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Muscle physiologists study the accumulation of lactic acid [CH3CH(OH)COOH] during exercise. Food chemists study its occurrence in sour milk, beer, wine, and fruit. Industrial microbiologists study its formation by various bacterial species from carbohydrates. A biochemist prepares a lactic acid-lactate buffer by mixing 225 mL of 0.85 M lactic acid (Ka = 1.38 × 10−4) with 435 mL of 0.68 M sodium lactate. What is the buffer pH?

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dsdrajlin

Answer:

4.1

Explanation:

First, we will calculate the moles of each species.

  • Lactic acid: 0.225 L × 0.85 mol/L = 0.19 mol
  • Lactate: 0.435 L × 0.68 mol/L = 0.30 mol

The volume of the mixture is 0.225 L + 0.435 L = 0.660 L

The concentration of the species in the buffer are:

  • Lactic acid: 0.19 mol/0.660 L = 0.29 M
  • Lactate: 0.30 mol/0.660 L =0.45 M

We can find the pH of the buffer using the Henderson-Hasselbach equation.

pH = pKa + log [lactate] / [lactic acid]

pH = -log 1.38 × 10⁻⁴ + log 0.45 M / 0.29 M

pH = 4.1

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