LAB 2
TITLE Hydrolysis of salt and pH of buffer solutions.
ABSTRACT This experiment has four objective to be achieved, that are to determine d etermine pH values of salts solutions by using different indicators, to calculate the K a or K b for each cation or anion that hydrolyzes, preparing an acetic acid-sodium ac etate buffer and to investigate the effect of acid on the buffer pH. For Part A, unboiled distilled water is added into each of the six test tubes. Then, three drops of different indicators that are methyl orange, methyl red, bromoth ymol blue, phenol red, phenolphthalein and alizarin yellow-R is added into each test tubes. The colours changes are then recorded. Approximate pH of these solutions is determined b y using indicator chart provided in the laboratory. After that, all the test tubes is emptied and rinsed by using boiled distilled water. All these steps are then repeated by b y changing unboiled distilled water with boiled distilled water, 0.1 M NaCl, 0.1 M NH4Cl, 0.1 M NaC2H3O2, 0.1 M ZnCl2, 0.1 M KAI(SO4)2, and 0.1 M Na2CO3. Then, Part B is the preparation of acetic acid-sodium acetate buffer and to determine the effect of acid and base on buffer pH. The pH of buffer solution prepared is 5.598 but after the addition of acid, the pH decreases to 5.141 and after the addition of base, the pH increases to 6.698.
OBJECTIVES 1. 2. 3. 4.
To determine pH values of salts solutions by using different indicators. To calculate the K a or K b for each cation or anion that hydrolyzes. To prepare acetic acid-sodium acetate buffer. To investigate the effect of acid and base on the buffer pH.
APPARATUS 500 mL Erlenmeyer flask, 150 mL beaker, 10 mL measuring cylinder, 100 mL measuring cylinder, 6 sets of test tubes, test tubes rack, stirring rod, pH meter, analytical balance, pipette, hot plate, plastic water bottle
CHEMICALS NaC2H3O2.3H2O, 0.1 M ZaCl2, 0.1 M NH4Cl, 0.1 M KAl (SO4)2, 0.1 M Na2CO3, 0.1 M NaCl, 0.1 M NaC2H3O2, 3.0 M HCl, 3.0 M HC2H3O2, 3.0 M NaOH Dropper bottles of : Methyl orange, Methyl red, Bromothymol blue, Phenolphthalein, Alizarin yellow-R, Phenol red, Standard buffer solution (pH 4.5)
PROCEDURE Initial step (prepare by lab technician) Boil 1L distilled water and allow it to cool to room temperature before starting the experiment.
Part A : pH solution and hydrolysis of salts 1. 5mL of unboiled distilled water is added to e ach of six test tubes. 2. Three drops of different indicators is added into each six test tubes. The indicators to use are: methyl orange, methyl red, bromothymol blue, phenol red, phenolphthalein, and alizarin yellow-R. 3. The colours obtained are recorded. 4. The approximate pH of unboiled distilled water samples to the nearest pH is determined by using provided indicator chart. 5. The test tubes is emptied and is rinsed three times with 3mL boiled distilled water. 6. Step 1 until step 5 is repeated by changing unboiled distilled water with boiled distilled water, 0.1 M NaCl, 0.1 M NH4Cl, 0.1 M NaC2H3O2, 0.1 M ZnCl2, 0.1 M KAl(SO4)2, and 0.1 M Na2CO3.
Part B : pH of buffer solution i.
Preparation of acetic acid-sodium acetate buffer 1. 3.50 g of NaC2H3O2.3H2O is weighed. 2. The mass is recorded and is put into a 150 mL beaker. The beaker is labelled as beaker 1. 3. 8.8 mL of 3 M acetic acid is measured and is added to beaker 1. 4. 55.6 mL of distilled water is then measured and also added to beaker 1. 5. The solution is stirred until all the sodium acetate is dissolved. 6. The pH of the solutions is measured using c alibrated pH meter. 7. The solutions are saved for Part (ii).
ii.
