ABSTRACT
The knowledge about acid and base in one of the fundamental branch in chemistry. According to the Brønsted-Lowry the Brønsted-Lowry theory definition, acid is a substance capable of donating a proton while base is a substance that capable of accepting proton. Every Br ǿnsted acid has a conjugate base and every Br ǿnsted base have a conjugate acid. In this experiment, students will be introduced to the concept of hydrolysis, acid-base indicator and the behaviour of buffer solutions. The epxriment is divided into two parts. The first part (part A) is about pH solution and hydrolysis of salt and part B is about the pH of buffer solution. In part A, students will start with unboiled water and place the water in 5 separate test tubes and a few drops of pH indicator will be put into the test tubes. The colour change chan ge of each indicator then will be matched with the pH scale that will determine the pH of unboiled water. The Th e experiment will be repeated with different substance consisting of acid and base or o r as known as salt solution. For part B, students will prepare acetic acid-sodium acetate buffer. The solution is poured into two different beakers and will be pipeted with HCl and NaOH with given concentration and volume. The pH reading will be taken after the procedure is done. OBJECTIVE
After this experiment, students should be able to: 1. 2. 3. 4.
Determine pH values of salts solutions by using different indicators Calculate the k a and k b of each cation or anion that hydrolyzes Prepare acetic acid-sodium acetate buffer Investigate the effect of acid and base on the buffer pH
MATERIAL AND APPARATUS
Apparatus 500 ml Erlenmeyer flask, 150 ml beaker, 10 ml measuring cylinder ,100 ml measuring me asuring cylinder, test tube, test tube rack, stirring rod, pH meter, ana lytical balance, pipette, hot plate, plastic water bottle Chemicals NaC2H3O2.3H2O, 0.1M ZnCl2, 0.1M NH4Cl, 0.1 M KAL(SO4)2 , 0.1M Na2CO3, 0.1M NaCl, 0.1M NaC2H3O2, 3.0M HCl, 3.0M HC2H3O2 , 3.0M NaOH, Methyl orange, Methyl red, Bromothymol blue, Phenolphthalein, Alizarin yellow-R
PROCEDURE
Initial step ( prepared by lab technician) 1L of distilled water is boiled and allowed to be cool to room temperature before the experiment is started. Part A: pH solution and hydrolysis salt 1. 5 ml of unboiled distilled water is added into each separate five test tubes 2. Three drops of different indicators is added into each test tube. The indicator used are Methyl orange, Methyl red, Bomothymol blue, Phenolphthalein, Alizarin yellow-R. 3. The colours obtained are recorded. 4. The approximate pH of unboiled distilled water sample is determined to the nearest pH by using the provided indicator chart. 5. The test tubes are emptied and rinsed three times with 3 ml of boiled distilled water 6. 1-5 is repeated by replacing the unboiled distilled water with boiled distilled water, 0.1M ZnCl2, 0.1M NH4Cl, 0.1 M KAL(SO4)2, 0.1M Na2CO3, 0.1M NaCl, 0.1M NaC2H3O2. Part B : pH of buffer solution i) Preparation of acetic acid-sodium acetate buffer 1. 2. 3. 4. 5. 6. 7.
3.50g of NaC2H3O2.3H2O is weighted. The mass is recorded and put into a 150 ml beaker and is labeled as beaker 1. 8.8 ml of 3.0 M acetic acid is measured and added into beaker 1. 55.6 ml of distilled water is measured and added to the beaker 1. The solution is stirred until sodium acetate is completely dissolved. The pH of the solutions is measured using a calibrated pH meter. The solution is saved for part B (ii)
ii) Effect of acid and base on buffer solution 1. 32 ml of prepared buffer solution is poured into two of 50 ml beakers. It is labeled as beaker. 2. 1 ml 3.0M HCl is pipeted into beaker 2 and the pH of resultant solution is measured. 3. 1 ml 3.0M NaOH is pipeted into beaker 3 and the pH of resultant solution is measured.
