Arrhenius acid/base theory was created by Swedish chemist Svante Arrhenius, and is the oldest acid/base classification. According to his classification, acids are compounds that increase the concentration of ions in a solution, while bases are compounds or elements that either decrease the concentration of ions in solution or increase the concentration of ions in a solution. The other two answers describe the Brønsted–Lowry theory of acids and bases.

Below is a generic acid-base neutralization reaction:

The products are always water, and a salt. This salt is produced from the resulting ions and . The from the acid replaces the from the base, and the from the acid replaces the from the base. Since there are two replacements, acid-base neutralizations are classified as double-replacement reactions.

The equation to use here is:

Here, is the molarity of the acid, is the volume of the acid, is the molarity of the base, and is the volume of the base. Don't forget to convert the volume to liters!