1
GRADE 12 PRESCRIBED EXPERIMENT 2: ACID-BASE REACTIONS
WORK SHEET
TOTAL MARK: 50
ACTIVITY
Titration of oxalic acid against sodium hydroxide to determine the concentration of the sodium
hydroxide.
In this investigation you will prepare an acidic solution accurately and thus you will know its exact
concentration. You will then react this acid with a base of an unknown concentration to determine
the concentration of the base.
What you will need:
Erlenmeyer flasks
Burettes
Burette clamp
Medicine dropper
Retort stand
White tile /paper
Measuring cylinders
Mass meter
Oxalic acid
Sodium Hydroxide
Phenolphthalein as indicator
Funnel
Beaker
Spatula
Glass rod
Pipette with sucker
What to do:
1. Prepare a standard solution of oxalic acid which has a concentration of
approximately 1mol.dm-3.
2. Now prepare a sodium hydroxide solution by dissolving approximately 2g of dry
sodium hydroxide in 500ml of water.
3. Add two drops of the indicator solution.
4. Place the burette in the clamp.
5. Using the funnel, fill the burette to above zero mark with the acid solution.
6. Then, holding the beaker, with which you used to pour the acid, beneath the
burette, gradually open the tap.
7. Allow the level of the base to come down to exactly zero (reading from the bottom
of the meniscus).
8. Pipette using the sucker exactly 25ml of oxalic acid solution in a volumetric flask.
9. Add a few drops of phenolphthalein to the acid.
10. Hold the conical flask beneath the burette with your right hand and gradually open
the tap with your left.
11. Swirl the conical flak continuously and watch it closely for the first sign of a colour
change.
12. As you see that you are approaching the point of neutralization, close the tap slightly
so that you are adding drop by drop.
13. When the colour changes completely the titration is finished.
14. Close the tap and read from the burette how much acid was used.
15. Repeat this procedure at least twice so that you have three readings for the volume
2
of NaOH (of unknown concentration) required to neutralize exactly 25ml of oxalic
acid (of known concentration).
16. Take an average of these three and use it to calculate the concentration of the
NaOH.
17. Now calculate the concentration of the sodium hydroxide solution.
18. Make a neat labeled sketch to represent the apparatus
19. Now write a report using the format learnt in class.
Questions
1. What is the appropriate concentration of NAOH (2g in 500ml of water)
2. Calculate the theoretical concentration of NaOH from the actual mass of NaOH you
measured.
3. How does your theoretical value for NaOH concentration (from the actual mass you
measured) differ from the actual concentration you calculated (from the titration
procedure)? Can you think of some reasons why your values may differ?
3
MEMORANDUM
1. Title
ACID- BASE TITRATION
2. AIM
To determine the concentration of Sodium Hydroxide (NaOH) using a standard
solution of Oxalic acid (H2C204)
3. MATERIALS (APPARATUS & CHEMICALS)
Apparatus:
Chemical
1. Erlenmeyer flasks
1. Oxalic acid
2. Burettes
2. Sodium Hydroxide
3. Medicine dropper
3. Phenopthalein as indicator
4. Retort stand
5. White tile /paper
6. Measuring cylinders
4.
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PRECAUTIONARY MEASURES
Safety glasses must be worn
The use of gloves is recommended
Test run before doing actual titration experiment
Contamination from dirty glassware
Oxalic acid solution becoming lumpy / forming precipitate
‘Overshooting’ the mark
Volumetric flask calibrated at a specific temperature (e.g. room temperature)
5. SAFETY AUDIT ON:
1. Oxalic acid
2. Sodium Hydroxide
(4)
6. DATA COLLECTION
Experiment 1, 2 & 3
Concentration of oxalic acid: 1mol/dm3
Concentration of base (sodium hydroxide): unknown
Experiment Number. Volume of Acid(mℓ) Volume of NaOH(mℓ)
1
25 mℓ
2
25 mℓ
3
25 mℓ
The average of the results obtained: _____________ ml of NaOH solution.
(2)
What
is
your
observation/colour
change
at
the
end
point?
________________________________________________________________________ (2)
7. CONCLUSION__________________________________________________________
_____________________________________________________________________
__________________________________________________________________ (1)
4
8. CALCULATIONS
(a) Write the balanced equation for the reaction
_________________________________________________________________________(3)
(b) Calculate the concentration of the unknown solution using the equation
nbCaVa = naCbVb
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
________________________________________________________________________ (7)
TOTAL [50]
SAFETY AUDITS ON:
Oxalic acid
√√
FIRE
Combustible.
Gives
off
irritating or toxic fumes (or
gases) in a fire.
NO open flames.
Powder,
alcohol-resistant
foam, water spray, carbon
dioxide.
