Multiple Choice
Which of the following best describes the entire alkaline range of the pH scale?
17. Acid and Base Equilibrium
The pH Scale
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Which of the following statements about aqueous solutions is/are true?
A
For an basic solution the concentration of H3O+ is greater than the concentration of OH-.
B
The pH of a neutral aqueous solution is 7.00 at all temperatures.
C
An acidic solution under normal conditions has a pH value less than 7.00.
D
If the concentration of H3O+ decreases then the concentration of OH- will also decrease.
E
The pH of aqueous solutions is less than 7.
Verified step by step guidance1
Step 1: Understand the concept of pH and its relation to H3O+ and OH- concentrations. pH is a measure of the acidity or basicity of a solution. It is defined as the negative logarithm of the hydrogen ion concentration: \( \text{pH} = -\log[\text{H}_3\text{O}^+] \). In pure water at 25°C, the concentration of \( \text{H}_3\text{O}^+ \) and \( \text{OH}^- \) are equal, making the solution neutral with a pH of 7.
Step 2: Analyze the statement about basic solutions. In a basic solution, the concentration of \( \text{OH}^- \) is greater than the concentration of \( \text{H}_3\text{O}^+ \). Therefore, the statement 'For a basic solution the concentration of \( \text{H}_3\text{O}^+ \) is greater than the concentration of \( \text{OH}^- \)' is false.
Step 3: Consider the effect of temperature on pH. The statement 'The pH of a neutral aqueous solution is 7.00 at all temperatures' is incorrect. The pH of pure water changes with temperature because the ionization of water is temperature-dependent. At temperatures other than 25°C, the pH of neutral water can be slightly above or below 7.
Step 4: Evaluate the statement about acidic solutions. An acidic solution has a pH less than 7.00 under normal conditions, which is true. This is because the concentration of \( \text{H}_3\text{O}^+ \) is greater than \( 1 \times 10^{-7} \) M, making the solution acidic.
Step 5: Examine the relationship between \( \text{H}_3\text{O}^+ \) and \( \text{OH}^- \) concentrations. The statement 'If the concentration of \( \text{H}_3\text{O}^+ \) decreases then the concentration of \( \text{OH}^- \) will also decrease' is false. According to the water dissociation equilibrium \( \text{H}_2\text{O} \rightleftharpoons \text{H}_3\text{O}^+ + \text{OH}^- \), if \( \text{H}_3\text{O}^+ \) decreases, \( \text{OH}^- \) must increase to maintain the equilibrium constant \( K_w = [\text{H}_3\text{O}^+][\text{OH}^-] = 1 \times 10^{-14} \) at 25°C.
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