Textbook Question
Does the pH increase, decrease, or remain the same when the substances are added to the solutions? (a) LiF to an HF solution(b) KI to an HI solution (c) NH4Cl to an NH3 solution
Ch.17 - Applications of Aqueous Equilibria
Chapter 17, Problem 58
Calculate the pH of a solution prepared by dissolving 0.10 mol of solid NH4Cl in 0.500 L of 0.40 M NH3. Assume that there is no volume change.
Verified step by step guidance1
Identify the components of the solution: NH4Cl is a salt that dissociates into NH4+ and Cl- ions, and NH3 is a weak base.
Recognize that NH4+ is the conjugate acid of NH3, and this forms a buffer solution. Use the Henderson-Hasselbalch equation for buffer solutions: \( \text{pH} = \text{pK}_a + \log \left( \frac{[\text{base}]}{[\text{acid}]} \right) \).
Find the \( \text{pK}_a \) of NH4+ by using the relationship \( \text{pK}_a + \text{pK}_b = 14 \), where \( \text{pK}_b \) is for NH3. Look up or calculate \( \text{pK}_b \) for NH3.
Calculate the concentration of NH4+ in the solution: Since 0.10 mol of NH4Cl is dissolved in 0.500 L, the concentration \([\text{NH4}^+]\) is \( \frac{0.10 \text{ mol}}{0.500 \text{ L}} \).
Calculate the concentration of NH3 in the solution: The initial concentration is 0.40 M, and since there is no volume change, \([\text{NH3}]\) remains 0.40 M. Substitute these values into the Henderson-Hasselbalch equation to find the pH.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Acid-Base Chemistry
Acid-base chemistry involves the study of substances that can donate protons (acids) or accept protons (bases). In this context, NH4Cl acts as an acid, providing NH4+ ions, while NH3 is a weak base. The interaction between these species is crucial for determining the pH of the solution.
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Arrhenius Acids and Bases
Henderson-Hasselbalch Equation
The Henderson-Hasselbalch equation is a formula used to calculate the pH of a buffer solution. It relates the pH to the pKa of the acid and the ratio of the concentrations of the conjugate base and acid. In this case, it helps to find the pH of the NH4+/NH3 buffer system formed by the dissolution of NH4Cl in NH3.
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Buffer Solutions
Buffer solutions are mixtures that resist changes in pH upon the addition of small amounts of acid or base. They typically consist of a weak acid and its conjugate base or a weak base and its conjugate acid. The NH4+/NH3 system acts as a buffer, allowing for the calculation of pH based on the concentrations of NH4+ and NH3.
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