chemistry paper 5 tips
TRANSCRIPT
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Post by RGBM211
ermm i found this FOR CHEMISTRY PAPER 5 on studyguide.pk looks extremly helpful.... here yu go
guys....
For planning section:
When asked to draw a diagram, ALWAYS mention the volume of the apparatus being used.
The one most often used is gas syringe. I think it is 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 250cm3.
Someone carrying out my expm 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 very important
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
Shared by zeebujha
Okay guys, just over with P5. Let me put down whatever I understood from the P5s of M/J 2002 to
O/N 2010 into words.
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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 wouldnt 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.
When using a magnesium ribbon or any dirty surface, clean it with SANDPAPER
How to prepare crystals (which is also linked with solubility)
Many methods are mentioned in MS but I found this to be 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:
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
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8. And evaporate as shown above to get the mass of crystals.
9. Measure the mass of crystal + beaker
10. Subtract mass of beaker from mass in 8 to get the mass of crystals
11. Subtract the mass of crystals from the mass of saturated solution to get the mass of
water in the solution
12. 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 dessicants such as:
ANHYDROUS sulphuric acid
ANHYDROUS calcium chloride
Silica gel
You have to pass the vapour from the beaker containing the dessicant
Also, a useful property of soda lime is that it absorbs BOTH water vapour and carbon
dioxide
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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
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, doesnt prevent it
totally)
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
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When preparing a solution of a fixed concentration from a given parent solution of concentration ,
say, 2.0 mol/dm3
We are required to prepare a solution of concentration of 1.0 mol/dm3 with volume 250
cm3 (the concentration cannot be greater than 2 !!!!!!!!!!!!!!)
a. Add 125 cm3 of the parent solution to a volumetric flask of marking at 250 cm3
[1/2 x 250 = 125 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 1 mol/dm3 solution of a crystal of Mr 50g
1. First realize that we need only 250 cm3, not 1 dm3
2. Now in 250 cm3 there will be: 50/4 = 12.5 g of the solid
3. So, first add 12.5 g of solid to 50 cm3 of water in a BEAKER ( not the volumetric flask yet).
4. Also note that the volume of water is less than 250 cm3.
5. Stir properly and if the solid doesnt dissolve add more water until it fully dissolves
6. 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
7. Stopper the flask and shake properly
8. Finally, add the required volume of water to make the solution upto 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
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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:
1. Distillation is a great way of determining boiling point of a liquid
2. Melting point can be measured using a Mel Temp
Some Random points:
We do not need to measure the mass of a reagent or volume of reagent that is in
excess
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!!!
Rather, use a divided flask (I would appreciate it greatly if someone explains what
that actually means) or 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
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
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For Question 2:
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 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.
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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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Titration:
Dont forget to:
- rinse the burette with acid
- rinse the pipette with alkali
- When emptying the pipette into the conical flask, allow it to empty under gravity, and then
touch the surface of the liquid with the pipette for approximately one second
- remove funnel before titrating
- add only two drops of indicator
- swirl mixture during titration
- Wash sides of the flask regularly using a wash bottle
- titrate dropwise near end-point
- read the burette accurately (eyes horizontal, bottom of meniscus)
- record burette reading to 2 decimal places (second 0 or 5)
For better observation:
1. Place a white tile under the conical flask
2. Illuminate the burette while taking the reading
Some more random points:
If asked in an enthalpy experiment what effect does incomplete value of ethanol
have on the measured enthalpy change of combustion:
1. Value decreased
2. Because less C=O bonds formed
Errors in measuring enthalpy change:
1. Maximum temperature difficult to determine (Measure temperatures often or
_ use a computer and datalogger to measure the temperature)
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2. The temperature of the solution is not uniform (Stir the solution throughout the l
_ 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
defintion of enthalpy requires formation of liquid water on combustion but in our
experiment the water is being turned into vapour!!!!!!!!
When asked "Which equipment should be changed so that the accuracy is improved
the most?", always choose the equipment which has the highest percentage error in
the experiment.
The volume of a polystyrene cup used in the lab is roughly 150 cm3