A biochemist needs 750 mL of an acetic acid–sodium acetate buffer with pH 4.50. Solid sodium acetate (CH3COONa) and glacial acetic acid (CH3COOH) are available. Glacial acetic acid is 99% CH3COOH by mass and has a density of 1.05 g/mL. If the buffer is to be 0.15 M in CH3COOH, how many grams of CH3COONa and how many milliliters of glacial acetic acid must be used?

Verified step by step guidance

Calculate the moles of CH3COOH needed using the formula: moles = molarity \times volume. Use the given molarity (0.15 M) and convert the volume from mL to L (750 mL = 0.750 L).

Determine the mass of CH3COOH required using the moles calculated and the molar mass of CH3COOH (60.05 g/mol).

Calculate the volume of glacial acetic acid needed using the mass of CH3COOH and the density of glacial acetic acid (1.05 g/mL). Remember that glacial acetic acid is 99% CH3COOH by mass.

Use the Henderson-Hasselbalch equation to find the ratio of [CH3COONa] to [CH3COOH] needed for the buffer: pH = pKa + log([CH3COONa]/[CH3COOH]). Use the given pH (4.50) and the pKa of acetic acid (4.76).

Calculate the moles of CH3COONa needed using the ratio from the Henderson-Hasselbalch equation and the moles of CH3COOH. Then, convert the moles of CH3COONa to grams using its molar mass (82.03 g/mol).

Key Concepts

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

Buffer Solutions

Buffer solutions are mixtures that resist changes in pH when small amounts of acid or base are added. They typically consist of a weak acid and its conjugate base, which in this case are acetic acid (CH3COOH) and sodium acetate (CH3COONa). Understanding how buffers work is essential for maintaining the desired pH in biochemical reactions.

Molarity and Dilution

Molarity (M) is a measure of concentration defined as the number of moles of solute per liter of solution. In this problem, the biochemist requires a specific molarity of acetic acid in the buffer solution. Calculating the required volume of glacial acetic acid involves using its density and the molarity formula to ensure the correct concentration is achieved.

Stoichiometry

Stoichiometry involves the calculation of reactants and products in chemical reactions based on balanced equations. In this context, it is used to determine the amounts of sodium acetate and acetic acid needed to create the buffer solution at the specified pH and concentration. Understanding stoichiometric relationships is crucial for accurate preparation of chemical solutions.

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A sample of 0.2140 g of an unknown monoprotic acid was dissolved in 25.0 mL of water and titrated with 0.0950 M NaOH. The acid required 30.0 mL of base to reach the equivalence point. (a) What is the molar mass of the acid?

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