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2021-23 Unit 13 LAA Acid or Base?
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- PEARSON (PEARSON)
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2021-23 Unit 13 LAA Acid or Base?Introduction
I am a quality control technician working for Severn Trent Water. An essential part of my
work is to accurately determine the pH of the water and dissolved chemicals at each stage
in the treatment process. For safety reasons, and so that pipework and pumps are not
damaged, the pH must always be within the 6 - 10 range. Because my department
regularly receives visits from schools, my manager has allocated me the task of designing
a training programme for the ‘A’ level and BTEC work experience students. I will be
expected to include theory notes, and demonstrate the use and action of buffer solutions
and practical acid-base titrations (including how to interpret the results of the titrations). I
will justify why certain indicators are used for certain titrations. My supervisor has also
provided you with pH calculations for the students to complete. I will need to work out the
correct answers, showing my workings, so I can help the students if they are unsure of
how to calculate the answers. I will need to evaluate different acid-base titration methods
as part of the design of my training programme. If my manager is happy that I am confident
in my technical ability and subject knowledge, you will become the official ‘placement
supervisor’ for future work experience visits.
✔
Determine the acid dissociation constant (Ka) for a weak acid
The aim of this experiment is to accurately demonstrate a reading of pH of a half-
neutralisation solution of ethanoic acid and to determine a value for Ka. First accurately
determine the titre of sodium hydroxide solution required to neutralise 25.0 cm3 of 0.1M
ethanoic acid. Then neutralise exactly half of a new neutralise exactly half of a new sample
of 25.0 cm3 of ethanoic acid with sodium hydroxide and accurately measure the pH of the
resulting solution in order to determine Ka.
Chemicals and equipment list-
-ethanoic acid (approx.1M)
-sodium hydroxide solution (0.1M)
-Safety spectacles
-1 burette, 50 cm3 , and stand
-1 funnel , small
-pipette, 25.0 cm3, and pipette filler
-1 beaker, 250 cm3
-1 conical flask, 250 cm3
-distilled water
-pH meter (accurately calibrated)
,2021-23 Unit 13 LAA Acid or Base?
Burett
e
Ethanoi
c acid Distilled
water
Sodium
hydroxid
Phenolphthalei e
Pipett
n
e filler Conic
al
Pipett flask
e
Stand
Risk assessment
Risk Hazard Precaution
Ethanoic acid Irritations to the skin and eye During the experiment, always
wear goggles, and if anything
gets in your eyes, wash it out
right away.
Sodium Corrosive and can cause irritation to Put the bottle in the centre of
hydroxide the eyes the table and secure the cap
solution when not in use to prevent
spills. Wear safety goggles .
Glassware Can break and cause cuts to the skin To stop the glass from
dropping and breaking, place it
in the centre of the table. Use
caution while handling
shattered glass because it
might break and result in cuts
to the skin.
Method steps
1. Clamp the stand with the 50 cm3 burette. Rinse and fill the burette with 0.1M
sodium hydroxide solution using a funnel.
2. Rinse the pipette with some ethanoic acid using a pipette filler, and then transfer
precisely 25.0 cm3 of the solution into a 250 cm3 conical flask.
3. The conical flask should contain a few drops of phenolphthalein indicator solution.
Stir to mix it together.
, 2021-23 Unit 13 LAA Acid or Base?
4. Swirl sodium hydroxide solution from the burette till it barely becomes pink and
becomes permanent.
5. Till you have concordant titres, repeat the titration. Do the mean titre (V cm3)
calculation.
6. This time, pipette precisely 25.0 cm3 of the ethanoic acid solution into a beaker
with a capacity of 250 cm3. No phenolphthalein indicator should be added.
7. Pour V/2 cm3 precisely of sodium hydroxide solution from the burette into the
ethanoic acid beaker. This amount is needed to completely neutralise the
ethanoic acid.
8. To make sure the solutions are well incorporated, gently swirl.
9. Insert a pH metre that has been precisely calibrated so that the tip of the probe is
fully submerged in the prepared solution into the beaker.
10. Read and note the final pH of the solution in the beaker after the reading is
steady.
11. To get a trustworthy average pH value, repeat the half-neutralisation procedures
once more.
Results
Calibration of ph meter result
PH buffer PH reading
2 2.5
4 4.7
7 7.8
Results table for titration of sodium hydroxide solution
Trial run 1st run 2nd run 3rd run 4th run
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