Preparing Buffer Solutions

This section discusses two approaches to prepare buffer solutions in the lab, which is an essential skill for A Level Chemistry students.

Approach 1: Mixing Solutions

The first method involves mixing a solution of a weak acid with a solution containing the salt of the weak acid.

Example: To create an acetate buffer, you might mix a solution of acetic acid (CH3COOH) with a solution of sodium acetate (CH3COONa).

Approach 2: Weak Neutralization

The second method involves adding an aqueous solution of alkali to an excess of a weak acid.

Highlight: In this approach, the weak acid is partially neutralized, resulting in a solution containing both unreacted HA and the A- salt.

Example: To prepare a phosphate buffer, you could add a calculated amount of sodium hydroxide (NaOH) to a solution of phosphoric acid (H3PO4).

Both methods result in a solution containing a weak acid and its conjugate base, which is the essence of a buffer solution.

Vocabulary: Conjugate base - The species formed when an acid donates a proton. In the case of acetic acid (CH3COOH), the conjugate base is the acetate ion (CH3COO-).

Practical Example: Calculating Buffer pH

This section provides a practical example of how to calculate the pH of a buffer solution, which is a common type of question in A Level Chemistry buffer calculations.

Example: Ethanoic acid buffer

Given:

• 125 cm³ of 0.8 mol dm⁻³ ethanoic acid (CH3COOH)
• 75 cm³ of 1 mol dm⁻³ sodium ethanoate (CH3COONa)
• Ka of ethanoic acid = 1.74 × 10⁻⁵ mol dm⁻³

Step 1: Calculate new concentrations after mixing

[CH3COOH] = (0.8 × 125) / 200 = 0.5 mol dm⁻³ [CH3COO⁻] = (1 × 75) / 200 = 0.375 mol dm⁻³

Step 2: Use the equation [H+] = Ka × [HA] / [A-]

[H+] = (1.74 × 10⁻⁵ × 0.5) / 0.375 = 2.32 × 10⁻⁵ mol dm⁻³

Step 3: Calculate pH

pH = -log(2.32 × 10⁻⁵) = 4.63

Highlight: This example demonstrates how to prepare a buffer solution with a specific pH and how to calculate pH of buffer solution given molarity and volume.

Example: If you needed to prepare a pH 4 buffer solution, you would adjust the ratio of acid to conjugate base to achieve this pH.

Understanding these calculations is crucial for mastering buffers in A Level Chemistry OCR and other exam boards.