(a) What is a strong acid? (b) A solution is labeled \(0.500 \mathrm{M}\) HCl. What is \(\left[\mathrm{H}^{+}\right]\) for the solution? (c) Which of the following are strong acids: \(\mathrm{HF}, \mathrm{HCl}, \mathrm{HBr}, \mathrm{HI?}\)

A strong acid is a substance that completely ionizes (or dissociates) in water to produce a high concentration of hydrogen ions (\(\mathrm{H^+}\)) or hydronium ions (\(\mathrm{H_3O^+}\)). In simple terms, a strong acid is an acid that easily loses a proton (\(\mathrm{H^+}\)) when dissolved in water.

Given a 0.500 M (\(\mathrm{M}\) stands for molarity) solution of HCl, we need to find the concentration of H+ ions in the solution. Since HCl is a strong acid, it will completely ionize in water, releasing one \(\mathrm{H^+}\) ion for every HCl molecule: \[\mathrm{HCl} \rightarrow \mathrm{H^+} + \mathrm{Cl^-}\] Since the reaction goes to completion, the concentration of \(\mathrm{H^+}\) ions is equal to the initial concentration of HCl. So, \(\left[\mathrm{H^+}\right] = 0.500\,\mathrm{M}\).

We are given four acids: \(\mathrm{HF}\), \(\mathrm{HCl}\), \(\mathrm{HBr}\), and \(\mathrm{HI}\). We are to identify which of these are strong acids. The strong acids are usually the ones that have a high tendency to lose a proton when dissolved in water. In the list, only hydrochloric acid (\(\mathrm{HCl}\)), hydrobromic acid (\(\mathrm{HBr}\)), and hydroiodic acid (\(\mathrm{HI}\)) are strong acids, as they dissociate completely in water. Hydrofluoric acid (\(\mathrm{HF}\)) is a weak acid because it does not ionize completely in water. In fact, the strength of these acids increases down the group, making \(\mathrm{HI}\) the strongest of them all. So, the strong acids in the given list are \(\mathrm{HCl, HBr,}\) and \(\mathrm{HI}\).