Effect of acid and base on buffer pH 1. 32 mL of prepared buffer solution is poured into two 150 mL beakers. The beakers are then labelled as beaker 2 and beaker 3. 2. 1 mL of 3.0 M HCl is pipette into beaker 2 and the pH of the resultant solution is measured. 3. 1 mL of 3.0 M NaOH is pipette into beaker 3 and the pH of the resultant solution is then measured.
RESULTS AND DISCUSSIONS Part A : pH solution and hydrolysis of salts Solution: unboiled distilled water INDICATOR pH COLOUR CHANGES
Methyl Orange 4.0 Orange
Methyl Red 5.4 Pale Pink
Bromothymol Blue 2.0 Pale Yellow
Phenol Red 7.0 Yellowish Orange
Phenolphthalein 8.0 Colourless
Alizarin Yellow-R 10.0 Yellow
Solution: boiled distilled water INDICATOR pH COLOUR CHANGES
Methyl Orange 4.0 Orange
Methyl Red 5.4 Light Pink
Bromothymol Blue 3.0 Very Light Yellow
Phenol Red 7.7 Yellow
Phenolphthalein
Methyl Red 5.4 Pale Pink
Bromothymol Blue 1.0 Very Light Yellow
Phenol Red 7.0 Light Orange
Phenolphthalein
7.0 Colourless
Alizarin Yellow-R 10.0 Yellow
Solution: 0.1 M NaCl INDICATOR pH COLOUR CHANGES
Methyl Orange 4.0 Orange
7.0 Colourless
Alizarin Yellow-R 9.0 Light Yellow
Solution: 0.1 M NH4Cl INDICATOR pH COLOUR CHANGES
Methyl Orange 4.0 Orange
Methyl Red 5.8 Pink
Bromothymol Blue 2.0 Very Light Yellow
Phenol Red 7.0 Yellow
Phenolphthalein
Methyl Red 6.5 Light Yellow
Bromothymol Blue 7.3 Light Blue
Phenol Red 8.4 Pink
Phenolphthalein
Methyl Red 5.8 Pale Yellow
Bromothymol Blue 5.8 Pale Green
Phenol Red 7.0 Orange
Phenolphthalein
Methyl Red 5.5 Very Light Pink
Bromothymol Blue 4.5 Very Light Yellow
Phenol Red 6.0 Light Yellow
Phenolphthalein
7.0 Colourless
Alizarin Yellow-R 10.0 Yellow
Solution: 0.1 M NaC2H3O2 INDICATOR pH COLOUR CHANGES
Methyl Orange 5.0 Yellow
8.0 Colourless
Alizarin Yellow-R 11.0 Orange
Solution: 0.1 M ZnCl2 INDICATOR pH COLOUR CHANGES
Methyl Orange 4.3 Pale Orange
8.0 Pale Pink
Alizarin Yellow-R 10.3 Slightly Orange
Solution: 0.1 M KAl(SO4)2 INDICATOR pH COLOUR CHANGES
Methyl Orange 3.6 BrickRed
8.0 Colourless
Alizarin Yellow-R 10.0 Light Yellow
Solution: 0.1 M Na2CO3 INDICATOR pH COLOUR CHANGES
Methyl Orange 4.0 Orange
Methyl Red 6.8 YellowOrange
Bromothymol Blue 7.3 Blue
Phenol Red 8.0 Pink
Phenolphthalein 9.3 Dark Pink
Part B : pH of buffer solution (i)
Preparation of acetic acid-sodium acetate buffer
Weight of NaC2H3O2.3H2O Volume of 3.0 M acetic acid Volume of distilled water pH of the buffer solution
(ii)
3.5014 g 8.8 mL 55.6 mL 5.598
Effect of acid and base on buffer pH
pH of the solutions in beaker 1 pH of the solutions in beaker 2 (addition of 1mL of 3.0 M HCl) pH of the solutions in beaker 3 (addition of 1mL of 3.0 M NaOH)
