Multiple Choice
A 230.0 mL buffer solution initially contains 3.0×10⁻² M of HCHO₂ and 3.0×10⁻² M of NaCHO₂. In order to adjust the buffer pH to 4.20, what mass of NaOH should you add?
17. Acid and Base Equilibrium
Acids Introduction
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In a 0.10 M KF solution, what is the concentration of H₃O⁺ given that the Ka for HF is 6.8 × 10⁻⁴?
A
0.10 M
B
6.8 × 10⁻⁴ M
C
1.0 × 10⁻⁷ M
D
1.3 × 10⁻³ M
Verified step by step guidance1
Identify the relevant chemical equilibrium: HF dissociates in water to form H⁺ and F⁻ ions. The equilibrium expression for this reaction is HF ⇌ H⁺ + F⁻.
Recognize that KF is a salt that dissociates completely in water to give K⁺ and F⁻ ions. Therefore, the initial concentration of F⁻ in the solution is 0.10 M.
Use the expression for the acid dissociation constant (Ka) of HF: \( K_a = \frac{[H^+][F^-]}{[HF]} \). Given that \( K_a = 6.8 \times 10^{-4} \), substitute the known values into this expression.
Assume that the change in concentration of HF is negligible compared to its initial concentration due to the common ion effect. Therefore, the concentration of HF remains approximately constant.
Solve the equilibrium expression for [H⁺] by substituting the known values and simplifying the equation. This will give you the concentration of H₃O⁺ in the solution.
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