DATE

DATE EXPERIMENT 20H NAME

BLOCK BLOCK

pH TITRATION CURVES

PROBLEM: to gain practical experience in the use of a pH meter and to measure changes in pH as a strong acid and a weak acid are titrated against a strong base.

APPARATUS / MATERIALS / PROCEDURE:

Reference: Heath Chemistry Laboratory Experiments Pages 262-265.

CALCULATIONS:

1. Refer to Data Tables I & II.

2. Sample Calculations for HCl/ NaOH Titration:

a. Volume of 0.100 M NaOH added between 0 and 50 mL ( XX mL )

Moles HCl: (0.l00 mol/L) (0.0500 L) = 5.00 X 10^-3 mol HCl

Moles NaOH: (0.100 mol/L) ( XX L) = XX mol NaOH

Initial 0.00500 mol XX mol

HCl + NaOH NaCl + H₂O

Change - XX mol - XX mol

Eqbrm XX mol XX mol

[HCl]: (XX mol) / (XX L) = XX M

[H₃O¹⁺] =

pH =

DATA:

TABLE I: HCl NaOH TITRATION pH DATA

VOLUME HCl pH HAc pH

0.10 M NaOH

(mL) THRTCL EXPTL THRTCL EXPTL

0.0 1.00 2.87

5.0 1.09 3.79

10.0 1.17 4.14

5.0 1.29 4.38

20.0 1.37 4.57

25.0 1.48 4.74

30.0 1.60 4.92

35.0 1.75 5.11

40.0 1.95 5.35

45.0 2.28 5.70

45.5 2.33 5.73

46.0 2.38 5.78

46.5 2.44 5.85

47.0 2.51 5.92

47.5 2.59 6.01

48.0 2.69 6.12

48.5 2.82 6.25

49.0 3.00 6.43

49.1 3.04 6.48

49.2 3.09 6.53

49.3 3.15 6.59

49.4 3.22 6.66

49.5 3.30 6.74

49.6 3.40 6.84

49.7 3.52 6.96

49.8 3.70 7.14

49.9 4.00 7.44

50.0 7.00 8.72

50.1 10.00 10.00

50.2 10.30 10.30

50.3 10.48 10.48

50.4 10.60 10.60

50.5 10.69 10.69

50.6 10.77 10.77

50.7 10.84 10.84

50.8 10.90 10.90

50.9 10.95 10.95

51.0 11.00 11.00

52. 0 11.29 11.29

53.0 11.44 11.44

54.0 11.59 11.59

55.0 11.68 11.68

56.0 11.75 11.75

57.0 11.82 11.82

58.0 11.87 11.87

59.0 11.92 11.92

60.0 11.96 11.96

65.0 12.12 12.12

70.0 12.22 12.22

75.0 12.30 12.30

b. Volume of 0.100 M NaOH added between 50 and 75 mL ( XX mL )

Moles HCl: (0.l00 mol/L) (0.0500 L) = 5.00 X 10^-3 mol HCl

Moles NaOH: (0.100 mol/L) ( XX L) = XX mol NaOH

Initial 0.00500 mol XX mol

HCl + NaOH NaCl + H₂O

Change - XX mol - XX mol

Eqbrm XX mol XX mol

[NaOH]: (XX mol) / (XX L) = XX M

[OH^1-] =

pOH = pH =

3/4. Refer to Graph I and Graph I I.

5. The STOICHIOMETRIC POINTS are:

Graph I: (HCl ) at XX mL NaOH added. Graph I I: (HAc) at XX mL NaOH added.

6. Concentration of the HCl solution:

a. Moles of NaOH solution required to reach the experimental stoichiometric point:

(0.100 mol/L) (XX L) = XX mol NaOH

b. Moles of HCl neutralized at the experimental stoichiometric point:

(1:1 mole ratio) = XX mol HCl

c. Concentration of HCl solution:

XX mol) / (XX L) = XX M

7. Concentration of the HCH₃COO solution:

a. Moles of NaOH solution required to reach the experimental stoichiometric point:

(0.100 mol/L) (XX L) = XX mol NaOH

b. Moles of HCH₃COO neutralized at the experimental stoichiometric point:

(1:1 mole ratio) = XX mol HAc

c. Concentration of HCH₃COO solution:

XX mol) / (XX L) = XX M

DISCUSSION:

1. The pH at the stiochiometric point is:

a. 7.00 for the HCl / NaOH titration because

b. Greater than 7 for the HCH₃COO / NaOH titration because

2. The pH changes so gradually in the HCH₃COO / NaOH titration especially in the volume ranges about half way to the stoichiometric point (between 10 and 45 mL of NaOH added) because.

3. There is more accuracy in locating the stoichiometric point from the titration curve graphs for the because

4. The choice of an indicator with a suitable endpoint wouldl be more difficult for the titration because

5. Refer to the Table of Indicators on Page 343 of "Heath Chemistry Laboratory Experiments ":

a. The names of THREE indicators that could be used for the HCl / NaOH titration but would probably not be useful for the HCH₃COO / NaOH titration are:

b. The names of THREE indicators that could be used for the HCH₃COO / NaOH titration. and would also be useable for the HCl / NaOH titration are:

6. The relationship between the KA of the acid and the relative steepness its titration curve in the region near the stiochiometric point is

7. pH titration curve for 0.100 M HCl added into 50.0 mL of 0.100 M NaOH:

SOURCES OF ERROR:

Instrumental:

Procedural: