You are asked to prepare a pH =4.00 buffer starting from...

You are asked to prepare a pH $=4.00$ buffer starting from 1.50 . of 0.0200$M$ solution of benzoic acid $\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\right)$ and any amount you need of sodium benzoate $\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COONa}\right) .(\mathbf{a})$ What is the pH of the benzoic acid solution prior to adding sodium benzoate? (b) How many grams of sodium benzoate should be added to prepare the buffer? Neglect the small volume change that occurs when the sodium benzoate is added.

Key Concepts

Henderson-Hasselbalch Equation

This is a key equation used to relate the pH of a buffer solution to the pKa of the weak acid and the ratio of the concentrations of its conjugate base and acid. It provides a straightforward method to calculate how much conjugate base is needed to achieve a desired pH in a buffer system.

Buffer Solutions

Buffer solutions are mixtures of a weak acid and its conjugate base that resist changes in pH when small amounts of acid or base are added. Their composition is critical in many chemical and biological systems, as they maintain a relatively constant pH by neutralizing added acids or bases.

Acid Dissociation Constant (pKa)

The pKa is the negative logarithm of the acid dissociation constant (Ka) and indicates the strength of a weak acid. It is essential for buffer calculations because it defines the equilibrium position between the acid and its conjugate base in solution, allowing one to predict and control the buffer's pH.

Stoichiometric Calculations

Stoichiometric calculations involve determining the number of moles of substances present based on their molarity and volume. In the context of buffer preparation, these calculations are used to compute the required amount of conjugate base that must be added to the weak acid solution to achieve the target pH, ensuring accurate buffer composition.

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