In the reaction NH3(aq) + H2O(aq) ⇌ NH4+(aq) + OH−(aq), which species acts as a conjugate base?

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Multiple Choice

In the reaction NH3(aq) + H2O(aq) ⇌ NH4+(aq) + OH−(aq), which species acts as a conjugate base?

Conjugate bases are what you get when an acid donates a proton. In this equilibrium, ammonium ion (NH4+) is the acid and donates a proton to water to become ammonia (NH3). Therefore, ammonia is the conjugate base of ammonium. Water acts as the acid in this reaction (it donates a proton to NH3, forming OH−), so its conjugate base would be hydroxide, which isn’t among the options. Ammonium hydroxide isn’t the conjugate base here, it’s the combination of the acid and its conjugate base present in solution.

Conjugate bases are what you get when an acid donates a proton. In this equilibrium, ammonium ion (NH4+) is the acid and donates a proton to water to become ammonia (NH3). Therefore, ammonia is the conjugate base of ammonium. Water acts as the acid in this reaction (it donates a proton to NH3, forming OH−), so its conjugate base would be hydroxide, which isn’t among the options. Ammonium hydroxide isn’t the conjugate base here, it’s the combination of the acid and its conjugate base present in solution.

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