5.598 5.141 6.698
Alizarin Yellow-R 11.0 Reddish Orange
Formula: Average pH =
+
K w = K a K b K a = K b =
-
-
K W = [H ] [OH ] = 1.0 x 10 +
pH = -log [H ]
-
[OH ] =
CALCULATIONS Unboiled distilled water Average pH
Average pH =
=
+
Concentration of [H ]
= 6.07
+
pH = -log [H ] + 6.07 = -log [H ] + -7 [H ] = 8.51 x 10 -
Concentration of [OH ]
-
[OH ] = =
Net ionic equation
K a
-8
= 1.18 x 10
+
-
H2O (l ) → H (aq) + OH (aq) CO2 (aq) + H2O (aq) → H2CO3 (aq) + H2CO3 (aq) → H (aq) + HCO3 (aq) -
+
K a
= [HCO3 ] [H ] [H2CO3]
K b
= 1.0 x 10 -7 1.2 x 10
K b
-14
-
= 4.2 x 10
-8
= 2.38 x 10
Boiled distilled water Average pH
Average pH =
=
=6.18
+
Concentration of [H ]
+
pH = -log [H ] + 6.18 = -log [H ] + -7 [H ] = 6.61 x 10 -
Concentration of [OH ]
-
[OH ] = =
+
-8
= 1.52 x 10 -
H2O (l ) → H (aq) + OH (aq)
Net ionic equation K a
+
-
-7
-8
K a = [H ] [OH ] = (6.61 x 10 ) (1.52 x 10 ) -14 = 1.0 x 10 = K w -
K b = 1.0 x 10 = 1.00 x 10 -14 1.0 x 10
K b
0.1M NaCl Average pH
Average pH =
=
+
Concentration of [H ]
=5.57
+
pH = -log [H ] + 5.57 = -log [H ] + -6 [H ] = 2.69 x 10 -
Concentration of [OH ]
-
[OH ] = =
-9
= 3.72 x 10
+
-
NaCl (l ) → Na (aq) + Cl (aq) + H2O (l ) → H (aq) + OH (aq) CO2 (aq) + H2O (aq) → H2CO3 (aq) + H2CO3 (aq) → H (aq) + HCO3 (aq)
Net ionic equation
-
K a K b
0.1M NH4Cl Average pH
Average pH =
=
=5.97
+
Concentration of [H ]
+
pH = -log [H ] + 5.97 = -log [H ] + -6 [H ] = 1.07 x 10 -
Concentration of [OH ]
-
[OH ] = =
Net ionic equation
+
-9
= 9.34 x 10 -
NH4Cl (l ) → NH4 (aq) + Cl (aq) +
+
NH4 (aq) + H2O (l ) ⇌ NH3(aq) + H3O (aq) +
-
K a
K a
= [NH3][H ] = 5.6 x 10 + [NH4 ]
K b
K b
= 1.0 x 10 = 1.79 x 10 -10 5.6x 10
-
-
0.1 M NaC2H3O2 Average pH
Average pH =
=
=
7.7
+
Concentration of [H ]
+
pH = -log [H ] + 7.7 = -log [H ] + -8 [H ] = 2.00 x 10 -
Concentration of [OH ]
-
[OH ] = =
-7
= 5 x 10 +
-
NaC2H3O2 (aq) → Na (aq) + C2H3O2 (aq)
Net ionic equation
-
-
C2H3O2 (aq) + H2O (l) ⇌ C2H4O2 (aq) + OH (aq) -
K a = 1.0 x 10 = 4.0 x 10 -19 2.5 x 10
K a
-
K b = [C2H4O2] [OH ] = 2.5 x 10[C2H3O2
K b
0.1 M ZnCl2 Average pH
Average pH =
=
=6.87
+
Concentration of [H ]
+
pH = -log [H ] + 6.87 = -log [H ] + -7 [H ] = 1.35 x 10 -
Concentration of [OH ]
-
[OH ] = =
Net ionic equation K a
-8
= 7.41 x 10
Zn (aq) + H2O (l ) Zn(OH)2 (aq) + H (aq) +
K a = [Zn(OH)2] [H ] = 2.5 x 10 2+ [Zn ]
-
-
-
K b = 1.0 x 10 = 4.0 x 10 -7 2.5 x 10
K b
0.1 M KAl(SO4)2 Average pH
Average pH =
=
=6.23
+
Concentration of [H ]
+
pH = -log [H ] + 6.23 = -log [H ] + -7 [H ] = 5.88 x 10 -
