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Ch.16 - Acid-Base Equilibria
Chapter 16, Problem 114b

The amino acid glycine 1H2N¬CH2¬COOH2 can participate in the following equilibria in water: H2N¬CH2¬COOH + H2OΔ H2N¬CH2¬COO- + H3O+ Ka = 4.3 * 10-3 H2N¬CH2¬COOH + H2OΔ+H3N¬CH2¬COOH + OH- Kb = 6.0 * 10-5 (b) What is the pH of a 0.050 M aqueous solution of glycine?

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1
Identify the relevant equilibria for glycine in water: the acid dissociation (Ka) and the base dissociation (Kb).
Recognize that glycine can act as both an acid and a base, making it an amphoteric species. Use the given Ka and Kb values to determine which equilibrium will dominate in a 0.050 M solution.
Calculate the concentration of hydronium ions \([H_3O^+]\) using the acid dissociation constant \(K_a\) and the initial concentration of glycine. Use the expression: \[ K_a = \frac{[H_3O^+][H_2N-CH_2-COO^-]}{[H_2N-CH_2-COOH]} \]
Assume that the change in concentration of glycine due to dissociation is small compared to the initial concentration, allowing simplification of the equilibrium expression.
Calculate the pH from the hydronium ion concentration using the formula: \( \text{pH} = -\log[H_3O^+] \).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Acid-Base Equilibria

Acid-base equilibria involve the transfer of protons (H+) between species in solution. In the case of glycine, it can act as both an acid and a base, participating in equilibria that define its behavior in water. Understanding the dissociation constants (Ka and Kb) is crucial for predicting the pH of the solution, as they indicate the strength of the acid and base forms of glycine.
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Arrhenius Acids and Bases

Dissociation Constants (Ka and Kb)

Dissociation constants, Ka and Kb, quantify the extent to which an acid or base dissociates in water. Ka represents the equilibrium constant for the dissociation of an acid, while Kb represents the equilibrium constant for the dissociation of a base. For glycine, the given values of Ka and Kb allow us to calculate the concentrations of the various species in solution, which is essential for determining the pH.
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Ka and Kb Relationship

pH Calculation

pH is a measure of the hydrogen ion concentration in a solution, calculated using the formula pH = -log[H+]. In the context of glycine, the pH can be determined by considering the contributions of both the acidic and basic forms of the amino acid in solution. By applying the Henderson-Hasselbalch equation or directly using the dissociation constants, one can find the pH of the glycine solution.
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pH Calculation Example
Related Practice
Textbook Question

The following observations are made about a diprotic acid H2A: (i) A 0.10 M solution of H2A has pH = 3.30. (ii) A 0.10 M solution of the salt NaHA is acidic. Which of the following could be the value of pKa2 for H2A: (i) 3.22, (ii) 5.30, (iii) 7.47, or (iv) 9.82?

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Open Question
Many moderately large organic molecules containing basic nitrogen atoms are not very soluble in water as neutral molecules, but they are frequently much more soluble as their acid salts. Assuming that the pH in the stomach is 2.5, indicate whether each of the following compounds would be present in the stomach as the neutral base or in the protonated form: nicotine, Kb = 7 * 10-7; caffeine, Kb = 4 * 10-14; strychnine, Kb = 1 * 10-6; quinine, Kb = 1.1 * 10-6.
Textbook Question

The amino acid glycine 1H2N¬CH2¬COOH2 can participate in the following equilibria in water: H2N¬CH2¬COOH + H2OΔ H2N¬CH2¬COO- + H3O+ Ka = 4.3 * 10-3 H2N¬CH2¬COOH + H2OΔ+H3N¬CH2¬COOH + OH- Kb = 6.0 * 10-5 (a) Use the values of Ka and Kb to estimate the equilibrium constant for the intramolecular proton transfer to form a zwitterion: H2N¬CH2¬COOH Δ +H3N¬CH2¬COO-

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Open Question
The pKb of water is _______. (a) 1 (b) 7 (c) 14 (d) not defined (e) none of the above
Open Question
Calculate the number of H3O+ ions in 1.0 mL of pure water at 25 °C.
Open Question
How many milliliters of concentrated hydrochloric acid solution (36.0% HCl by mass, density = 1.18 g/mL) are required to produce 10.0 L of a solution that has a pH of 2.05?