Question

(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: \(\mathrm{HF}, \mathrm{HCl}, \mathrm{HBr}, \mathrm{HI?}\)

Short answer

(a) A strong acid is a substance that completely ionizes in water to produce a high concentration of \(\mathrm{H^+}\) or \(\mathrm{H_3O^+}\) ions. In simple terms, it easily loses a proton when dissolved in water. (b) \(\left[\mathrm{H^+}\right] = 0.500\,\mathrm{M}\) for the given HCl solution. (c) The strong acids in the list are \(\mathrm{HCl, HBr,}\) and \(\mathrm{HI}\).

Step-by-step solution

(a) Definition of a strong acid

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.

(b) H+ concentration in a 0.500 M HCl solution

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, \(\left[\mathrm{H^+}\right] = 0.500\,\mathrm{M}\).

(c) Identifying strong acids among the given options

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 \(\mathrm{HI}\).

Key concepts

Acid Dissociation

Acid dissociation is the process where an acid releases its protons (H+) into the solution. This process is crucial as it determines how strong or weak an acid is. Strong acids dissociate completely, releasing all available protons. This results in a high concentration of hydrogen ions in the solution. For example, hydrochloric acid (HCl) is a strong acid because, when dissolved in water, it dissociates completely:

• HCl → H⁺ + Cl^-

This means every molecule of HCl releases one hydrogen ion (H+). The ability of an acid to dissociate fully is a defining feature of strong acids. Conversely, weak acids only partially dissociate, meaning not all acid molecules release their protons into the solution. This characteristic creates a significant difference in the behavior and strength of acids in different solutions.

Hydronium Ions

Hydronium ions (H₃O⁺) are formed when a hydrogen ion (H+) is transferred from an acid to a water molecule. The equation representing this process is:

• H₂O + H⁺ → H₃O⁺

This shows that the presence of H₃O⁺ ions is directly linked with the acidity of a solution. The formation of hydronium ions is an important measure of the acid's strength in the solution. In the case of strong acids, since they donate all their protons, the hydronium ion concentration is equal to the initial concentration of the acid. In a 0.500 M solution of HCl, the concentration of hydronium ions will also be 0.500 M. This full conversion enhances the acidic nature of the solution. Hydronium ions play a significant role in influencing the pH level, which measures the acidity or basicity of a solution.

Molarity of Solutions

Molarity is a unit of concentration that measures the number of moles of solute per liter of solution. It is represented in units of moles per liter (M). Understanding molarity is crucial for calculating concentrations and reactions in chemistry. For example, when a solution is labeled as 0.500 M HCl, it indicates that there are 0.500 moles of HCl dissolved in one liter of solution. Since HCl is a strong acid, it completely dissociates into H+ ions. Therefore, in this solution, the molarity of H+ ions is also 0.500 M, showing a direct way to calculate the concentration of ions based on the acid's molarity. Having this understanding allows students to predict how altering the molarity would affect the solution’s properties, such as pH and conductivity, especially in strong acid scenarios where dissociation is complete.