Question 14.3: The food industry uses the acetic acid/sodium acetate buffer...
The food industry uses the acetic acid/sodium acetate buffer to control the pH of some prepared foods. When NaOH is added to acetic acid (HAc) to prepare the buffer, the following reaction occurs:
HAc(aq) + OH^-(aq)\longrightarrow Ac^-(aq) + H_2O
a Show by calculation whether a solution made up of 0.300 mol of NaOH and 0.500 mol of HC_2H_3O_2 is a buffer.
ANALYSIS
Information given: mol NaOH = mol OH^- (0.300); mol HAc (0.500)
Information implied: K_a for HAc (Table 13.2)
Asked for: Is the solution a buffer?
STRATEGY
1. Fill in a table like the one shown in the preceding discussion.
2. Recall that for a solution to be a buffer, the solution must have a weak acid and its conjugate base.
b Is a buffer produced when 25.00 mL of 0.100 M NaOH is added to 35.00 mL of 0.125 M HC_2H_3O_2?
ANALYSIS
Information given: NaOH: V (25.00 mL); M (0.100) HAc: V (35.00 mL); M (0.125)
Information implied: K_a for HAc
Asked for: Is the solution a buffer?
STRATEGY
1. Find mol OH^-.
2. Find mol HAc.
3. Make a table as in part (a).
4. Check for the presence of the weak acid (HAc) and its conjugate base (Ac^-) after reaction is complete.
c When 5.00 g of NaOH are added to 150.0 mL of 0.500 M HC_2H_3O_2 (without a volume change), is the resulting solution a buffer?
ANALYSIS
Information given: NaOH: mass (5.00 g) HAc: V (150.0 mL); M (0.500)
Information implied: K_a for HAc
Asked for: Is the solution a buffer?
STRATEGY
1. Find mol OH^-.
2. Find mol HAc.
3. Make a table as in part (a).
4. Check for the presence of the weak acid (HAc) and its conjugate base (Ac^-) after reaction is complete.
| Table 13.2 Equilibrium Constants for Weak Acids and Their Conjugate Bases | ||||
| Acid | K_a | Base | K_b | |
| Sulfurous acid | H_2SO_3 | 1.7 \times 10^{-2} | {HSO_3}^- | 5.9 \times 10^{-13} |
| Hydrogen sulfate ion | {HSO_4}^- | 1.0 \times 10^{-2} | {SO_4}^{2-} | 1.0 \times 10^{-12} |
| Phosphoric acid | H_3PO_4 | 7.1 \times 10^{-3} | {H_2PO_4}^- | 1.4 \times 10^{-12} |
| Hexaaquairon(III) ion | {Fe(H_2O)_6}^{3+} | 6.7 \times 10^{-3} | Fe(H_2O)_5OH^{2+} | 1.5 \times 10^{-12} |
| Hydrofluoric acid | HF | 6.9 \times 10^{-4} | F^- | 1.4 \times 10^{-11} |
| Nitrous acid | HNO_2 | 6.0 \times 10^{-4} | {NO_2}^{-} | 1.7 \times 10^{-11} |
| Formic acid | HCHO_2 | 1.9 \times 10^{-4} | {CHO_2}^- | 5.3 \times 10^{-11} |
| Lactic acid | HC_3H_5O_3 | 1.4 \times 10^{-4} | {C_3H_5O_3}^- | 7.1 \times 10^{-11} |
| Benzoic acid | HC_7H_5O_2 | 6.6 \times 10^{-5} | {C_7H_5O_2}^- | 1.5 \times 10^{-10} |
| Acetic acid | HC_2H_3O_2 | 1.8 \times 10^{-5} | {C_2H_3O_2}^- | 5.6 \times 10^{-10} |
| Hexaaquaaluminum (III) ion |
{Al(H_2O)_6}^{3+} | 1.2 \times 10^{-5} | Al(H_2O)_5OH^{2+} | 8.3 \times 10^{-10} |
| Carbonic acid | H_2CO_3 | 4.4 \times 10^{-7} | {HCO_3}^- | 2.3 \times 10^{-8} |
| Dihydrogen phosphate ion | {H_2PO_4}^- | 6.2 \times 10^{-8} | {HPO_4}^{2-} | 1.6 \times 10^{-7} |
| Hydrogen sulfite ion | {HSO_3}^- | 6.0 \times 10^{-8} | {SO_3}^{2-} | 1.7 \times 10^{-7} |
| Hypochlorous acid | HClO | 2.8 \times 10^{-8} | ClO^- | 3.6 \times 10^{-7} |
| Hydrocyanic acid | HCN | 5.8 \times 10^{-10} | CN^- | 1.7 \times 10^{-5} |
| Ammonium ion | {NH_4}^+ | 5.6 \times 10^{-10} | NH_3 | 1.8 \times 10^{-5} |
| Tetraaquazinc(II) ion | {Zn(H_2O)_4}^{2+} | 3.3 \times 10^{-10} | Zn(H_2O)_3OH^+ | 3.0 \times 10^{-5} |
| Hydrogen carbonate ion | HCO_3^- | 4.7 \times 10^{-11} | {CO_3}^{2-} | 2.1 \times 10^{-4} |
| Hydrogen phosphate ion | HPO_4^{2-} | 4.5 \times 10^{-13} | {PO_4}^{3-} | 2.2 \times 10^{-2} |
| HB(aq) \rightleftharpoons H^+(aq) + B^-(aq) K_a= \frac{[H^+] \times [B^-]}{[HB]} | ||||
| B^-(aq) + H_2O \rightleftharpoons HB(aq) + OH^-(aq) K_b= \frac{[HB] \times [OH^-]}{[B^-]} | ||||
