객관식
A student titrated a 100.0 mL sample of 0.100 M acetic acid with 0.050 M NaOH. (For acetic acid, Ka = 1.8 × 10^-5 at room temperature.) Calculate the initial pH of the acetic acid solution before any NaOH is added.
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17. Acid and Base Equilibrium
pH of Weak Acids
Multiple Choice
Calculate the concentration of the lactate ion in a solution that is 0.100 M in lactic acid (CH₃CH(OH)COOH, pKa = 3.86) and 0.080 M in HCl.
A
0.180 M
B
0.020 M
C
0.100 M
D
0.080 M
검증된 단계별 안내1
Identify the relevant chemical equilibrium: Lactic acid (CH₃CH(OH)COOH) dissociates in water to form lactate ions (CH₃CH(OH)COO⁻) and hydrogen ions (H⁺). The equilibrium expression is: \( \text{CH}_3\text{CH(OH)COOH} \rightleftharpoons \text{CH}_3\text{CH(OH)COO}^- + \text{H}^+ \).
Recognize that HCl is a strong acid and will fully dissociate in solution, contributing additional \( \text{H}^+ \) ions. This will affect the equilibrium position of the lactic acid dissociation.
Use the Henderson-Hasselbalch equation to find the concentration of lactate ions. The equation is: \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \), where \([\text{A}^-]\) is the concentration of lactate ions and \([\text{HA}]\) is the concentration of lactic acid.
Calculate the pH of the solution. Since HCl is a strong acid, its concentration (0.080 M) will be the concentration of \( \text{H}^+ \) ions. Use the formula \( \text{pH} = -\log[\text{H}^+] \) to find the pH.
Substitute the known values into the Henderson-Hasselbalch equation to solve for the concentration of lactate ions \([\text{A}^-]\). Rearrange the equation to solve for \([\text{A}^-]\): \( [\text{A}^-] = [\text{HA}] \times 10^{(\text{pH} - \text{pKa})} \).
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