Polyprotic acids Focus Question: What are the major species in solution during t
ID: 957865 • Letter: P
Question
Polyprotic acids Focus Question: What are the major species in solution during the titration of citric acid with a strong base? Citric acid is a weak organic acid that occurs naturally in citrus fruits and is mainly used as a flavoring and preservative in food and beverages, especially soft drinks. Citric acid is a triprotic acid. Although its formula C_6 H_8 O_7 indicates the presence of 8 hydrogens, only the hydrogens attached to the three carboxylic groups (COOH) are acidic. In this part of the lab, you will calculate 8 data points of a titration of 100 mL of 0.50 M citric acid and 0.50 M NaOH and then draw the graph. Acid dissociation constants of citric acid. Acid dissociation: Start by writing the equations for the three steps of dissociation of citric acid. Make sure you give each species the correct charge. Initial pH: Calculate the initial pH of the 0.50 M citric acid solution. The initial pH is determined by k_a; and the initial concentration of the acid. Midpoints: Determine the pH of the 3 midpoints of the titration. At each midpoint, the pH = pK_a of the species you are titrating. Equivalence points: Calculate the pH of the 3 equivalence points of the titration. At the first and the second equivalence point, the pH is the average of the pK_a values above and below. At the third equivalence point (the end point), the pH is determined by the K_b of the conjugate base of the weakest acid and its concentration. Past the equivalence point: Calculate the pH after 400 mL of NaOH were added. At this point the pH depends only on the concentration of excess NaOH. Titration curve: the titration curve is a pH vs. volume of 0.50 M NaOH graph. Plot the 8 points and connect them to determine the titration curve of citric acid. The curve should be relatively flat around each of the midpoints (buffering region) and should sharply increase around the equivalence points. For each of the 8 points write the major species (other than H_2 O and Na^+) next to the point in the graph.Explanation / Answer
citric acid formula = C6H8O7
1) acid dissociations :
H3C6H5O4 < -------------------> H2C6H5O4- + H+ , Ka1
H2C6H5O4- <-------------------> HC6H5O4-2 + H+ , Ka2
HC6H5O4-2 <---------------------->C6H5O4--3 + H+ , Ka2
2)
H3C6H5O4 < -------------------> H2C6H5O4 - + H+
0.50 0 0 --------------> initial
0.50-x x x ---------------> equilibrium
Ka1 = [H2C6H5O4 ][H+]/[H3C6H5O4 ]
Ka1 = x^2 / 0.50-x
7.5 x 10^-4 = x^2 / 0.50-x
x^2 + 7.5 x 10^-4 - 3.75 x 10^-4 = 0
x = 0.0160
x = [H+] = 0.0160 M
pH = -log [H+] = -log (0.0160)
pH = 1.80
3)
mid points :
first mid point : pH = pKa1
pKa1 = -logKa = -log (7.5 x 10^-4) = 3.12
pH = 3.12
at second midpoint : pH = pKa2 = 4.77
pH = 4.77
at third midpoint : pH = pKa3 = 6.40
pH = 6.40