answer:
The strongest conjugate base is: S²⁻ (sulfide ion)
Explanation
To determine the strongest conjugate base among the conjugate bases of the given acids, we first need to identify the conjugate base of each acid. A conjugate base is formed when an acid donates a proton (H⁺).
1. Nitrous Acid (HNO₂)
Conjugate Base: NO₂⁻ (nitrite ion)
Nitrous acid is a weak acid, so its conjugate base (NO₂⁻) is relatively strong but still weaker than the conjugate base of a weaker acid.
2. Carbonic Acid (H₂CO₃)
Carbonic acid is diprotic (can lose two protons), but we’ll consider only the first deprotonation:
H₂CO₃ → H⁺ + HCO₃⁻
Conjugate Base: HCO₃⁻ (bicarbonate ion)
Carbonic acid is weak, but slightly stronger than nitrous acid, so HCO₃⁻ is a slightly weaker conjugate base than NO₂⁻.
3. Hydrogen Sulfide Ion (HS⁻)
HS⁻ can act as an acid by losing one more proton:
HS⁻ → H⁺ + S²⁻
Conjugate Base: S²⁻ (sulfide ion)
HS⁻ is a much weaker acid than the previous two. In fact, it is already a conjugate base itself (of H₂S), and losing another proton forms S²⁻, an extremely strong base.
Strength of Conjugate Base
The strength of a conjugate base is inversely related to the strength of its corresponding acid. The weaker the acid, the stronger its conjugate base.
Among the three:
- HNO₂ and H₂CO₃ are weak acids.
- HS⁻ is a weak acid but much weaker, meaning S²⁻ is the strongest base.
Thus, S²⁻ (sulfide ion), being the conjugate base of the very weak acid HS⁻, is the strongest conjugate base among the three.
