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IMPORTANT NOTES FOR PAPER 5 SKILLS
For planning section: PAPER 5
1. When asked to draw a diagram, ALWAYS mention the volume of the apparatus being used. The one
most often used is gas syringe. Its better to limit the volume of syringe to less than 500 cm3 . Also, the
volume of a commonly used small test tube is around 16 cm3 and that of a boiling tube is about 25 cm3.
We always have to keep the volume of the apparatus we are using while choosing the volume of the
solution for the planning. Saying that we are going to place 50cm3 of aqueous HCl in a test tube is of
course not going to please the examiners. I think I ended up doing the same in one of the papers where
we had to prepare different concentrations of a given solution. I chose volume of water to be added to a
beaker to be 250 cm3. However, the ER stated that the volume should be less than 200cm3 that so being
since the volume of a commonly used glass beaker is around 250 cm3. Someone carrying out this would
have a fully filled beaker. However, we can use greater volume if we state that the volume of the beaker
is 400cm3 or 500 cm3 or so on. So, stating the volume of the apparatus is very important
2. Always while measuring volume of solutions use burette or pipette because they have low
PERCENTAGE error. The question of percentage errors are usually asked in P5. The percentage error
becomes very unacceptable if we are measuring SMALL volumes of solution or small masses. So, a 3dp
balance is much better for measuring masses than a 2dp balance as it would have much less % error
when small masses are being measured
3. While heating crystals strongly, do not use a simple test tube or any apparatus with sharp edges as
they are liable to crack at the sharp edges. For very strong heating to a constant mass, a crucible placed
on a pipe clay triangle is appropriate. And of course to measure the mass of the crucible + solid, you
would have to place it on a balance. But we wouldn't want to fry our poor balance by placing a very hot
crucible on top of it. So, allow the crucible to cool for a few minutes by placing it on a heat mat.
How to prepare crystals (which is also linked with solubility)
Many methods are mentioned in MS but this is the simplest:
CIE usually demands us to prepare crystal after preparing a saturated solution of the crystal.
So, we have to prepare a saturated solution first:
1. Take a fixed volume of water in a beaker of appropriate volume
2. Add the crystal to the water and stir continuously. You have to allow some time for the crystal to
dissolve as it is an equilibrium process
3. After 5 min of stirring, if no solid crystals appear, add further mass of crystal
4. And repeat the process until solid appears in the beaker
5. Filter the solution using a filter paper and funnel so that the saturated solution is collected in a
beaker(whose mass has been measured previously) beneath the funnel
Now we have a saturated solution in a beaker.
How to get the crystals:
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1. We have to place the beaker in a warm water bath. We could use a burner as well but there is a risk of
overheating the solution which could decompose the crystal. If the heat is appropriate, the water of the
solution should evaporate and we should have dry crystals ready.
How to measure solubility:
6. Measure the mass of beaker + solution from step 5
7. Subtract the mass of beaker from the mass in 6 to get the mass of saturated solution
8. And evaporate as shown above to get the mass of crystals.
8. Measure the mass of crystal + beaker
9. Subtract mass of beaker from mass in 8 to get the mass of crystals
10. Subtract the mass of crystals from the mass of saturated solution to get the mass of water in the
solution
11. I have assumed that all the masses are in grams. So, to get the solubility: Mass of crystal x 100/Mass
of water
When you have to remove moisture from:
1. Surface:
Wash the surface with a stream of propanone. The water gets dissolved in the propanone and repeat it
multiple times. Then gently heat the surface to evaporate the propanone from the surface
2. Vapour:
Use desiccants such as:
1. ANHYDROUS sulphuric acid 2. ANHYDROUS calcium chloride 3. Silica gel
You have to pass the vapour from the beaker containing the desiccant Also, a useful property of soda
lime is that it absorbs BOTH water vapour and carbon dioxide
We are also regularly asked to measure enthalpy changes
Most of us already know that we use a plastic cup and thermometer for this purpose However this has
many disadvantages (asked regularly) and here are some of them with the required measures:
1. Heat loss to the surroundings from the beaker:
To avoid this:
a. Cover the plastic cup with a lid
b. Place the cup in a beaker. The air in the beaker acts a good insulator.