Concentration of [OH ]
-
[OH ] = =
-8
= 1.7 x 10
+
+
KAI (aq) + H2O (l ) KAI(OH)4 (aq) + H (aq)
Net ionic equation
+
= [KAI(OH) 4] [H ] = 6.3 x 104+ [KAI ]
K a
K a
K b
K b = 1.0 x 10 -7 6.3 x 10
-
= 1.59 x 10-
0.1 M Na2CO3 Average pH
Average pH =
= +
Concentration of [H ]
+
pH = -log [H ] + 7.73 = -log [H ] + -8 [H ] = 1.86 x 10
=7.73
-
Concentration of [OH ]
-
[OH ] = =
Net ionic equation
-7
= 5.38 x 10
+
-
Na2CO3 (l ) → 2Na (aq) + CO3 (aq) 2-
-
-
CO3 (aq) + H2O (l ) ⇌ HCO3 (aq) + OH (aq) -
-
K a
K a = 1.0 x 10 -4 2.1 x 10
K b
K b = [HCO3 ] [OH ] 2[CO3 ]
-
= 4.76 x 10
-
-
= 2.1 x 10
DICUSSION For the part A, the experiment is done by adding 5ml of unboiled water to six test tubes.Three drops of six different indicator (methyl orange, methyl red, bromothymol blu e, phenol red, phenolphthalein and alizarin yellow-R) is then added into each of the test tubes.From the changes of colour of the indicator, the pH value can be determine by referring the colour change to the indicator chart.The experiment is repeated by changing the unboiled water with boiled water, 0.1M NaCl, 0.1M NH4Cl, 0.1M NaC2H3O2, 0.1M ZnCl2, 0.1M KA(SO4)2, and Na2CO3. For unboiled distilled water, the pH that we get is 6.07. The concentration of H+ and OH- is 8.51 -7 -8. x 10 and 1.18 x 10 The net equation for this hydrolysis is, H2O (l )
+
-
H (aq) + OH (aq)
CO2 (aq) + H2O (aq) → H2CO3 (aq) +
-
H2CO3 (aq) → H (aq) + HCO3 (aq) The expression for equilibrium constant is, -
+
K a = [HCO3 ] [H ] [H2CO3] -7
-8
And the value of K a and K b that we get is 4.2 x 10 and 2.38 x 10 .
For boiled distilled water, the pH that we get is 6.18. The concentration of H+ and OH- is 6.61 -7 -8 x 10 and 1.52 x 10 The net equation for this hydrolysis is, +
-
H2O (l ) → H (aq) + OH (aq) The expression for equilibrium constant is, +
-
K a = [H ] [OH ] -14
0
And the value of K a and K b that we get is 1.0 x 10 and 1.00 x 10 . -6
For 0.1 M NaCl, the pH that we get is 5.57. The concentration of H+ and OH- is 2.69 x 10 and -9 3.72 x 10 The net equation for this hydrolysis is, +
-
H2O (l ) → H (aq) + OH (aq) CO2 (aq) + H2O (aq) → H2CO3 (aq)
+
-
H2CO3 (aq) → H (aq) + HCO3 (aq) There is no Ka or Kb for this hydrolysis.
-6
For 0.1 M NH4Cl, the pH that we get is 5.97. The concentration of H+ and OH- is 1.07 x 10 -9 and 9.34 x 10 The net equation for this hydrolysis is, +
-
NH4Cl (l ) → NH4 (aq) + Cl (aq) +
+
NH4 (aq) + H2O (l ) ⇌ NH3(aq) + H3O (aq) The expression for equilibrium constant is, +
K a = [NH3] [H ] + [NH4 ] -10
-5
And the value of K a and K b that we get is 5.6 x 10 and 1.79 x 10 .
