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Unit 22 – Assignment 1 – P1 & P2

Results
Mass of filter paper: 1
...
99g
Mass of dried crystals of product: 2
...
05
N of CuSO4 = 0
...
05 * 159
...
975g

Percentage yield = (actual / theoretical) x 100
= (2
...
975) x 100
= 31%

Calculating the percentage purity of hydrated copper(II) sulfate:

1
...

3
...

5
...
1 x 12
...
00124
0
...
0062
0
...
2232
(0
...
32%

Percentage yield = (actual yield/theoretical yield) x 100

Theoretical = n = mass/mr = 2/138 = 0
...
0145 x 180 = 2
...
161/2
...
36 x 100 = 236%

Percentage mass = (overall weight / mass of two tablets) x 100
= (0
...
543) x 100
= 11
...
Firstly, the solution is heated in an evaporating basin;
therefore allowing the solvent to evaporate, which leaves a saturated solution
...
The crystals are then collected and dried
...

Filtration - Passing a solid and liquid mixture through filter paper gets rid of the solvent and allows
excess solvent to evaporate
...
The fine, porous material of the filter
paper does not allow the solid, insoluble particles to go through
...


Determining the percentage purity of aspirin:
Titration – To determine the volume that has been consumed when the endpoint (the point at which
the titration is complete) is reached, a calibrated burette must be used to add the titrant
...
It is also when
the solution permanently changes colour; this is due to an indicator
...
This point is identified from the first faint pink colour that persists in
the solution being titrated
...
There is a
slight difference between the change in indicator colour and the actual equivalence point of the
titration
...
It leaves the solid residue behind
...
It isolated the
product
...

Drying – This is carried out to remove water from a compound
...
A desiccator
can also be used to absorb any moisture and keeping the product dry, from the evaporated solvent,
from the surrounding air
...
This is either
done by converting the substance into an insoluble form or by changing the composition of the
solvent to decrease the solubility of the substance in it
...
This causes the purity to decrease
whilst causing the analysis to be inaccurate
...
An effective technique

is that called homogeneous precipitation, in which the precipitating agent is synthesized in the
solution instead of adding mechanically
...
It leaves the solid residue behind
...
It isolated the product
...
Firstly, the solution is heated in an evaporating basin;
therefore allowing the solvent to evaporate, which leaves a saturated solution
...
The crystals are then collected and dried
...

Evaporation - Evaporation is used to separate a soluble solid from a liquid
...
During evaporation, the water evaporates away leaving solid crystals
behind
...
Then the
volume of the solution would be decreased
...
Once the water has been evaporated, the solid crystals
are left behind
...
It leaves the solid residue behind
...
It isolates the product
...
Crystallisation is normally
done from a hot concentrated solution
...
The solid crystals will become bigger, as the solution is too
concentrated for the solute to remain dissolved at the temperature
...
When cooling the hot,
concentrated solution, more crystals form
...

Re-crystallisation – Reactions will often yield impure products which need to be purified by recrystallisation
...
The technique of re-crystallisation dissolving the product in
the minimum amount of a hot solvent, hot filtration to remove any insoluble impurities and then
allowing crystals of the purified product to form
...
Firstly, the hot solvent is added to the compound
...
Then, the solution is
filtered while is it hot through a warmed filter funnel
...
As
the temperature of the solution decreases, the solubility of the compound also decreases
...
the
compound can then be collected by filtration and dried
...
If the glassware was not
washed or dried properly and there was leftover solution, it would contaminate and dilute the
solution causing the recordings to be inaccurate, invalid and unreliable
...
This means that you will get an accurate
reading and therefore reduce the error
...
So when adding the sulfuric acid to the measuring cylinder, the bottom of the meniscus must
be on the line
...
This would cause the recordings to be
inaccurate and the experiment to be invalid and unreliable
...
Whilst you are doing this, look at the cylinder at eye-level so you can be sure that you
have added the right amount, once the bottom of the meniscus has fallen on the line
...

Furthermore, an error can also be caused by the misjudgement of the amount of liquid evaporated
on a hotplate
...
If no
crystals were to form, the experiment would not be complete, as you will not be able to work out
the percentage yield of the product
...
To
improve the method before putting the filtrate on the hotplate, pour it into a measuring cylinder
and note the volume down
...
If not, pour it back into the evaporating basin and onto the hot plate and
check the volume again when you think it has reduced enough
...
This means that you will minimise the
error and therefore get a higher percentage yield
...
This means that the recorded mass of the crystals would be less than the actual mass of
the crystals
...
This would cause the recorded percentage yield to be inaccurate
...
By doing this, you can ensure that your
percentage yield would be as high as you can make it
...


Determining the percentage purity of hydrated copper(II) sulfate:
During the titration, one can misjudge the colour of the indicator near the end point - this is the
most common error during this procedure
...
To reduce the percentage
error, a number of people should observe the colour and then come to a final result
...

Another error in the titration could be misreading the volume due to parallax error; this is when
someone reads the volume looking at an angle
...

Parallax error is when we misinterpret a value when measuring something, not looking at it at eyelevel
...
To improve the method, when reading the level at which the meniscus fall on, ensure
that you are looking at the burette at eye level
...

When using solutions of the wrong concentration the titrant used may have a different
concentration
...
To reduce
these errors, record the concentration correctly so that it does not affects the later results
...
Do not keep the
titrant for too long and make sure the solution should be kept where it belongs and is fully sealed so
no water is evaporated
...
If all the
solids and liquids are not fully transferred, for example part of the solid was left in the funnel during
transferring it into the flask or it was lost
...
In addition, if the pipette is not clean, come of the solution
would be left inside in the glass in the form of droplets
...


Preparation of aspirin:
Also, when transferring the crystals from the filter paper used in the suction filtration, to the preweighed filter paper there would be a loss of product, as some would still be on the previously used
filter paper
...
Therefore, the using this mass, the calculated percentage yield would be less than the
actual yield that was made
...
To
improve the method, when transferring the crystals from the filter paper used in the suction
filtration, to the pre-weighed filter paper, use a spatula and maybe a few drops of water to ensure
that you scrape and rinse off as many crystals as possibly
...
This means that you will minimise the error
and therefore get a higher percentage yield
...
This would cause the ratio of the liquids added to the
flask to be inaccurate
...
If no crystals were to form, the
experiment would not be complete, as you will not be able to work out the percentage yield of the
product
...
To improve the method, add a
certain amount of drops to the flask (for example, 4), note it down then add the same number again
until you have added 16 drops in total to the flask
...
This means that you will minimise the error
and therefore get a higher percentage yield
...
This could be caused by parallax error
when pouring ethanoic anhydride up to the 4cm3 line on the measuring cylinder
...
So when
adding the ethanoic anhydride to the measuring cylinder, the bottom of the meniscus must be on
the line

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