A strong acid, such as hydrochloric acid, ionizes quantitatively (completely) in water to form ions. This means that, for example, in 0.10 mol/L HCl(aq) there is really 0.10 mol/L of both H+(aq) and Cl−(aq). Some common strong acids: HCl, HClO4, HBr, HI, HNO3, and H2SO4 (you can remember most of them with the mnemonic brinoso).
A strong base, such as sodium hydroxide, also dissociates quantitatively in water. This means that, for example, in 0.10 mol/L NaOH(aq) there is really 0.10 mol/L of both Na+(aq) and OH−(aq). All group 1 hydroxides (LiOH, NaOH, KOH, RbOH, CsOH) are strong bases. There are also strong bases from group 2: Mg(OH)2, Ca(OH)2, Ba(OH)2, and Sr(OH)2.
When we are working with strong acids and bases, it is easy to find the concentrations of H+(aq) and OH−(aq) as in the example from the previous section. Always make sure you don’t have a strong acid or base before using the more complicated method for weak acids and bases.