What is the correct formula for hydroxide?

The formula for hydroxide is represented as OH-. This indicates that the hydroxide ion consists of one oxygen atom and one hydrogen atom. The negative charge signifies that it is an anion, which means it carries a surplus of electrons. Hydroxide ions play a significant role in chemistry, particularly in acid-base reactions, as they can neutralize acids and contribute to pH balance in various solutions.

In contrast, the other options represent different ions. The ammonium ion, NH4+, has a positive charge and consists of one nitrogen atom and four hydrogen atoms. HPO4 2- is the hydrogen phosphate ion, containing one phosphorus atom and four oxygen atoms, and carries a -2 charge. Lastly, PO4 3- represents the phosphate ion, which has one phosphorus atom and four oxygen atoms with a -3 charge. None of these represent the hydroxide ion, which distinctly has the composition of one oxygen and one hydrogen with a negative charge.