In case of fire: keep drums,
etc., cool by spraying with
water.
EXPLOSION
EXPOSURE
AVOID ALL CONTACT!
IN ALL CASES CONSULT A
DOCTOR!
Inhalation
Sore throat. Cough. Burning
sensation.
Shortness
of
breath. Laboured breathing.
Symptoms may be delayed
(see Notes).
Local exhaust or breathing
protection.
Fresh air, rest. Half-upright
position. Artificial respiration
if indicated. Refer for medical
attention.
Skin
Redness. Skin burns. Pain.
Blisters.
Protective clothing.
First rinse with plenty of
water,
then
remove
contaminated clothes and
rinse again. Refer for medical
attention.
Eyes
Redness. Pain. Loss of vision.
Severe deep burns.
Face shield, or eye protection
in
combination
with
breathing
protection
if
First rinse with plenty of
water for several minutes
(remove contact lenses if
5
FIRE
Not combustible. Contact
with moisture or water may
generate sufficient heat to
ignite
combustible
substances.
In case of fire in the
surroundings: use appropriate
extinguishing media.
EXPLOSION
AVOID ALL CONTACT!
EXPOSURE
Inhalation
Ingestion
IN ALL CASES CONSULT A
DOCTOR!
Corrosive. Burning sensation. Local exhaust or breathing Fresh air, rest. Half-upright
Sore throat. Cough. Laboured protection.
position. Artificial respiration
breathing.
Shortness
of
may be needed. Refer for
breath. Symptoms may be
medical attention.
delayed (see Notes).
Sore
throat.
Burning
sensation. Abdominal pain.
Vomiting. Drowsiness. Shock
or collapse. Convulsions.
powder.
easily possible), then take to
a doctor.
Do not eat, drink, or smoke
during work. Wash hands
before eating.
Rinse mouth. Rest. Refer for
medical attention.
6
Skin
Corrosive. Redness. Pain. Protective gloves. Protective Remove
contaminated
Serious skin burns. Blisters.
clothing.
clothes. Rinse skin with plenty
of water or shower. Refer for
medical attention.
Eyes
Corrosive. Redness. Pain. Face shield or eye protection First rinse with plenty of
Blurred vision. Severe deep in combination with breathing water for several minutes
burns.
protection if powder.
(remove contact lenses if
easily possible), then take to a
doctor.
Ingestion
Corrosive. Burning sensation. Do not eat, drink, or smoke Rinse mouth. Do NOT induce
Abdominal pain. Shock or during work.
vomiting. Give plenty of water
collapse.
to drink. Refer for medical
attention.
SODIUM HYDROXIDE (NaOH)
√√
APPROPRIATE SKETCH
burette
Retort stand
Erlenmeyer
flask
White tile
(6)
Burette with oxalic acid
Retort stand
Erlenmeyer flask with NaOH
White tile
√√
√
√√
√
6. DATA COLLECTION
Experiment 1,2 & 3
Concentration of Oxalic acid: 1mol/dm3
Concentration of base (NaOH): unknown
7
No.
1
2
3
Volume of NaOH(mℓ)
50 mℓ
50 mℓ
50 mℓ
Volume of Oxalic acid (ml)
43 mℓ
√
44 mℓ
√
41 mℓ
√
The average of the results obtained: 42 mℓ √
Colour of phenopthalein changes from pink to colourless
7. CONCLUSION:
50ml NaOH solution of 0.42 mol.dm-3 neutralises 42ml oxalic acid solution of 1
mol.dm-3
(1)
8. CALCULATIONS
a) Write the balanced equation for the reaction
2NaOH + H 2C 2O 4 √
→ Na 2C 2O 4 √ + 2H 2O √
b) Calculate the concentration of the unknown solution using the equation
n bC aV a = n aC bV b
1x1x 0.042 = 2 X Cb x 0.05
C b = 0.42 mol/dm3
(7)
TOTAL [25]
HOW SECTION 1.3 (CAPS) CAN FEATURE IN THE EXPERIMENT
Refer to CAPS page 5 d, bullet 1-5
 identify and solve problems and make decisions using critical and creative
thinking;
 work effectively as individuals and with others as members of a team;
 organise and manage themselves and their activities responsibly and
effectively;
 collect, analyse, organise and critically evaluate information;
 communicate effectively using visual, symbolic and/or language skills in
various modes;
Suggestions on how to improve:
 You can let them write an equation before they perform the experiment
 Don’t ask questions like ‘is it a weak or strong acid: because there is 50%
chance of guessing
 Pour the oxalic acid in the burette & NaOH in the Erlenmeyer flask because
cleaning the NaOH from burette is difficult & burette might break