In a Brønsted-Lowry acid-base reaction, which species loses a proton?

• A. The conjugate base
• B. The conjugate acid
• C. The base
• D. The acid

Answer: (D)

In a Brønsted-Lowry acid-base reaction, an acid donates a proton and a base accepts it. The species that loses a proton is the acid, which becomes its conjugate base after donation. The base, by contrast, accepts the proton and becomes its conjugate acid. So the correct choice is the acid, because it is the one that donates the proton (for example, HCl donates H+ to water, becoming Cl−, while water becomes H3O+). The conjugate base is what’s left after the proton is lost, the conjugate acid is the species formed after the base gains a proton, and the base is the proton acceptor.