Unit 8: Acids & Bases
Chemistry · Unit 8 · Paper 3

Acids & Bases unit test

A test on this unit alone, marked as a percentage and a letter grade — for the test your class is actually sitting, rather than for May. Answer everything, then submit once: seeing the answer to question 3 before attempting question 4 makes the final percentage meaningless.

Paper 1Paper 2Paper 3
Each paper is built from this unit’s 20 terms and is the same for everyone, so a teacher can assign “Unit 8, Paper 3” and every student sits the identical test. Multiple choice is marked objectively; the written sections you mark yourself against the model answer and rubric.
Suggested time 37 min 34 points0/17 attempted
1

Henderson-Hasselbalch equation

2

Titration curve of a weak acid

3

Brønsted-Lowry definition

4

Structure and acid strength

5

Strong vs weak acids

6

pH, pOH and their relationship

7

Autoionization of water

8

Buffer capacity

9

Amphoteric species

10

Salt hydrolysis

11

Why pH = pKa at half-equivalence

12

Relationship of Ka and Kb

Short answer 1. Define or explain: Ka and acid strength

3 pts

Short answer 2. Define or explain: Percent ionization

3 pts

Short answer 3. Define or explain: Choosing an indicator

3 pts

Short answer 4. Define or explain: Titration curve features to label

3 pts

Free response

10 pts

Answer the following questions about ascorbic acid (vitamin C). A student combusts a sample of ascorbic acid, CxHyOx, to determine its chemical composition. The only products of the reaction are 0.2400 mol of CO2 and 2.883 g of H2O. The molar mass of H2O is 18.02 g/mol. Ascorbic acid, HAsc(aq), acts as a weak acid: HAsc(aq) + H2O(l) ⇌ H3O+(aq) + Asc−(aq) TITRATION CURVE — a 10.0 mL sample of HAsc(aq) titrated with 0.0550 M NaOH(aq). pH against volume of NaOH added (mL): 0.0 mL → 2.4 2.0 mL → 3.6 4.0 mL → 4.0 6.0 mL → 4.1 8.0 mL → 4.15 10.0 mL → 4.2 12.0 mL → 4.3 14.0 mL → 4.6 16.0 mL → sharp vertical rise through pH 7 to pH 10 18.0 mL → 11.0 20.0 mL → 11.3 Dehydroascorbic acid (DHAsc) can be produced by reacting ascorbic acid with the triiodide ion: HAsc + I3− → DHAsc + 3 I− + H+ Trial [HAsc]i (M) [I3−]i (M) Initial rate of DHAsc formation (M/s) 1 0.450 1.200 2.457 × 10−4 2 0.450 0.600 1.229 × 10−4 3 0.900 1.200 4.914 × 10−4 Lewis diagrams are provided for I2 (two iodine atoms joined by a single bond, each with three lone pairs) and for I3− (a linear ion: a central iodine with three lone pairs joined by single bonds to two terminal iodine atoms, each of which has three lone pairs).

(a)(i) Calculate the number of moles of H2O produced.

(a)(ii) The mole ratio of carbon to oxygen is 1:1 in ascorbic acid. Based on this information and your answer to part (a)(i), determine the empirical formula of ascorbic acid.

(b)(i) Calculate the molar concentration of the ascorbic acid solution.

(b)(ii) From the titration curve, determine the approximate pKa of ascorbic acid.

(b)(iii) What is the value of the ratio [Asc−]/[HAsc] when the pH of the solution is 4.7?

(c)(i) The rate law for the reaction is rate = k[HAsc]^a[I3−]. Explain how the data in the table support the conclusion that the reaction is first order with respect to [HAsc].

(c)(ii) Calculate the value of the rate constant, k, for the reaction. Include units with your answer.

(d) The triiodide ion is significantly more soluble in water than elemental iodine is. Identify an intermolecular force between I3− and water that is not present between I2 and water, which could explain the difference in solubility.