RESULT
Part A: pH solution and hydrolysis of salt 1) unboiled distilled water Indicator Colour pH
Methyl orange Orange 4.0
Methyl red
Methyl orange Orange 3.0
Methyl red
Pale orange 5.0
Bromothymol Phenolpthalein Alizarin blue yellow-R Pale Blue Colourless yellow 7.0 10.0 7.0
2) 0.1M NaCl Indicator Colour pH
Bromothymol Phenolpthalein Alizarin blue yellow-R Light brown Pale green Colourless pale yellow 5.0 5.0 3.0 5.0
4) 0.1 M NH4Cl Indicator Colour pH
Methyl orange Orange 4.0
Methyl red Pale orange 4.0
Bromothymol Phenolpthalein Alizarin blue yellow-R Pale yellow Colourless Light yellow 4.0 4.0 4.0
5) 0.1 mol NaC2H3O2 Indicator Colour
Methyl orange Orange
pH
3.0
Methyl red Yellowish brown 4.0
Bromothymol Phenolpthalein Alizarin blue yellow-R Blue Colourless Pale yellow 8.0
6.0
6.0
6) 0.1M ZnCl2 Indicator Colour
Methyl orange Pale orange
pH
6.0
Methyl red Yellowish brown 5.0
Bromothymol Phenolpthalein Alizarin blue yellow-R Pale green Colourless Light yellow 5.0
5.0
5.0
7) 0.1M KAl(SO4)2 Indicator Colour pH
Methyl orange Light red 1.0
Methyl red Purple 3.0
Bromothymol Phenolpthalein Alizarin blue yellow-R Pale yellow Colourless Yellow 4.0 4.0 4.0
8) 0.1M Na2CO3 Indicator Colour pH
Methyl orange Orange 5.0
Methyl red
Bromothymol Phenolpthalein Alizarin blue yellow-R Light brown Blue Purple Brick red 8.0 8.0 8.0 11.0
Part B Mass of NaC2H3O2.3H2O
= 3.5012 g
pH of buffer solution in beaker 1 = 5.327 pH of buffer solution added with 3.0 M HCl in beaker 2 = 5.002 pH of buffer solution added with 3.0 M NaOH in beaker 3 = 5.813 CALCULATION
The calculation of the average of pH Average pH =
1. Average pH of the unboiled water = 4.0 + 5.0 + 7.0 + 10.0 + 7.0 5 = 6.6
3. Average pH of the 0.1M NaCl
= 3.0 + 5.0 + 5.0 + 3.0 + 5.0 5 = 4.2
4. Average pH of the 0.1 M NH4Cl
= 4.0 + 4.0 + 4.0 + 4.0 + 4.0 5 = 4.0
5. Average pH of the 0.1M NaC2H3O2 = 3.0 + 4.0 + 8.0 + 6.0 + 6.0 5 = 5.4 6. Average pH of the 0.1M ZnCl2
= 6.0 + 5.0 + 5.0 +5.0 + 5.0 5 = 5.2
7. Average pH of the 0.1M KAl(SO4)2 = 1.0 + 3.0 + 4.0 + 4.0 + 4.0 5 = 3.2 8. Average pH of the 0.1M Na2CO3
= 5.0 + 8.0 + 8.0 +8.0 + 11.0 5 = 8.0
To calculated the Hydrogen ion +
pH = -log [H ] +
-pH
So, [H ] = 10
1) pH unboiled distilled water = 6.6 +
pH = -log [H ] +
-pH
[H ] = 10
-6.6
= 10
-7
= 2.512 X 10
2) pH 0.1M NaCl
= 4.2 +
pH = -log [H ] +
-pH
[H ] = 10
-4.2
= 10
-5
= 6.3096 X 10 3) pH 0.1 M NH4Cl
= 4.0 +
pH = -log [H ] +
-pH
[H ] = 10
-4.0
= 10
-4
= 1.0 X 10 4) pH 0.1M NaC2H3O2 = 5.4 +
pH = -log [H ] +
-pH
[H ] = 10
-5.4
= 10
-6
= 3.981 X 10 5) pH 0.1M ZnCl2
= 5.2 +