-8
For 0.1 M Na2C2H3O2, the pH that we get is 7.7. The concentration of H+ and OH- is 2 x 10 -7 and 5x 10 . The net equation for this hydrolysis is, +
-
NaC2H3O2 (aq) → Na (aq) + C2H3O2 (aq) -
-
C2H3O2 (aq) + H2O (l) ⇌ C2H4O2 (aq) + OH (aq) The expression for equilibrium constant is, -
K b = [C2H4O2] [OH ] -
[C2H3O2 ] 4
-19
And the value of K a and K b that we get is 4.0 x 10 and 2.5 x 10
-7
For 0.1 M ZnCl2, the pH that we get is 6.87. The concentration of H+ and OH- is 1.35 x 10 and -8 7.41 x 10 The net equation for this hydrolysis is, 2+
+
Zn (aq) + H2O (l ) Zn(OH)2 (aq) + H (aq)
The expression for equilibrium constant is, +
K a = [Zn(OH)2] [H ] 2+
[Zn ] -7
-8
And the value of K a and K b that we get is 2.5 x 10 and 4.0 x 10 .
-
For 0.1 M KAI (SO4)2, the pH that we get is 6.23. The concentration of H+ and OH- is 5.88 x 10 7 -8 and 1.7 x 10 . The net equation for this hydrolysis is, 4+
+
KAI (aq) + H2O (l ) KAI(OH)4 (aq) + H (aq) The expression for equilibrium constant is, +
K a = [KAI(OH)4] [H ] 4+
[KAI ] -7
-8
And the value of K a and K b that we get is 6.3 x 10 and 1.59 x 10
-8
For 0.1 M Na2CO3, the pH that we get is 7.73. The concentration of H+ and OH- is 1.86 x 10 -7 and 5.38 x 10 . The net equation for this hydrolysis is, +
2-
Na2CO3 (l ) → 2Na (aq) + CO3 (aq) 2-
-
-
CO3 (aq) + H2O (l ) ⇌ HCO3 (aq) + OH (aq) The expression for equilibrium constant is, -
-
K b = [HCO3 ] [OH ] 2-
[CO3 ] -11
-4
And the value of K a and K b that we get is 4.76 x 10 and 2.1 x 10 .
For experiment in part B, acetic acid is added to the NaC2H3O2.3H2O .The pH that we get after measuring the pH of the solution using a calibrated pH meter is 5.598.
After the addition of 3.0 M HCl, the pH increase to 5.141. This pH value can be said valid because the number of pH is decrease as the solution is becoming more acidic. However. after the addition of 3.0 M NaOH, the pH is increasing to 6.698. This pH value is valid because the number of pH is increase as the solution becomes more alkaline.
CONCLUSION From the experiment, we get the pH value of unboiled distilled water 6.07, boiled distilled water 6.18, 0.1 M NaCl 5.57, 0.1 M NH4Cl 5.94, 0.1 M NaC2H3O2 7.7, 0.1 M ZnCl2 6.87, 0.1 M KAI (SO4)2 6.23, 0.1 M Na2CO3 7.73. For salts derived from a weak acid and a weak base, both the cation and anion hydrolyze. However, whether a solution containing such a salt is acidic, basic or neutral depends on the relative strengths of the weak acid and weak base. If K a is more than K b, then the solution is acidic solution and vice versa. A buffer solution resists large changes in pH upon the addition of small amounts of strong acid or strong base. The pH value will decrease when buffer solution react with acid and will increase when react with base. But in this experiment, some errors had occurred that make the pH value after the addition of acid increase to 4.948 and not decrease. All the objectives of this experiment which are to determine pH values of salts solutions by using different indicators, to calculate the K a or K b for each cation or anion that hydrolyzes, preparing acetic acid-sodium acetate buffer and to investigate the effect of acid on the buffer pH has been achieved.
REFERENCES
th
Chang R. (2008). Chemistry (9 Ed) New York, McGraw-Hill Companies, Inc. (Page 660-607)
Carnegie Mellon University (2001), Handout On Buffer Solutions, 30 July 2010 from http://www.chem.cmu.edu/courses/09-106/notes/buffers.pdf
Jim clark (2002), Buffer Solutions, 31 July 2010 from http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html
Dhanlal De Lloyd (2000), Preparation of pH Buffer Solutions, 31 July 2010 from http://delloyd.50megs.com/moreinfo/buffers2.html
Brad Cole (2010), What Is Buffer Solutions?, 28 July 2010 from http://www.wisegeek.com/what-is-a-buffer-solution.htm