c. Use multiple cups so as to thicken the lateral layer of plastic
2. Instability of the cup
a. Place the cup in a glass beaker
3. For exothermic reactions, spray of the solution very likely
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a. Use a large beaker to carry out the experiment rather than the small plastic cup (which has a small
volume)
b. Put a lid on the top of the beaker (this only MINIMISES the spray, doesn't prevent it totally)
4. When we are heating a volume of water in glass beaker, there are two cases of heat loss to consider:
a. Heat loss from the beaker of water
b. Heat loss from the burner heating the volume of water
When preparing a solution of a fixed concentration from a given parent solution of concentration ,
say, 2.0 mol/dm3
A. We are required to prepare a solution of concentration of 1.0 mol/dm3 with volume 250 cm3 ( the
conc cannot be greater than 2. [a]. Add 100 cm3 of the parent solution to a volumetric flask of marking
at 250 cm3 [b]. Top off with water to the mark of 250 cm3. Use a wash bottle for this purpose to have
greater control over the addition process
When required to prepare 250 cm3 of 0.5 mol/dm3 solution of a crystal of Mr 50g
First realize that we need only 250 cm3, not 1 dm3 Now in 250 cm3 there will be: 50/4 = 12.5 g of the
solid So, first add 12.5 g of solid to 50 cm3 of water in a BEAKER (not the volumetric flask yet). Also note
that the volume of water is less than 250 cm3. Stir properly and if the solid doesn't dissolve add more
water until it fully dissolves then transfer the solution from the beaker to a volumetric flask. Remember
to rinse the beaker with water and transfer the solution to the volumetric flask, Stopper the flask and
shake properly. Finally, add the required volume of water to make the solution up to 250 cm3 USE A
FUNNEL OR YOU RISK OVERSHOOTING THE MARK
Titration is accurate because:
1. Standard solution of acid/base is used
2. We obtain concordant titres
3. % error in pipette and burette is very small
4. The end point of a titration is sharp
For filtration:
1. If you require a great separation , use a fluted filter paper
2. If you require a quick filtration, use vacuum filtration
Distillation is a great way of determining boiling point of a liquid
Melting point can be measured using a Mel Temp
We do not need to measure the mass of a reagent or volume of reagent that is in excess
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If you want to flush oxygen out of a system, use an inert gas that doesn't oxidise. Very useful when we
are conducting a reduction experiment of metal oxides
If you want to collect water vapour as water droplet, collect the vapour in a beaker placed in ICE BATH
During magnesium and hydrochloric acid experiment, to avoid loss of gas do not start the experiment by
dropping a piece of Mg in the container of acid!!! Place a test tube containing the magnesium metal in
the solution and pull the string to start the reaction
If a syringe gives wrong volume, it could be because the syringe got stuck during the experiment
For question 2, explanation in O/N 2010 ER is the best ever:
The ER states:
Two points about data tables are common every session. Firstly, candidates need to label each new
column used. These labels require three items – a written label, a correct unit and an expression to be
used to calculate the column data. All of these items are required for credit to be awarded (often the
expression is omitted). Secondly, the data needs to be correctly quoted, to either a specified number of
decimal places or significant figures. In this examination, the data requirement was for 3 significant
figures. Usually, the table head and data requirements are given below the table, so candidates should
be advised to look there before calculating any data. In scaling the axes on a graph plot, candidates
should always consider if the true origin (0,0) would be a point on the line. If so, it should always be
included in the scales since it would always be a reliable, correct point. Two factors need to be taken
into account when scaling the axes. The points need to be plotted within at least half of each axis and
the scale should be reasonable to facilitate accurate plotting. For example data of 1, 2, 4 or 5 to 1 large
square (of 10 small squares) would be reasonable but 3 or 6 or 7 to 1 large square (of 10 small squares)
would not. Some candidates would benefit from practice in fitting a reasonable scale into the grid first
that uses at least half of each axis.
When referring to the reason for an anomalous result, it should be stressed to candidates that they
unambiguously define the point (e.g. Experiment 4) before stating a reason. This is vital when there is
more than one anomaly.
