Which compound can act as both a brønsted-lowry acid and a brønsted-lowry base?

water

Explanation:

In 1923, Bronsted and Lowry independently proposed a theory to explain the acid-base behavior of substances, in which acids are proton donors while bases are proton acceptors.

NH3+H2O⇌NH+4+OH−

Water acts as an acid, yielding a proton to ammonia, which in turn acts as a base. Water becomes its conjugate base; the hydroxide ion. While ammonia becomes its conjugated acid; the ammonium ion.

On the other hand, when water reacts with nitric acid, it is water that accepts protons to form the hydronium ion and nitric acid that donates them. In this case the water acts as the Brønsted-Lowry base.

15.3 cos 65 degrees trig