2. Answer the following questions about ascorbic acid (vitamin C). — Stoichiometry, Acid-Base Equilibria, Kinetics, Intermolecular Forces Chemistry Question
Problem Context
- Answer the following questions about ascorbic acid (vitamin C).
A. 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.
i. Calculate the number of moles of H2O produced.
Model Answer
2.883 g H2O × (1 mol H2O / 18.02 g H2O) = 0.1600 mol H2O [1].
ii. The mole ratio of carbon (C) to oxygen (O) is 1:1 in ascorbic acid. Based on this information and your answer to part A (i), determine the empirical formula of ascorbic acid.
Model Answer
0.1600 mol H2O × (2 mol H / 1 mol H2O) = 0.3200 mol H [1]. Examples
of acceptable responses for the empirical formula include setting up a molar
ratio x : y : z = (moles of C):(moles of H):(moles of O), which yields x : y : z
= 0.2400 : 0.3200 : 0.2400 = 3 : 4 : 3 [1]. Therefore, the empirical formula of
ascorbic acid is C3H4O3 [1]. Another acceptable justification notes that 0.3200
mol H / 0.2400 mol C = 4 H / 3 C, and given that the ratio of C:O is 1:1, the
empirical formula of ascorbic acid is C3H4O3 [2].
B. Ascorbic acid, HAsc(aq), acts as a weak acid, as shown in the equation.
HAsc(aq) + H2O(l) ⇌ H3O+(aq) + Asc-(aq)
The following titration curve was produced when a 10.0 mL sample of HAsc(aq) was titrated using 0.0550 M NaOH(aq).
i. Calculate the molar concentration of the ascorbic acid solution.
Model Answer
0.0160 L NaOH × (0.0550 mol NaOH / 1 L NaOH) × (1 mol HAsc / 1 mol
NaOH) = 8.80 × 10^-4 mol HAsc [2]. Then, 8.80 × 10^-4 mol HAsc / 0.0100 L =
0.0880 M HAsc [2].
ii. From the titration curve, determine the approximate pKa of ascorbic acid.
Model Answer
4.1 (acceptable range: 4.0−4.3) [2].
iii. What is the value of the ratio [Asc-]/[HAsc] when the pH of the solution is 4.7?
Model Answer
Using the Henderson-Hasselbalch equation: pH = pKa +
log([Asc-]/[HAsc]) [2, 3]. Substituting the values gives 4.7 = 4.1 +
log([Asc-]/[HAsc]), which simplifies to 0.6 = log([Asc-]/[HAsc]) [3]. Therefore,
[Asc-]/[HAsc] = 10^0.6 = 4.0 [3].
C. Dehydroascorbic acid (DHAsc) can be produced by reacting ascorbic acid with the triiodide ion, I3-, as represented by the following equation.
HAsc + I3- → DHAsc + 3 I- + 2 H+
The student runs three trials of the reaction with different initial concentrations of HAsc and I3-, producing the following data.
i. The rate law for the reaction is rate = k[HAsc][I3-]. Explain how the data in the table support the conclusion that the reaction is first order with respect to HAsc.
Model Answer
(4.914 × 10^-4) / (2.457 × 10^-4) = (0.900 / 0.450)^a, thus a = 1
[3]. An acceptable written explanation is: Comparing trials 1 and 3, the rate
doubles when the concentration of ascorbic acid is doubled and the triiodide ion
concentration is constant, indicating that the process is first order with
respect to ascorbic acid [3].
ii. Calculate the value of the rate constant, k, for the reaction. Include units with your answer.
Model Answer
rate = k[HAsc][I3-] [4]. Using trial 1 data: k = rate /
([HAsc][I3-]) = (2.457 × 10^-4) / ((0.450 M)(1.200 M)) = 4.55 × 10^-4 [4]. The
correct units are M^-1 s^-1 [4].
D. The triiodide ion, I3-, is significantly more soluble in water than elemental iodine, I2, is. Identify an intermolecular force between I3- and water that is not present between I2 and water, which could explain the difference in solubility. Lewis diagrams for I2 and I3- are provided.
Model Answer
Ion-dipole attractions are present between I3- ions and water but not
between I2 molecules and water [4].
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