Question 14.1: Lactic acid, C3H6O3, is a weak organic acid present in both ...
Lactic acid, C_{3}H_{6}O_{3}, is a weak organic acid present in both sour milk and buttermilk. It is also a product of carbohydrate metabolism and is found in the blood after vigorous muscular activity (Figure 14.3). A buffer is prepared by dissolving lactic acid, HLac (K_{a} = 1.4 \times 10^{-4}), and sodium lactate, NaC_{3}H_{5}O_{3}, NaLac.
a Calculate [H^{+}] and the pH of the buffer made up of 1.00 mol of sodium lactate and 1.00 mol of lactic acid in enough water to form 550.0 mL of solution.
ANALYSIS
Information given: K_{a}\> HLac\> (1.4 \times 10^{-4});n_{HLac} (1.00\> mol); n_{Lac^{-}} (1.00\> mol) volume of solution (550.0 mL)
Asked for: [H^{+}] and pH of the buffer
STRATEGY
1. Substitute into Equation 14.2 to obtain [H^{+}].
[H^+] = K_{a} \times \frac{n_{HB}}{n_{B^{-}}}
2. Find pH: pH = -\log_{10}[H^{+}]
b Calculate [H^{+}] and the pH of the buffer made up of 34.6 g of NaLac dissolved in 550.0 mL of a 1.20 M aqueous solution of HLac. (Assume no volume change after addition of NaLac.)
ANALYSIS
Information given: K_{a}\> HLac\> (1.4 \times 10^{-4}); mass of NaLac (34.6 g); M HLac (1.20) volume of solution (550.0 mL)
Information implied: MM NaLac
Asked for: [H^{+}] and pH of the buffer
STRATEGY
1. Find n_{HLac} in solution: n = V \times M
2. Find n_{Lac^{-}} in solution. Recall 1 mol {Lac^{-}}/1 mol NaLac. Thus
n_{NaLac} = n_{Lac^{-}} = mass\> NaLac/MM\> NaLac
3. Substitute into Equation 14.2 to find [H^{+}].
4. pH = -\log_{10}[H^{+}]
