Multiple Choice
Calculate the pH of a 0.03 M aqueous solution of a weak acid, HA, with a pKa of 3.39.
17. Acid and Base Equilibrium
pH of Weak Acids
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Calculate the pH of a 0.800 M NaCH3CO2 solution. Ka for acetic acid, CH3CO2H, is 1.8 × 10^-5.
A
7.00
B
5.13
C
2.87
D
8.87
Verified step by step guidance1
Identify the nature of the solution: NaCH3CO2 is the sodium salt of acetic acid, which means it is a basic solution because the acetate ion (CH3CO2-) will react with water to form acetic acid and hydroxide ions.
Write the equilibrium expression for the hydrolysis of acetate ion: CH3CO2- + H2O ⇌ CH3CO2H + OH-. The equilibrium constant for this reaction is Kb, which can be calculated using the relation Kb = Kw / Ka, where Kw is the ion-product constant of water (1.0 × 10^-14 at 25°C) and Ka is the acid dissociation constant for acetic acid.
Calculate Kb using the formula: Kb = Kw / Ka = (1.0 × 10^-14) / (1.8 × 10^-5). This will give you the equilibrium constant for the reaction of acetate ion with water.
Set up an ICE table (Initial, Change, Equilibrium) for the hydrolysis reaction to determine the concentration of OH- ions at equilibrium. Assume the initial concentration of CH3CO2- is 0.800 M, and let x be the change in concentration due to the reaction.
Use the expression for Kb: Kb = [CH3CO2H][OH-] / [CH3CO2-]. Substitute the equilibrium concentrations from the ICE table into this expression and solve for x, which represents the concentration of OH- ions. Finally, calculate the pOH from the concentration of OH- ions and convert it to pH using the relation pH = 14 - pOH.
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