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Purpose: To successfully
titrate a weak acid against a strong base and determine the
concentration of the weak
acid as well as picking an apporpriate acid-base indicator that is
suitable for this particular reaction.
The acid will be either formic or ethanoic acid which will
have been prepared for you. The base will be potassium hydroxide
which you will have to prepare.
Part 1. Generation of the KOH solution
| 1. |
Weigh out between 2-4 grams of KOH accurately to at
least 2 decimal places. |
| 2. |
Dissolve in distilled water in a volumetric flask.
Swirl the contents to fully dissolve the KOH. |
| 3. |
When dissolved, top up the volumetic flask to the
mark. |
Part 2. Standardization of the
KOH solution with benzoic acid.
| 1. |
Fill a clean, rinsed burette with the KOH solution. |
| 2. |
Mass out approximately 0.2-0.5 grams of benzoic acid
and record
the mass used accurately to two decimal places. |
| 3. |
Benzoic acid is a weak acid. This means that
the endpoint
will not be at 7. Using the Ka for benzoic acid
estimate
the approximate pH of the endpoint and determine an appropriate
indicator
for you to use. |
| 4. |
Ask your teacher for the appropriate
indicator. If the one you request is not available be prepared
with 2 other alternate choices! |
| 5. |
You will have to make up a solution of indicator for
your use. Either water or alcohol will be needed as a solvent. |
| 6. |
Dissolve the benzoic acid in a minimum amount of
water in an Erlenmeyer flask and add enough of your indicator to colour
the solution. |
| 7. |
Titrate the benzoic acid solution with your KOH. |
| 8. |
Repeat the titration with fresh samples of benzoic
acid twice more. |
| 9. |
Calculate the average molarity of the KOH |
Part 3. Titration of the weak acid
against the known KOH solution
| 1. |
Get a sample of either formic or ethanoic acid from
your teacher. |
| 2. |
You are again titrating a weak acid against a strong
base. The endpoint will not the 7 again but it will also not be
the same as the benzoic acid endpoint. Re-estimate the endpoint
using the Ka of either the formic or ethanoic acid to
determine an appropriate indicator to use. |
| 3. |
Using a pipette, accurately transfer some of your
acid sample to a clean Erlenmeyer flask. Record the amount of
acid transferred. |
| 4. |
Titrate your sample of weak acid. |
| 5. |
Repeat with two more samples of your weak acid. |
| 6. |
Calculate the average molarity of your weak acid. |
Data Table:
Part 1
1. Mass of KOH used: ____________ grams
2. Volume of volumetric flask: ___________ mL
Part 2
3. Ka of benzoic acid:
________________ Kb of benzoic
acid:_______________
pKa of benzoic
acid: _______________ pKb of benzoic acid:
______________
Indicators: Choice
#1 ______________
Choice #2 ______________
Choice #3 ______________
Choice #4 ______________
|
Trial
#1
|
Trial
#2
|
Trial
#3
|
| 4.
Mass of Benzoic acid used: |
g
|
g
|
g
|
| 5.
Volume of KOH solution used: |
mL
|
mL
|
mL
|
| 6.
Calculated molarity of the KOH solution: |
M
|
M
|
M
|
7. Average molarity of the KOH solution:
_________
Part 3
8. Acid used: ______________________
9. Ka of acid:
_____________________ Kb of acid:
___________
10. pKa of acid:
____________________ pKb of acid: _________
11. Approximate pH of the final
solution: __________
Indicators: Choice #1 ______________
Choice #2 ______________
Choice #3 ______________
Choice #4 ______________
|
Trial
#1
|
Trial
#2
|
Trial
#3
|
| 12. Volume
of acid used: |
mL
|
mL
|
mL
|
| 13. Volume
of KOH solution used: |
mL
|
mL
|
mL
|
| 14.
Calculated molarity of the acid: |
M
|
M
|
M
|
15. Average molarity of your sample acid:
___________
|