You are asked to prepare a pH = 3.00 buffer solution starting from 1.25 L of a 1.00 M solution of hydrofluoric acid (HF) and any amount you need of sodium fluoride (NaF). (a) What is the pH of the hydrofluoric acid solution prior to adding sodium fluoride?

검증된 단계별 안내

Identify the chemical species involved: Hydrofluoric acid (HF) is a weak acid, and its dissociation in water can be represented as: \( \text{HF} \rightleftharpoons \text{H}^+ + \text{F}^- \).

Use the acid dissociation constant (\( K_a \)) for HF to set up the equilibrium expression: \( K_a = \frac{[\text{H}^+][\text{F}^-]}{[\text{HF}]} \).

Look up the \( K_a \) value for HF, which is typically around \( 6.8 \times 10^{-4} \).

Assume that the initial concentration of HF is 1.00 M and that the change in concentration due to dissociation is \( x \). Set up the expression: \( K_a = \frac{x^2}{1.00 - x} \).

Solve for \( x \), which represents the \([\text{H}^+]\) concentration, and then calculate the pH using the formula: \( \text{pH} = -\log[\text{H}^+] \).

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주요 개념

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pH and Acid-Base Chemistry

pH is a measure of the hydrogen ion concentration in a solution, indicating its acidity or basicity. A pH of 7 is neutral, below 7 is acidic, and above 7 is basic. Understanding pH is crucial for analyzing solutions, especially in buffer systems where the pH remains relatively stable despite the addition of acids or bases.

Weak Acids and Their Dissociation

Hydrofluoric acid (HF) is a weak acid that partially dissociates in water to produce hydrogen ions (H⁺) and fluoride ions (F⁻). The degree of dissociation is characterized by the acid dissociation constant (Ka), which helps determine the concentration of H⁺ ions in the solution and thus the pH. This concept is essential for calculating the pH of the HF solution before any buffer components are added.

Buffer Solutions

A buffer solution is a system that resists changes in pH upon the addition of small amounts of acids or bases. It typically consists of a weak acid and its conjugate base, which in this case is HF and NaF. Understanding how buffers work is important for preparing solutions with specific pH values, as they maintain stability in pH during chemical reactions or external changes.