Chemo P5 tips

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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. Hopefully someone can correct me when I make mistakes and yeah add to what I have written (of course!)

For planning section:

1. 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
 
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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
 
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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.
 
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The points are coming in a random way, so there is not going to be any particular pattern of my posts
 
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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:
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
 
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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:
1. ANHYDROUS sulphuric acid
2. ANHYDROUS calcium chloride
3. 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, 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
 
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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 concn 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 upto 250 cm3 USE A FUNNEL OR YOU RISK OVERSHOOTING THE MARK
 
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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
 
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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. 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
 
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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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