According to the Brønsted–Lowry theory, an acid is a proton donor, and a base is a proton acceptor

According to the Brønsted–Lowry theory, an acid is a proton donor, and a base is a proton acceptor. Thus, HBr is an acid because it reacts with water according to the equation HBr + H?O → H?O? + Br?

In the process, the HBr has donated a proton (H?) to the water, and the water has accepted the proton. The HBr is a Brønsted acid, and the water is a Brønsted base.

In the Brønsted-Lowry theory, a conjugate base is whatever is left over after the proton has left. Thus, HBr lost a proton to become Br?, so Br? is the conjugate base of HBr.

A conjugate base always has one less H atom and one more negative charge than the original acid. Thus, for example, you can easily say that the conjugate base of H?O is OH?.