Transcript Weak Acid - St John Brebeuf

Acids & Bases
Lesson 12
Titration Curves &
Weak Acid/Strong Base calculations
Titration Curves
A titration curve is a graph of the pH changes that occur during an
acid-base titration versus the volume of acid or base added.
The equivalence point is the end of a titration where the stoichiometry
of the reaction is exactly satisfied, or moles H+ = moles OH-.
The transition point refers to when an indicator changes color and
[HInd] = [Ind-].
Three types of Titrations
Strong acid and strong base
Weak acid and STRONG BASE
Weak base and STRONG ACID
Choosing an Indicator
When you choose an indicator, you must pick one so that the transition
point of the indicator matches the equivalence point of the titration.
Look at the salts produced, if
Rule of thumb
Salt
Equivalence Point
SA + SB
Neutral
7
WA + SB
Basic
9
WB + SA
Acidic
5
1.
Titration Curve: Strong Acid and Strong Base
HCl + KOH
→
KCl
+ HOH neutral salt
Indicator
pH = 7
Bromothymol Blue- see Ind Chart
50 mL of 0.10 M KOH is added to 25 mL of 0.10 M HCl
The pH of 0.10 M KOH is 13.0
pH
0.10 M HCl
We start here
The pH of 0.10 M HCl is 1.0
0.10 M KOH
14
7
Neutral Salt pH = 7.0
0
0
25
Volume .10 M KOH added
50
2.
Titration Curve: Weak Acid and Strong Base
HCN + KOH → KCN + H2O basic salt
Indicator
pH = 9
Phenolphthalein- see ind. chart
20 mL of 1.0 M HCN isis added
added to 10 mL of 1.0 M KOH
We end here pH = 3
pH
We start here pH = 14
14
1.0 M KOH
Basic Salt pH = 9
7
0
0
10
Volume 1.0 M HCN added
20
1.0 M HCN
3.
Titration Curve: Strong Acid and Weak Base
HCl + NH3 → NH4+ +
Clacid salt
Indicator
pH = 5
Methyl Red- see ind chart
60 mL of 1.0 M NH3 is added to 30 mL of 1.0 M HCl
pH
1.0 M NH3 pH  10
14
7
Acid Salt pH  5
0
0
1.0 M HCl pH = 0
30
Volume 1.0 M NH3 added
60
4.
Match the Curve with the Reaction
pH
14
7
0
0
A.
B.
C.
25
Volume .10 M KOH added
3HCl + Al(OH)3
HCl + KOH
HCN + KOH
→
→
→
50
AlCl3 +
KCl +
KCN +
3HOH
HOH
HOH
5.
Match the Curve with the Reaction
pH
14
7
0
0
A.
B.
C.
30
Volume 1.0 M NH3 added
3HCl + Al(OH)3
HCl + KOH
HCN + KOH
→
→
→
60
AlCl3 +
KCl +
KCN +
3HOH
HOH
HOH
6.
Match the Curve with the Reaction
pH
14
7
0
0
A.
B.
C.
30
Volume 1.0 M NH3 added
3HCl + Al(OH)3
HCl + KOH
HCN + KOH
→
→
→
60
AlCl3 +
KCl +
KCN +
3HOH
HOH
HOH
3 Types of Titrations
Strong acid and strong base
Weak acid and STRONG BASE
Weak base and STRONG ACID
Strong acid and Strong base
• They produce neutral salts and water.
• We have practiced these calculations
before.
Weak acid and STRONG BASE
•
•
•
•
We expect to produce a basic solution.
Therefore, equivalence point at pH > 7
Indicators around pH = 9; between (8-10)
Rules found through experiments to make
your life EASY.
Weak acid and STRONG BASE
Weak acid and STRONG BASE
Rules:
pKa = pH ½  means pH when half Volume is used, NOT HALF pH!!!!
Ka = 10 –pKa
or Ka = 10-pH ½
Ka = [ H3O+]1/2
H3O+]1/2 can be found from pH 1/2
[Acid]equilibrium = [ H3O+]2 …..
this [H3O+] is found from pH initial
Ka
Ka found from above equation
[Acid]original= [Acid]eq + [H3O] from pH initial
where [
Weak acid and STRONG BASE
The following data was obtained during the titration of 25.0 mL of Furoic acid with
NaOH.
28.8mL = volume of NaOH required to get to equivalence point
2.021 = initial pH of furoic acid solution
3.170 = pH at 14.4mL point of titration
A) Calculate the Ka value for furoic acid
Weak acid and STRONG BASE
The following data was obtained during the titration of 25.0 mL of Furoic acid with
NaOH.
28.8mL = volume of NaOH required to get to equivalence point
2.021 = initial pH of furoic acid solution
3.170 = pH at 14.4mL point of titration
B) Calculate the initial concentration of the furoic acid solution
Weak acid and STRONG BASE
The following data was obtained during the titration of 25.0 mL of Furoic acid with
NaOH.
28.8mL = volume of NaOH required to get to equivalence point
2.021 = initial pH of furoic acid solution
3.170 = pH at 14.4mL point of titration
C) Calculate the concentration of the NaOH used.
Weak acid and STRONG BASE
The following data was obtained during the titration of 25.0 mL of Furoic acid with
NaOH.
28.8mL = volume of NaOH required to get to equivalence point
2.021 = initial pH of furoic acid solution
3.170 = pH at 14.4mL point of titration
D) Is the titration mixture acidic, basic or neutral at the equivalence point?
Weak acid and STRONG BASE
The following data was obtained during the titration of 25.0 mL of Furoic acid with
NaOH.
28.8mL = volume of NaOH required to get to equivalence point
2.021 = initial pH of furoic acid solution
3.170 = pH at 14.4mL point of titration
E) Suggest a suitable indicator for the titration
Weak acid and STRONG BASE
TRY: The following data was obtained during the titration of 25.0 mL of propanoic acid,
with 0.100 M NaOH.
Initial pH= 2.950
Volume of NaOH used= 23.8 mL
pH at 11.9 mL = 4.873
A) What is Ka for propanoic acid?
Weak acid and STRONG BASE
The following data was obtained during the titration of 25.0 mL of propanoic acid,
with 0.100 M NaOH.
Initial pH= 2.950
Volume of NaOH used= 23.8 mL
pH at 11.9 mL = 4.873
B) What is the concentration for propanoic acid based on the [NaOH] and the volumes of the reactants?
Weak acid and STRONG BASE
The following data was obtained during the titration of 25.0 mL of propanoic acid,
with 0.100 M NaOH.
Initial pH= 2.950
Volume of NaOH used= 23.8 mL
pH at 11.9 mL = 4.873
C) What is the concentration of propanoic acid as calculated from your value of Ka, found in part,a and the
initial pH value?
Homework
#129 on page 177