Calculate the pH of a solution that is 0.25 M in HF and 0.10 M in NaF.

Verified step by step guidance

Identify that the solution is a buffer solution, consisting of a weak acid (HF) and its conjugate base (F^- from NaF).

Use the Henderson-Hasselbalch equation for buffer solutions: \( \text{pH} = \text{pK}_a + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \), where \([\text{A}^-]\) is the concentration of the conjugate base and \([\text{HA}]\) is the concentration of the weak acid.

Look up or calculate the \(\text{pK}_a\) of HF. The \(\text{K}_a\) of HF is typically around \(6.8 \times 10^{-4}\), so \(\text{pK}_a = -\log(\text{K}_a)\).

Substitute the given concentrations into the Henderson-Hasselbalch equation: \( [\text{A}^-] = 0.10 \text{ M} \) and \( [\text{HA}] = 0.25 \text{ M} \).

Calculate the pH using the equation: \( \text{pH} = \text{pK}_a + \log \left( \frac{0.10}{0.25} \right) \).

Key Concepts

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

Weak Acid and Conjugate Base

HF (hydrofluoric acid) is a weak acid that partially dissociates in solution, while NaF (sodium fluoride) provides the conjugate base F-. The presence of both a weak acid and its conjugate base in solution creates a buffer system, which helps maintain a relatively stable pH when small amounts of acid or base are added.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is a mathematical formula used to calculate the pH of a buffer solution. It is expressed as pH = pKa + log([A-]/[HA]), where pKa is the negative logarithm of the acid dissociation constant (Ka) of the weak acid, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid.

Acid Dissociation Constant (Ka)

The acid dissociation constant (Ka) quantifies the strength of a weak acid in solution. It is defined as the equilibrium constant for the dissociation of the acid into its ions. For HF, the Ka value is essential for determining the pKa, which is needed in the Henderson-Hasselbalch equation to calculate the pH of the buffer solution.