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tatuchka [14]
3 years ago
12

Muscle physiologists study the accumulation of lactic acid [CH3CH(OH)COOH] during exercise. Food chemists study its occurrence i

n 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?
Chemistry
1 answer:
Alexus [3.1K]3 years ago
3 0

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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<h3>Explanation</h3>

Note that the group number in this answer refers to the new IUPAC group number, which ranges from 1 to 18. Counts from the left. Start with the first two column (group 1 and 2), go on to the transition elements (Sc, Ti, etc. in group 3 through 12), and continue with the nonmetals (group 13 through 18).  

Li is a group 1 metal. As a metal, it tends to form positive ions ("cations"). Metals in group 1 and 2 are <em>main group</em> metals. The charge on main group metal ions tends to be the same as the group number of the metal. Li is in group 1. The charge on an Li ion will be +1. Formula of the Li ion will be \text{Li}^{+}.

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All the ions in an ionic compound carry charge. However, some of the ions like \text{Li}^+ are positive. Others ions like \text{Br}^{-} are negative. Charge on the two types of ions balance each other. As a result, the compound is <em>overall</em> neutral.

1 × (+1) + 1 × (-1) = 0. The positive charge on one \text{Li}^{+} ion balances the negative charge on one \text{Br}^{-} ion. The two ions would pair up at a 1:1 ratio.

The empirical formula for an ionic compound shows all the ions in the compound. Positive ions are written in front of negative ions. \text{Li}^{+} is positive and \text{Br}^{-} is negative. The formula shall also show the simplest ratio between the ions. For the compound between Li and Br, a 1:1 ratio will be the simplest. The "1" subscript in an empirical formula can be omitted. Hence the formula: LiBr.

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