Unit 13A - : Investigate acid-base equilibria in order to understand buffer action and to optimise acid-base titration procedures
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Unit 13 - Applications of Inorganic Chemistry
Instelling
PEARSON (PEARSON)
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Unit 13A: Application of Inorganic Chemistry
Half-titrations (Task 2)
Equipment
50ml burette
25ml volumetric pipette
0.1M of sodium hydroxide
A burette stand
Funnel
pH probe
250ml beaker
Ethanoic acid
Buffer solution
Method
1. First, carefully make sure the 50ml burette is clamped safely into the burette sand and
make sure to fill the 50ml burette with the sodium hydroxide, with the bottom of the
meniscus touching directly on the line that states zero. Turn the tap on to let the air bubbles
out and then re-fill back to the line.
2. Then, using the 25ml volumetric pipette fill it with the ethanoic acid until it reaches the
line that will engraved faintly into the pipette.
3. After that, pour the 25ml of ethanoic acid into the 250ml beaker, the beaker should be at
a 45° angle and the pipette held vertically.
4. Once that is done, place the beaker directly under the bottom of the burette and have a
pH probe inside it.
5. Start the tap and pour 1cm³ of sodium hydroxide into the beaker, swirl it and check the
pH that is stated on the pH probe.
6. Record the value down into a table and continue until 15cm³ of sodium hydroxide has
been poured in. Continue by pouring 0.5cm³ of sodium hydroxide and recording the value of
the pH that is being stated.
7. Keep repeating this until the pH reaches approximately 12.
8. Once you have done a rough run, you can repeat the experiment two more times to get
an average. If the results are not concordant make sure to do an additional one to make sure
that they become concordant.
9. Once the additional runs have been done and averages worked out, make sure to plot
your data into a table and work out the half-equivalence point. The pH that is read will then
be equal to pKa.
Health and safety
As you will be working with acids and bases, you will need to ensure that the correct PPE is worn
such as: gloves, a lab coat and goggles. These will be used to reduce the chance of any irritation
that may occur if the substances end up getting onto the individual’s skin or eyes.
,The lab coat will be used to make sure that the any substances do not end up getting onto the
clothes of anyone and to protect everyone from any chemical spillages.
NaOH 1 2 3 Mean
0 2.95 2.95 2.95 2.95 The gloves will be used to protect anyone’s hands as
1 3.27 3.27 3.28 3.275 they will be using the chemicals and if the substances
2 3.51 3.51 3.52 3.515 do end up spilling onto their hands, they will be
3 3.70 3.70 3.70 3.70 protected, and it is very unlikely any form of irritation
4 3.85 3.85 3.85 3.85 will come to them. If the chemicals do spill onto the
5 3.97 3.97 3.98 3.975 person’s skin for any reason, make sure that they
6 4.07 4.07 4.06 4.065 inform the practical lead immediately and run their
7 4.15 4.15 4.15 4.15 hand under the tap for at least 2-3 minutes.
8 4.24 4.24 4.25 4.245
The goggles are used to protect the person’s eyes from
9 4.34 4.34 4.35 4.345
the chemicals spillage and if any chemicals were to end
10 4.40 4.40 4.40 4.40
up in someone's eyes it could cause painful irritation,
11 4.45 4.45 4.45 4.45
but the chance of eye damage is low due to the
12 4.56 4.56 4.56 4.56
concentration of the acid and base being very low,
13 4.65 4.65 4.65 4.65
however, making sure goggles are worn is imperative to
14 4.70 4.70 4.70 4.70
ensure they stay as safe as possible. If the chemical gets
15 4.75 4.75 4.75 4.75 into their eyes, make sure the practical lead is informed
15.5 4.81 4.81 4.81 4.81 immediately and they will most likely use the eye
16 4.85 4.84 4.82 4.83 cleaning station to clean the eye/eyes out thoroughly to
16.5 4.88 4.88 4.87 4.875 make sure no further damage will come to them.
17 4.98 4.91 4.92 4.915
17.5 5.00 4.98 4.98 4.98 Result table
18 5.05 5.00 5.01 5.005
18.5 5.08 5.05 5.05 5.05
19 5.17 5.08 5.07 5.075
How to work out the half-equivalence point
19.5 5.22 5.17 5.17 5.17
1. First start of by drawing a tangent on the
20 5.36 5.22 5.23 5.225
points where the function changes its direction
20.5 5.45 5.36 5.36 5.36
These lines will need to be parallel.
21 5.64 5.45 5.42 5.43
21.5 5.79 5.48 5.48 5.48
2. Then, draw a line between the two parallel
22 6.00 5.64 5.64 5.64
lines and measure the distance between them in
22.5 6.01 5.79 5.78 5.785
cm.
23 6.02 6.00 6.00 6.00
23.5 6.20 6.01 6.01 6.01
3. We will half the measured distance of the
24 6.55 6.02 6.03 6.025
two parallel lines and draw a line from that point
24.5 7.35 6.20 6.21 6.205 directly to the point on the line of the points that
25 8.74 6.55 6.54 6.545 have been plotted.
25.5 9.40 7.35 7.35 7.35
26 10.10 8.74 8.75 8.745 4. Finally, you can read off the graph and
26.5 11.30 9.40 9.41 9.405 this will give you the volume of sodium hydroxide
27 11.38 10.10 10.10 10.10 solution that was poured in. Half this value and
27.5 11.59 11.30 11.30 11.30 read off the graph. This will give you the pH, which
28 11.67 11.38 11.39 11.385 is equal to the pKa, giving you the value of the pKa.
28.5 11.72 11.59 11.60 11.595
29 11.73 11.67 11.68 11.675 Graph
29.5 11.79 11.72 11.70 11.71
30 11.82 11.79 11.80 11.795
30.5 11.88 11.82 11.83 11.825
31 11.91 11.88 11.89 11.885
31.5 11.94 11.91 11.91 11.91
32 11.96 11.94 11.94 11.94
32.5 12.1 11.96 11.96 11.96
33 12.2 12.1 12.1 12.1
, Derivation
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