pH = -log [H ] +
-pH
[H ] = 10
-5.2
= 10
-6
= 6.3096 X 10 6) pH 0.1M KAl(SO4)2
= 3.2 +
pH = -log [H ] +
-pH
[H ] = 10
-3.2
= 10
-4
= 6.3096 X 10
7) pH 0.1M Na2CO3
= 8.0 +
pH = -log [H ] +
-pH
[H ] = 10
-8.0
= 10
-8
= 1.0 X 10 the concentration of hydroxide ion +
-
[H ][OH ] = k w +
-
-14
[H ] [OH ] = 1.0 x 10 -
[OH ] =
[ ] +
-7
1. unboiled distilled water, [H ] = 2.512 x 10
-
[OH ] =
[ ] -8
= 3.980 x 10
2. 0.1M NaCl,
+
-5
[H ] = 6.309 x 10
-
[OH ] =
[ ] -10
= 1.585 x 10
3. 0.1 MNH4Cl,
+
[H ] = 1.0 x 10 -
[OH ] =
-4
[ ] -10
= 1.0 x 10
4. 0.1 M NaC2H3O2,
+
-5
H]
= 6.310 x 10 -
[OH ] =
[ ] -10
= 1.585 x 10
+
5. 0.1M ZnCl2,
-6
[H ] = 3.981 x 10 -
[OH ] =
[ ] -9
= 2.512 x 10 +
6. 0.1 M KAl(SO4)2
-4
[H ] = 6.3096 x 10 -
[OH ] =
[ ] -11
= 1.58 x 10 +
7. 0.1M Na2CO3,
-8
[H ] = 1.0 x 10 -
[OH ] =
[ ] -6
= 1.0 x 10 Net ionic equation 1) unboiled distilled water -
H2O ⇌ H+ + OH K c =
[ ][ ] [ ] -14
K c = K w = 1.0 x 10 2) 0.1M NaCl NaCl
+
Na
+
-
Cl
Initial
0.1
0.0
0.0
Change
-0.1
0.1
0.1
0.0
0.1
0.1
Equilibrium
Equilibrium constant expression: no K a or K b for strong acid Equilibrium constant value : no value
3) 0.1M NH4Cl
Initial
+
⇌
NH4Cl
H
0.1
-4
+ 1.0 x 10 -4
+ 1.0 x 10
-4
0.1-1.0 x 10
-4
1.0 x 10
Equilibrium constant expression: K a =
-
0.0
-4
-1.0 x 10
Equilibrium
NH3Cl
0.0
-4
Change
+
1.0 x 10
[ ][ ] [ ] -7
Equilibrium constant value : K a = 1.001 X 10
4) 0.1 M NaC2H3O2
⇌
NaC2H3O2 + H2O
Initial Change Equilibrium
HNaC2H3O2
0.1
+
-
+
OH
0.0 -5
0.0 -5
-6.310 x 10
-5
+ 6.310 x 10
-5
+ 6.310 x 10
-5
0.1-6.310 x 10
-5
6.310 x 10
6.310 x 10
Equilibrium constant expression: K b =
[ ][ ] []
Equilibrium constant value : K b = 3.98 X 5) 0.1M ZnCl2 2+
⇌
Zn Initial Change Equilibrium
-
Zn(OH)
0.1
0.0 -6
-6.3096 X 10
-6
Equilibrium constant expression: K a =
-6
+ 6.3096 X 10
-6
6.3096 X 10
6.3096 X 10
[ ][ ] [ ] -10
Equilibrium constant value : K a = 3.98 X 10
H 0.0
+6.3096 X 10
0.1-6.3096 X 10
+
+
-6
-6
6) 0.1 M KAl(SO4)2 Al Initial
3-
⇌
+ 3H20
Al(OH)
0.1
+
+
3H
0.0 -4
Change Equilibrium
3+
0.0 -4
-6.3096 X 10
-4
+6.3096 X 10 -4
-4
0.1- 6.3096 X 10
Equilibrium constant expression: K a =
+ 6.3096 X 10
-4
6.3096 X 10
6.3096 X 10
[ ] [ ] [ ] -12
Equilibrium constant value : K a =1.59. X 10
7) 0.1 M Na2CO3 CO3
2-
+ H20
Initial
0.1
Change
-1.0 x 10
Equilibrium
⇌
HCO3
+
+
0.0 -6
0.0 -6
+1.0 x 10 -6
-6
0.1-1.0 x 10
-
OH
-6
+1.0 x 10 -6
1.0 x 10
1.0 x 10
b
[ ][ ]
Equilibrium constant expression: K =
[] -11
Equilibrium constant value : K b = 1.00 X 10
Part B: pH of buffer solution
Acetic Acid-Sodium Acetate Buffer: i. ii. iii. iv. v.
