Question

What is meant by pH? True or false: A strong acid solution always has a lower pH than a weak acid solution. Explain.

Short answer

pH is a measure of the acidity or basicity of a solution, ranging from 0 to 14, with 7 being neutral. Strong acids, such as hydrochloric acid, fully dissociate in water and have higher concentrations of hydrogen ions, resulting in lower pH values. Weak acids, like acetic acid, partially dissociate and generally have higher pH values. The statement "A strong acid solution always has a lower pH than a weak acid solution" is generally true, but exceptions may occur when comparing solutions with vastly different concentrations. An example: a 1M HCl solution has a pH of 0, while a 1M acetic acid solution has a pH of approximately 2.4, demonstrating that a strong acid solution generally has a lower pH than a weak acid solution.

Step-by-step solution

Explain the concept of pH

pH is a measure of the acidity or basicity of a solution. It is represented by a numerical scale ranging from 0 to 14, where a pH value of 7 indicates a neutral solution, values less than 7 represent acidic solutions, and values greater than 7 represent basic (alkaline) solutions. The pH of a solution can be calculated using the formula: pH = -log10[H+], where [H+] represents the concentration of hydrogen ions (protons) in the solution.

Define strong and weak acids

Strong acids are acids that completely ionize (dissociate) in water, resulting in a high concentration of hydrogen ions (protons) in the solution. Examples of strong acids include hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3). Weak acids, on the other hand, do not fully ionize in water, resulting in a lower concentration of hydrogen ions. Some examples of weak acids are acetic acid (CH3COOH) and citric acid (C6H8O7).

Evaluate the statement

The statement "A strong acid solution always has a lower pH than a weak acid solution" is generally true. Strong acids have a higher concentration of hydrogen ions than weak acids, which results in a lower pH value. However, it's important to note that the relative concentrations of the acid solutions and their relative strengths can affect this general rule. For example, a highly concentrated weak acid solution may have a pH lower than a very diluted strong acid solution.

Provide an example to support the answer

Let's take a 1M hydrochloric acid (HCl, a strong acid) solution and a 1M acetic acid (CH3COOH, a weak acid) solution. In the case of HCl, it will completely dissociate into H+ and Cl- ions, resulting in a 1M concentration of H+ ions. Using the formula for pH, we have: pH = -log10[1] = 0 For acetic acid, only a small fraction will ionize into H+ and CH3COO- ions. In a typical 1M acetic acid solution, approximately 0.004M of H+ ions will be present. Using the pH formula, we obtain: pH = -log10[0.004] ≈ 2.4 This example demonstrates that, for equal concentrations, a strong acid solution has a lower pH than a weak acid solution. However, this general rule may not apply in cases where the concentrations of the acid solutions are vastly different.