Question

1. Use the data given to calculate the following parts:

Suggested Setup for Lab Notebook (All lab data goes in the notebook; do not record lab data here) Molarity of NaOH (determined in Experiment 8) 0.2202 PART A: Titration of Phosphoric Acid TRIAL 1 TRIAL 2 Volume of phosphoric acid used 15.00 mL 15.00 mL Volume at equivalence point NaOH 6.87 mL 13.73 mL 6.80 mL 13.60 mL pH at half the 1s equivalence point 1.72 1.84 pH at the point halfway between the 19 and 2nd equivalence points 7.41 7.16 Average value of pka1 Average value of pKaz PART B: Titration of Unknown Acid Code of Acid B TRIAL 1 TRIAL 2 Mass of acid used 0.1415 g 0.1631 a Volume at equivalence points NaOH 5.13 mL 10.25 mL 6.63 mL 13.25 ml pH at half the 1st equivalence point 1.71 1.76 pH at the point halfway between the 1st and 2nd equivalence points 6.44 6.25 Average value of pka Average value of pKa2 7

a. Using your data for the value for VEP1, calculate the molarity of H3PO4 at the 1st equivalence point, EP1. Show work clearly, with units and correct significant figures.

b. Using your data for the value for VEP2, calculate the molarity of H3PO4 at the 2nd equivalence point, EP2. Show work clearly, with units and correct significant figures.

c. Calculate the percent error between these two molarity values, % error = 100 x |value1 – value2| / (average of value 1 and value 2). Show work clearly with correct significant figures.

d. Tabulate your average values of pKa1 and pKa2, and the accepted values. Calculate the percent error of pKa1 and pKa2. Show work clearly with correct significant figures.

e. Your unknown acid is oxalic acid (H2C2O4*2 H2O, pKa1 = 1.27, pKa2 = 4.27), or citraconic acid (C5H6O4, pKa1 = 2.29. pKa2 = 6.15), or maleic acid (C4H4O4, pKa1 = 1.92, pKa2 = 6.09). ——— These three acids are diprotic.

f. Calculate the molar mass of your unknown acid two different ways:
i. Use the average mass from trials 1 and 2, and the average volume of the first equivalence point
ii. Use the average mass from trials 1 and 2, and the average volume of the second equivalence point. Show work clearly, with units and correct significant figures.
iii. Using the average mass at the 2nd equivalence point, and your average values of pKa1 and pKa2, identify your unknown acid. Explain your answer.

Answer 1

a) The molarity of the acid is calculated, with respect to its first equivalence point:

Ca = Cb * Vb / Va = 0.2202 M * 6.87 mL / 15 mL = 0.1009 M

Trial 2: 0.0998 M

b) The molarity of the acid is calculated with respect to its second equivalence point:

Ca = 0.2202 M * 13.73 mL / 15 mL * 2 = 0.1008 M

Trial 2: 0.0998 M

c) The error is calculated:

% error = 100 * (0.1009 - 0.1008) / (0.1009 + 0.1008) / 2 = 0.1%

Trial 2: 0%

d) The pKa errors are calculated:

% error pKa1 = (1.78 - 2.12) * 100 / 2.12 = 16%

% error pKa2 = (7.21 - 7.29) * 100 / 7.21 = 1.1%