Work in groups on these problems. You should try to answer the questions without referring to your textbook. If you get stuck, try asking another group for help.
Use a penny stacked on top of a poker chip to represent a monoprotic weak acid molecule. Let the penny be the ionizable hydrogen ion and the quarter be the conjugate base. Ignore water molecules in this exercise.
- Starting from ten de-ionized molecules, develop a model of a solution that is 10% ionized. Is the equilibrium static or dynamic? Explain.
- Model a weak acid solution that is 40% ionized.
- Model a strong acid solution.
- Determine the value of the acid equilibrium constant for each case.
2. Consider the particulate-level composition of the weak acid above at the moment of addition to water before molecular dissociation.
a) What is the ratio of weak acid molecules to water molecules in a 0.10 M solution? Determine the answer mathematically.
b) Draw a proportionally-correct sketch of a representative sample of the solution, using open spheres to represent water molecules and filled spheres to represent weak acid molecules. (Hints: Water has a density of 1.0 g/mL and a molar mass of 18 g/mol. What is the molar concentration of water?)
c) How does the autoionization of water affect the hydrogen ion concentration of a weak acid solution? (Hints: Assume a typical weak acid has a Ka of about 10–5. What is the K for water?)
d) What is the pH of HCl solutions of the following concentrations: 10–3 M, 10–6 M, 10–10 M? How does the autoionization of water affect the hydrogen ion concentration of a strong acid solution?
3. The following table includes some common acids and bases. Complete the table, using your textbook as a reference.
|Name||Aqueous Equilibrium Equation (Ionization)||Strong or Weak||Acid or Base||Ka or Kb||Expression|
|Butanoic acid (C3H7COOH)|
4. Consider an acetic acid solution at equilibrium
\[ H_2O \; (l) + CH_3CO_2H \; (aq) \rightleftharpoons H_3O^+ \;(aq) +CH_3CO_2^- \; (aq) \]
There are three quantities commonly measured and/or known for a weak acid solution:
- the value of the equilibrium constant from reference sources,
- the solution pH from experimental measurement, and
- the initial acid concentration, also from experimentation.
It is possible that one, two, or all three of these quantities may be known in any given situation, yielding seven possible combinations.
Complete the following table by checking off with an x which of the missing values including those in the equilibrium constant expression can be calculated in each case? (Be able to provide the algebra to justify each case.)
5. Without making any simplifying assumptions, determine the [H3O+] and pH of a 0.0010 M weak acid solution with Ka = 9.5 x 10–5.
6. Determine the [H3O+] and pH of the weak acid solution from Question 5, making the assumption that the equilibrium concentration of the acid is the same as the initial concentration. What is the percent difference from your answer in #5 when using this simplifying assumption? How accurate does a pH calculation need to be? Consider a real-life situation in which pH is an important factor, such as the 7.35-7.45 normal pH range of human blood.
7. Determine the percentage dissociation of each of the following weak acid solutions. What is the relationship between concentration and percentage dissociation for a given weak acid?
a) A 1.0 M solution of a weak acid with Ka = 5.0 x 10–5
b) A 0.10 M solution of a weak acid with Ka = 5.0 x 10–5
c) A 0.0010 M solution of a weak acid with Ka = 5.0 x 10–5
8. Draw a series of illustrations that show what occurs at the particulate level (a) before solid sodium acetate and water are combined, (b) just after the sodium acetate is added to the water, and (c) after the solution comes to equilibrium.
9. Write the equilibrium reaction and outline the respective mathematical process that you would use use to find the pH of each of the following aqueous solutions.
- 0.10 M sodium hydroxide
- 0.10 M sodium acetate (Kb is known)
- 0.10 M ammonium chloride (Ka is known)
- 0.10 M hydrochloric acid