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Context for question 2

Answer the following questions about ascorbic acid (vitamin C).

2(A)Formulae, Equations and the MoleMedium3 marks

A student combusts a sample of ascorbic acid,

C_{x} H_{y} O_{z}

, to determine its chemical composition. The only products of the reaction are 0.2400 mol of CO

_{2}

and 2.883 g of H

_{2}

i. Calculate the number of moles of H

_{2}

O produced.

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.

Context for 2(B)

Ascorbic acid, HAsc(aq), acts as a weak acid, as shown in the equation.

HAsc (aq) + H_{2} O (l) ⇌ H_{3} O^{+} (aq) + As c^{-} (aq)

The following titration curve was produced when a 10.0 mL sample of HAsc(aq) was titrated using 0.0550 M NaOH(aq).

Titration curve: pH against the volume of 0.0550 M NaOH(aq) added, in mL, from 0.0 to 20.0 mL

2(B)(i)Solution Concentration and Titration CalculationsEasy1 mark

Calculate the molar concentration of the ascorbic acid solution.

2(B)(ii)pH, Weak Acids, Buffers and Titration CurvesEasy1 mark

From the titration curve, determine the approximate pK

_{a}

of ascorbic acid.

2(B)(iii)pH, Weak Acids, Buffers and Titration CurvesEasy1 mark

What is the value of the ratio [Asc

^{-}

]/[HAsc] when the pH of the solution is 4.7?

2(C)Rate Equations, Orders and Rate ConstantsEasy3 marks

Dehydroascorbic acid (DHAsc) can be produced by reacting ascorbic acid with the triiodide ion, I

_{3}^{-}

, as represented by the following equation.

HAsc + I_{3}^{-} \to DHAsc + 3 I^{-} + 2 H^{+}

The student runs three trials of the reaction with different initial concentrations of HAsc and I

_{3}^{-}

, producing the following data.

Trial	[HAsc] (M)	[I $_{3}^{-}$ ] (M)	Initial Rate of DHAsc Formation (M/s)
1	0.450	1.200	$2.457 \times 1 0^{- 4}$
2	0.450	0.600	$1.229 \times 1 0^{- 4}$
3	0.900	1.200	$4.914 \times 1 0^{- 4}$

i. The rate law for the reaction is rate = k[HAsc][I

_{3}^{-}

]. Explain how the data in the table support the conclusion that the reaction is first order with respect to [HAsc].

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

2(D)Intermolecular Forces, Structure and Properties of SubstancesEasy1 mark

The triiodide ion, I

_{3}^{-}

, is significantly more soluble in water than elemental iodine, I

_{2}

, is.

Identify an intermolecular force between I

_{3}^{-}

and water that is not present between I

_{2}

and water, which could explain the difference in solubility. Lewis diagrams for I

_{2}

and I

_{3}^{-}

are provided.