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p.01
Acid-Base Eqm (6):
(5):
Titration Curves
Plotting the Titration Curves with
your calculated results.
E.g. (1a) 25 cm3 HCl + 20 cm3 NaOH
new [HCl] =
25/1000 0.1 – 20/1000 0.1
= 0.011 M
(25+20)/1000
[H3O+] = 0.011 M
pH = 1.95
C. Y. Yeung (CHW, 2009)
p.02
E.g. (1b) 25 cm3 HCl + 30 cm3 NaOH
[NaOH] =
30/1000 0.1 – 25/1000 0.1
(30+25)/1000
[OH-] = 9.0910-3 M
pOH = 2.04
pH = 11.96
= 9.0910-3 M
p.03
E.g. (2a) 25 cm3 CH3COOH + 20 cm3 NaOH
new [CH3COOH] =
25/1000 0.1 – 20/1000 0.1
= 0.011 M
(25+20)/1000
[CH3
COO-]
=
20/1000 0.1
= 0.044 M
(25+20)/1000
pH = - log(1.76
10-5)
+ log
= 5.36
[H3O+] = 10-5.36 = 4.37 10-6 M
(0.044)
(0.011)
p.04
E.g. (2b) 25 cm3 CH3COOH + 25 cm3 NaOH
[CH3
COO-]
=
25/1000 0.1
(25+25)/1000
CH3COO- + H2O
Kb =
1.00 10-14
1.76
= 0.05 M
10-5
=
CH3COOH + OH-
x2
(0.05 – x)
x = 5.33 10-6 = [OH-]
pOH = 5.27
pH = 8,73, [H3O+] = 10-8.73 = 1.86 10-9 M
p.05
E.g. (2c) 25 cm3 CH3COOH + 45 cm3 NaOH
[NaOH] =
45/1000 0.1 – 25/1000 0.1
(45+25)/1000
[OH-] = 0.0286 M
pOH = 1.54
pH = 12.5
= 0.0286 M
p.06
E.g. (3a) 25 cm3 HCl + 10 cm3 NH3
new [HCl] =
25/1000 0.1 – 10/1000 0.1
(25+10)/1000
[H3O+] = 0.043 M
pH = 1.37
= 0.043 M
p.07
E.g. (3b) 25 cm3 HCl + 25 cm3 NH3
+]
[NH4 =
25/1000 0.1
(25+25)/1000
NH4+ + H2O
Ka =
1.00 10-14
1.74
10-5
= 0.05 M
=
NH3 + H3O+
x2
(0.05 – x)
x = 5.36 10-6 = [H3O+]
pH = 5.27
pOH = 8,73, [OH-] = 10-8.73 = 1.86 10-9 M
p.08
E.g. (3c) 25 cm3 HCl + 40 cm3 NH3
[NH3] =
[NH4+] =
40/1000 0.1 – 25/1000 0.1
= 0.0231 M
(40+25)/1000
25/1000 0.1
= 0.0385 M
(40+25)/1000
pOH = - log(1.74 10-5) + log
(0.0385)
(0.0231)
pOH = 4.98, [OH-] = 10-4.98 = 1.04 10-5 M
pH = 9.02
Titration Curves: Strong Acid VS Strong Base
Titration Curve: Strong Acid Vs Strong Base
14
12
10
pH
8
abrupt change of pH
6
4
2
0
0
2
4
6
8
vol. of NaOH added / cm 3
10
12
p.09
Titration Curves: Weak Acid VS Strong Base
Titration Curve: Weak Acid Vs Strong Base
Due to hydrolysis of
conjugate base of weak acid:
14
A- + H2O
HA + OH-
12
pH of salt > 7
10
abrupt change of pH
pH
8
6
4
2
0
0
2
4
6
8
vol. of NaOH added / cm
10
3
12
p.10
Titration Curves: Strong Acid VS Weak Base
Titration Curve: Strong Acid Vs Weak Base
14
12
Due to hydrolysis of conjugate acid of weak base:
BH+ +10 H2O
B + H3O+
8
pH
pH of salt < 7
6
abrupt change of pH
4
2
0
0
2
4
6
8
vol. of NaOH added / cm 3
10
12
p.11
Titration Curves: Comparison
Titration Curve: Comparison
14
12
weak acid
VS strong base
strong acid VS strong base
10
pH
8
strong acid VS weak base
6
4
2
0
0
2
4
6
8
vol. of NaOH added / cm 3
10
12
p.12
Choosing a Suitable Indicator (1)
p.13
the abrupt change on the pH curve must fall
across the “working range” of the indicator.
pKIn ± 1
8.15
9.15
10.15
Phenolphthalein
7
pH
pink
colourless
pale pink
Methyl orange
2.70
3.70
4.70
7
pH
red
yellow
orange
p.14
Choosing a Suitable Indicator (2)
End point: The sudden change in colour seen
in a titration.
Equivalent point: The mixture in which amount of
acid and base are exactly balance.
If the correct indicator has been chosen, the end point
will be very close to the equivalent point.
end pt.
pale pink
10
Strong Acid
VS Strong Base
pink
12
phenolphthalein
methyl orange
14
pH
eqv. pt.
6
yellow
8
orange
4
0
red
2
25
vol. of alkali
added / cm3
colourless
end pt.
p.15
14
phenolphthalein
methyl orange
pink
Weak Acid
VS Strong Base
12
pale pink
10
pH
yellow
8
6
0
red
2
25
vol. of alkali
added / cm3
colourless
orange
4
p.16
14
phenolphthalein
methyl orange
pink
Strong Acid
VS Weak Base
12
pale pink
10
pH
yellow
8
6
0
red
2
25
vol. of acid
added / cm3
colourless
orange
4
p.17
Explain why phenolphthalein turns pink
in a solution of sodium carbonate, but
remains colourless in a solution of
sodium hydrogencarbonate. [1990]
p.18
Explain why at 298K, in a solution of pH7.0, the
indicator methyl orange shows its alkaline
colour (yellow), while phenolphthalein shows
its acidic colour (colourless). [1994]
p.19
Acid-base indicators are weak acids or bases.
The dissociation of which can be represented by
HIn(aq) + H2O(l)
H3O+(aq) + In-(aq)
The colour of an indicator depends on the relative
concentrations of HIn and In- which are of different colours.
The dissociation constant KIn of different indicators are
different, thus they change colour over different pH range.
The pH range of methyl orange is below 7, while that of
phenolphthalein is above 7.
HKALE:
p. 233 Q.15(a),(b)
p.20
HKALE: Q.20(a)
p.21
Double Indicator Titration
p.22
[Phenolphthalein & Methyl Orange]
For mixtures containing TWO
BASES.
11.2 cm3 0.1M HCl: Phenolphthalein changes colour.
28.8 cm3 0.1M HCl: Methyl Orange changes colour.
E.g.
25cm3 mixture containing
to be neutralized first
NaHCO3 & Na2CO3
11.2 cm3 0.1M HCl
NaHCO3
no. of mol of Na2CO3
= 1.1210-3 mol
28.8 cm3 0.1M HCl
NaCl
[Na2CO3] = 0.0448 M
[NaHCO3] = 0.0704 M
total no. of mol of NaHCO3
= 2.8810-3 mol
Original no. of mol of NaHCO3
= 2.8810-3 – 1.1210-3
= 1.76 10-3 mol
p.23
Assignment
p.229 Q.3(c), 4, 11, 18, 26, 29
p.171 Check Point 18-4
[due date: 29/4(Wed)]
Next ….
Solubility Product (Ksp) [p.172-176]