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Hi all!
Since there are only a few months left in CIEs so it would be worth sharing some notes for Chemistry P3.
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Titration:
Burette can be started from any point. The first titre must be from about 26 cm3 because we don't know how much volume will be needed. but rest of them can be started from any value which allows you to measure the volume. Let's say that rough titre was 23 cm3 you can start the next titration from 15 cm3 because you know 15+23=38 which is inside the 50 cm3 range of burette. But if the rough titre is 38 cm3 and you start from 15 cm3 then you will be in trouble.
The two best titre must be consecutive as well as within 0.1 cm3. If first titre was 23.40, second was 23.10 and third was 23.50 then first and second can not be best titres. the reason is obvious. the second titre was more accurate than first and third.
If first two titres are within 0.1 cm3 then there is no need for third titration. If you think that any reading is not correct because it is out of pattern or too far away from line of graph circle that point on graph, again take that reading on the same table but on new row, and then plot new point.
All readings on table must be consistent. All your burette readings must be given to the nearest 0.05 cm3. For example if your volume comes out to be 24.1 cm3, then don't write it as 24.1 cm3 rather write it as 24.10 cm3.
Always beware of endpoints. Whenever you know the end point is near, add the solution in burette drop by drop and close tap when the end point is reached without overly-titrating. An example of good titraion (#1) and overly-titration (#2) (bad titration) is shown on the left.
All readings should be in a single organized table. 1 MARK would be deducted otherwise. An example of a good table for recording readings while performing titrations is given below (you may memorize this table!) :
Initial Burette Reading / cm3
Final Burette Reading / cm3
Titre / cm3
Best Results (✓)
In the table, the headings and their units have separate marks. Write Temperature/C or temperature (C) but T/C or temperature C are not allowed. Full name of quantity must be used. Writing V for volume might be acceptable but ‘T’ is not acceptable as it may have been used for temperature or for time.
Measuring Mass:
Using the Electronic Balance:
Mass of empty test-tube / g
Mass of empty test-tube + Sample X / g
Mass of test-tube + residual Sample X / g
Mass of sample X used / g
Measuring Temperature:
There're only a few tips relating to this measurement. First, always stir the mixture before recording the temperature. Second, when measuring temperature, make sure that the thermometer is not taken out from the solution.
Calculations:
This is an easier yet trickier part of the paper. Mostly, it will involve calculations regarding moles, enthalpy changes etc.
In this part of the the paper, keep the following things in mind:
1 Heat loss to surrounding
* Please note that these are not the only possible errors & improvements. There may be more as well depending upon the nature of the experiment.
Since there are only a few months left in CIEs so it would be worth sharing some notes for Chemistry P3.
-----
Titration:
Burette can be started from any point. The first titre must be from about 26 cm3 because we don't know how much volume will be needed. but rest of them can be started from any value which allows you to measure the volume. Let's say that rough titre was 23 cm3 you can start the next titration from 15 cm3 because you know 15+23=38 which is inside the 50 cm3 range of burette. But if the rough titre is 38 cm3 and you start from 15 cm3 then you will be in trouble.
The two best titre must be consecutive as well as within 0.1 cm3. If first titre was 23.40, second was 23.10 and third was 23.50 then first and second can not be best titres. the reason is obvious. the second titre was more accurate than first and third.
If first two titres are within 0.1 cm3 then there is no need for third titration. If you think that any reading is not correct because it is out of pattern or too far away from line of graph circle that point on graph, again take that reading on the same table but on new row, and then plot new point.
All readings on table must be consistent. All your burette readings must be given to the nearest 0.05 cm3. For example if your volume comes out to be 24.1 cm3, then don't write it as 24.1 cm3 rather write it as 24.10 cm3.
Always beware of endpoints. Whenever you know the end point is near, add the solution in burette drop by drop and close tap when the end point is reached without overly-titrating. An example of good titraion (#1) and overly-titration (#2) (bad titration) is shown on the left.
All readings should be in a single organized table. 1 MARK would be deducted otherwise. An example of a good table for recording readings while performing titrations is given below (you may memorize this table!) :
Initial Burette Reading / cm3
Final Burette Reading / cm3
Titre / cm3
Best Results (✓)
In the table, the headings and their units have separate marks. Write Temperature/C or temperature (C) but T/C or temperature C are not allowed. Full name of quantity must be used. Writing V for volume might be acceptable but ‘T’ is not acceptable as it may have been used for temperature or for time.
Measuring Mass:
Using the Electronic Balance:
- Before measuring any mass, press the "tare" button to make sure that the intial reading is zero.
- Make sure that the electronic balance is clean with no residues on it. Blow lightly if you feel there are any.
- The precision of the electronic balance is up to 2 decimal places e.g. 13.45 g.
- Units: Electronic balance measure in grams, so always write units ("g", "/g", or "grams") beside every reading.
- All recordings should be made in a single table. Otherwise your 1 Mark will be on stake.
- As already stated, the headings and their units have separate marks. Write mass/g or mass (g) but m/g or mass g are not allowed. Full name of quantity must be used.
Mass of empty test-tube / g
Mass of empty test-tube + Sample X / g
Mass of test-tube + residual Sample X / g
Mass of sample X used / g
Measuring Temperature:
There're only a few tips relating to this measurement. First, always stir the mixture before recording the temperature. Second, when measuring temperature, make sure that the thermometer is not taken out from the solution.
Calculations:
This is an easier yet trickier part of the paper. Mostly, it will involve calculations regarding moles, enthalpy changes etc.
In this part of the the paper, keep the following things in mind:
- 3 significant figures:Calculation should be done to 3 significant figures. Remember that there’s a difference
between 3 significant figures and 4 significant figures. 25.6 is 3 significant figures and 25.60 is 4 significant figures & 2 decimal places. 25.659 is 25.7 when read to 3 significant figures. (1 Mark) - Working: Show your working in calculations, and the key steps in your reasoning.
- Calculations involving enthlapy: In calculations involving enthalpy, never forget to put a negative sign (-) before exothermic, and positive sign (+) before endothermic reactions.
1 Heat loss to surrounding
- Lid – prevents convection or evaporation
- Insulation – prevents conduction
- Use plastic beaker – provides insulation
- Use thermometer at 0.5°C or better, gives smaller %error.
- Polystyrene cup – lower heat capacity
- Use larger quantities of reacting substances. e.g. Larger mass of NH4Cl or smaller water volume. Gives greater temperature change
* Please note that these are not the only possible errors & improvements. There may be more as well depending upon the nature of the experiment.