C3H6O3(aq) + NaOH(aq) → NaC3H5O3(aq) + H2O(l) 1. A student is studying the reaction between lactic a — Acids and Bases, Titration, Thermodynamics Chemistry Question
Problem Context
C3H6O3(aq) + NaOH(aq) → NaC3H5O3(aq) + H2O(l)
1. A student is studying the reaction between lactic acid, C3H6O3, and sodium hydroxide, NaOH, as represented in the balanced equation above.
(a) The structural formula of lactic acid is shown in the following diagram. Circle the hydrogen atom that most readily participates in the chemical reaction with sodium hydroxide.
Model Answer
The rightmost hydrogen atom should be circled [1].
(b) The student begins the experiment by dissolving 10.22 g of sodium hydroxide (molar mass 40.00 g / mol) in enough water to produce 500. mL of solution. Calculate the molarity of the sodium hydroxide solution.
Model Answer
(10.22 g) × (1 mol / 40.00 g) / 0.500 L = 0.511 M [2].
The student uses the sodium hydroxide solution from part (b), a buret, a pH meter, and a 100 mL Erlenmeyer flask to titrate a 25.0 mL sample of lactic acid solution. The student’s data are shown in the following graph.
(c) Use the information in the graph to determine the approximate pKa of lactic acid. ____________
Model Answer
3.9 (acceptable range: 3.7 – 4.0) [2].
(d) The preceding diagram represents the relative amounts of major species in a sample of the solution in the flask at one point during the titration. (Note that water molecules are omitted.)
(i) Draw an X on the preceding titration curve at a point in the titration where the reaction mixture would be represented by this diagram.
Model Answer
The X should be at a point greater than or equal to 3 mL and less
than 8 mL [2].
(ii) Justify your answer.
Model Answer
More acid particles are present than conjugate base particles,
meaning that the titration is before the half-equivalence point [2].
(iii) The student repeats the experiment but uses a solution of NaOH(aq) with twice the concentration, as shown in the preceding table. On the following graph, draw the titration curve that would be expected for experiment 2.
Model Answer
The equivalence point should be at 8 mL [2]. The drawn curve should
begin at the same pH, gradually increase, rise sharply at a volume different
than 16 mL, and end at a pH similar to the first curve [3].
(e) In a third experiment, the student investigates the enthalpy of the reaction between lactic acid and sodium hydroxide. The student combines 100.0 mL of a 0.500 M lactic acid solution at 20.0°C with 100.0 mL of a 0.500 M NaOH solution at 20.0°C in a calorimeter. The final temperature of the resulting combined solution is 23.2°C. Assume that the density of each solution before combining is 1.00 g / mL and that the specific heat capacity of the combined solution is 4.2 J / (g · °C).
(i) Calculate the quantity of heat produced in the reaction, in J.
Model Answer
q = mcΔT = (200.0 g)(4.2 J/(g ⋅ °C))(23.2°C − 20.0°C) = 2700 J [3].
(ii) Calculate the molar enthalpy of reaction, in kJ / molrxn. Include the sign in your answer.
Model Answer
q_rxn = −q_soln = −2700 J = −2.7 kJ, and ΔH_rxn = q_rxn / mol = −2.7
kJ / ((0.100 L)(0.500 mol/L)) = −54 kJ/mol_rxn [3, 4].
(iii) The student claims that if heat is lost from the calorimeter to the surrounding air during the reaction, then the experimental value of the molar enthalpy of reaction will be smaller in magnitude than the actual value. Do you agree or disagree with the student’s claim? Justify your answer.
Model Answer
Agree. The heat lost from the system would result in a lower final
temperature, which results in values of ΔT, q_soln, and ΔH that are smaller than
the actual value [4].
New conversation: e58311e8-5544-453d-aa46-7d2a30dcb458