Weight of NaC2H3O2.3H2O Volume of acetic acid Concentration of acetic acid Volume of distilled water pH of the prepared buffer
=3.50 g = 8.80 mL = 3.0 M = 55.6 mL = 5.222
Effect of Acid and Base on the buffer pH i. ii. iii. iv. v.
pH of solution in beaker 1 pH of solution in beaker 2 pH of solution in beaker 3 pH values after the addition of the HCl pH values after the addition of the NaOH
+
HCl
H
= 5.222 = 5.222 = 5.222 = 4.896 = 6.544
-
+
Cl
Initial
3.0
0.0
0.0
Change
-3.0
3.0
3.0
Equilibrium
0.0
3.0
3.0
-
Buffer action :
CH3COO
Initial
+
+
H
3.0M x 0.032L = 0.096 mol
3.0M x 0.032L = 0.096 mol
Addition Change After Neutralization Final Concentration
pH = pK a + log
CH3COOH
3.0 x 0.001L = 0.003mol -0.003 mol 0 mol
-0.003 mol 0.093 mol 2.82M
0M
=
[ ] [ ]
= - log (1.8 x ) + log
+0.003 mol 0.099 mol
= 4.71 When NaOH is added to the solution +
+
-
NaOH
Na
OH
Initial
3.0
0.0
0.0
Change
-3.0
3.0
3.0
= 3M
Equilibrium
0.0
Buffer action :
3.0
CH3COOH
Initial
+
+
-
OH
CH3COO
3.0M x 0.032L = 0.096 mol
Addition Change After Neutralization Final Concentration
pOH = pK b + log
3.0
-0.003 mol 0.093 mol 2.82M
=
3.0M x 0.032L = 0.096 mol 3.0 x 0.001L = 0.003mol -0.003 mol 0 mol 0M
+0.003 mol 0.099 mol
= 3M
[ ] [ ] -10
= - log 5.6 x 10 + log
= 9.25 - 0.0269 = 9.2231 pH = 14 - pOH pH = 4.78
Beaker 2 Beaker 3
Theoretical value 4.71 4.78
Actual (experiment) value 4.896 6.544
DISCUSSION
In part A, the colour indicator showed inconsistence with some of the substances. Some indicators give a reading that is higher than th e pH limit of acid or base. The resultant colour is important for pH determination and there are few reasons why the indicator give different colour. First of all, the test tubes are not thorou ghly washed before another substance are put into it. This will result the substance to mix up with the previous substance and will affect the colour change of the indicator. To overcome this problem, students need to rinse the test tubes thoroughly with boiled distilled water for at least three times to remove any remaining
substances thus give a better result. The second pro blem is there are some student who p ut too much colour indicator into the test tubes. This will lead the colour of indicator to overcome the resultant colour of the pH scale. To avoid this problems, students need to put the indicator with the amount mentioned in the manual and watch the colour carefully so they can obtain a consistent result The observation that can be made after part B is done is the experimented pH values is not the same with the theoretical values. The difference of the pH value may be caused by a few factors. First of all, The pH meter may did not cleaned completely and it have been been immersed multiple times simultaneously which may cause the buffer solution from another group mixed together and may affect the reading. To resolve this problem, students need to immersed the sensor in the rinsing solution and let it stay for a while to rinse any substance that left on the sensor from previous solution. There are also some students who did not measure the volume of acetic acid with precision. If the observer’s are not parallel with the meniscus, it will show a different reading thus affecting the volume of acetic acid. The difference maybe small, but it will result a different pH reading CONCLUSION
The results and calculations show that students have managed to determine pH values of salts solutions by using different indicators. The calculations of calculate the k a and k b of each cation or anion that hydrolyzes are also shown. Students also have learnt how to prepare acetic acid-sodium acetate buffer. Besides, students also mana ged to investigate and analyse the effect of acid and base on the buffer pH. All the objectives for this experiment have been achieved thus conclude the experiment REFERENCE
Raymond Chang. ( 2006 ). Chemistry. (9 th ed.). Mc Graw Hill Faculty of Chemical Engineering. (2011). Chemical Laboratory Manual CHE 235. UiTM Dungun, Terengganu http://en.wikipedia.org/wiki/Acid http://en.wikipedia.org/wiki/Base_(chemistry)