For tables:
Each column heading needs three things, a description (e.g. concentration of acid in water), a unit
(e.g. /mol dm-3) and an expression to calculate the data (e.g. B x 0.1/20) as required in the text
below the table. Also below the table is stated a requirement for a specific number of significant figures
or decimal places. There is a general misunderstanding of the difference between significant figures,
decimal places and trailing zeros. In this case the requirement was for three significant figures. There
was a frequent misunderstanding of this requirement. For example, 0.0105 (3sig figs.) was given as
0.011 (3dp), even to the extent of seeing 0.0105 written but crossed out and replaced by 0.011
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where, presumably, the candidate believes three decimal places is the same as three significant
figures. Also a calculated value of 0.05 needs to reported as 0.0500 if three significant figures are
required. Care needs to be taken with the reporting of data. The two required columns involved a great
deal of calculation. Each column required the calculation of moles of sodium hydroxide, converting this
into moles of succinic acid then a concentration of acid. Only the more able candidates had any success
here. Many managed two parts of these calculations but usually not the 2:1 relationship between
succinic acid and sodium hydroxide. This may have been due to unfamiliarity with succinic acid or its
formula. Quite a significant minority labelled the columns “moles of sodium hydroxide” rather than
concentration in each solvent even though this was stated below the table.
For graph plotting:
The scaling of the axes seems to cause misjudgement in the minds of some candidates. There
seems to be a determination to cover the whole span of the grid regardless of the implications of
the axis scaling necessary to achieve that. An example of difficult scaling is 1 large square being
0.03. This would make each small square (1 mm) 0.003. This creates difficulties for the candidate
in correctly plotting data of the nature of 0.00552 so mis-plotting is often seen when such
inconvenient scales are chosen. Another example of difficult scaling was a 1 mm square being
0.00218 on the x-axis and 0.000223 on the y-axis. Such scales require the extensive use of a
calculator to adequately plot the data which in the very least may take some time. Some
candidates chose a scale for the acid concentration in water that did not accommodate all the
points giving the last point outside the grid. The scaling chosen must adequately cover the
available grid but the minimum requirement is for the plot to cover at least half of the grid in both
directions. So the candidates must make a compromise between scaling and grid coverage to use
at least half of the grid in both directions with a reasonable scale that can be easily used. Much
better scaling for the concentration in the water layer would have been 1 large square for 0.0125 or
0.02 mol dm-3 (depending on which axis) and for the concentration in the ether layer, 1 large square
for 0.001 mol dm-3.
Two more factors need to be considered in graph work. Firstly it is the independent variable that is
to be plotted on the x-axis and the dependent on the y-axis. Almost uniquely in this particular
question either variable could be plotted on either axis. Secondly the data/relationships should
always be examined to see if the origin (0,0) is a valid point. If so (as in this question) then the
scaling used must include the origin and any line drawn would include the origin since it would be a
definite point not subject to any experimental error as the other points may be.
A requirement for determining a slope is the use of construction lines on the plot to produce
co-ordinates for the slope calculation. Many candidates did not draw construction lines.
Hope all this helps with question 2
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If the percentage difference between the measured value and the true value is more than the maximum
apparatus error, this means the experimenter's technique needs modification. If not, the error could be
considered to be entirely due to the intrinsic error or the apparatus used
if the experiment involves plotting a pH against volume curve, the accuracy of the curve can be
increased by using data logger interface and computer combined together to plot the graph
If you want to flush oxygen out of a system, use an inert gas that doesn't oxidise. Very useful when we
are conducting a reduction experiment of metal oxides- EXPLANATION
Suppose you had to reduce CuO. You would have to heat the solid in a container with H2 gas. But we
already have some air (which contains oxygen) present in the container initially. So, the reduced Cu will
keep on getting oxidised until all of the O2 in the container has not reacted and this will mean you will
have to use greater volume of Hydrogen to reduce a given mass of CuO due to re-reduction of the
oxidised Cu. So, it would be best if you could flush O2 out of the apparatus on the first place. If you pass
a stream of H2 into the container, the volume of H2 will eventually displace all of O2. So, there will be no
O2 in the system to re-oxidise Cu
Errors in measuring enthalpy change:
1. Maximum temperature difficult to determine
Measure temperatures often or use a computer and data logger to measure the temperature
2. The temperature of the solution is not uniform
Stir the solution throughout the experiment or use a magnetic stirrer
Also, even if there are no experimental error while measuring the enthalpy change of combustion of
ethanol, we cannot find the accurate value of the enthalpy. The definition of enthalpy requires
formation of liquid water on combustion but in our experiment the water is being turned into
vapour!!!!!!!!
How to purify an organic liquid (such as cyclohexene) that is contaminated with acid:
1. Add NaHCO3 in excess. This reacts with the acid to neutralize it
2. Wash off the NaHCO3 from the organic liquid using water (NaHCO3 is aqueous)
3. Add drying agent to the solution to remove water
4. Distil the solution