Multiple Choice
What is the pH of a solution that is 8.0 × 10⁻² M in H₂CO₃, given that the first dissociation constant (Ka1) of H₂CO₃ is 4.3 × 10⁻⁷?
17. Acid and Base Equilibrium
pH of Weak Acids
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If a solution of HF (Ka = 6.8 × 10⁻⁴) has a pH of 2.50, what is the initial concentration of hydrofluoric acid in the solution?
A
1.0 M
B
0.10 M
C
0.001 M
D
0.01 M
Verified step by step guidance1
Start by understanding that HF is a weak acid, and its dissociation in water can be represented by the equation: HF ⇌ H⁺ + F⁻.
Use the given pH to find the concentration of H⁺ ions in the solution. Recall that pH = -log[H⁺], so [H⁺] = 10^(-pH). Substitute the given pH value to find [H⁺].
Set up the expression for the acid dissociation constant (Ka) for HF: Ka = [H⁺][F⁻]/[HF]. Since HF is a weak acid, assume that the initial concentration of HF is approximately equal to the concentration of HF at equilibrium.
Assume that the concentration of H⁺ ions is equal to the concentration of F⁻ ions at equilibrium, as they are produced in a 1:1 ratio from the dissociation of HF.
Rearrange the Ka expression to solve for the initial concentration of HF: [HF] = [H⁺]²/Ka. Substitute the values for [H⁺] and Ka to find the initial concentration of HF.
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