Question 13.5: Aspirin, a commonly used pain reliever, is a weak organic ac...
Aspirin, a commonly used pain reliever, is a weak organic acid whose molecular formula may be written as HC_9H_7O_4. An aqueous solution of aspirin has total volume 350.0 mL and contains 1.26 g of aspirin. The pH of the solution is found to be 2.60. Calculate K_a for aspirin.
ANALYSIS
Information given: molecular formula for aspirin (HC_9H_7O_4); mass of aspirin (1.26 g); volume of solution (350.0 mL); pH of solution (2.60)
Information implied: molar mass of aspirin; [H^+]
Asked for: K_a
STRATEGY
1. Determine the original concentration, [ ]_o, of aspirin.
2. pH = [H^+]_{eq}
3. Draw a table as illustrated in Example 12.4. Substitute [ ]_o for P_o, Δ[ ] for ΔP, and [\> \> ]_{eq} for P_{eq}.
Since only one H atom ionizes at a time, Δ[ ] for all species is the same.
Recall that [ ] stands for the concentration in molarity.
4. Write the K expression for the ionization and calculate K_a.
