A2 ISA OCR CHEMISTRY
Quantitative Tasks
5 Practical work for A2 Unit F326
This section provides a summary of the practical experience and skills that will be acquired by the
use of the experiments suggested in the modules or by the use of equivalent Tasks devised by the
centre.
It should be noted that the practical experience acquired at AS may be tested at A2.
F324 Rings, Polymers and Analysis
(a) The preparation and purification of a solid organic compound and the measurement of its
melting point as a test for purity.
(b) The preparation of an organic liquid compound (e.g. the preparation of an ester). It
is also expected that candidates will be aware of the procedures involved in the purification and
re-distillation at an appropriate temperature of the product obtained.
(c) Qualitative tests to distinguish between:
(i) saturated and unsaturated compounds using bromine water;
(ii) primary and tertiary alcohols using acidified potassium dichromate;
(iii) phenols and aliphatic alcohols using bromine;
(iv) carbonyl and other functional groups using 2,4-dinitrophenylhydrazine;
(v) aldehydes and ketones using Tollens’ reagent;
(vi) acidic, neutral and basic compounds using indicators and sodium carbonate.
(d) An experiment to illustrate the use and interpretation of thin-layer or paper
chromatography.
(e) The interpretation of IR, mass and NMR spectra (proton and carbon-13)
(Data Sheets will be allowed).
F325 Equilibria, Energetics and Elements
(a) Rate experiments:
(i) involving a measurement of an initial rate and the use of an appropriate graph to determine an order of reaction for a reagent;
(ii) requiring continuous monitoring and the use of an appropriate graph to determine the rate of reaction and the order of reaction for a reagent.
(b) Candidates should be familiar with the procedures involved in an experiment to determine an equilibrium constant.
(c) Experiments to illustrate enthalpy changes of solution and neutralisation.
(d) Experiments to illustrate the use of electrode potentials to predict the likelihood of a reaction taking place. (e.g. simple test-tube experiments such as metal displacement reactions or simple redox processes) .
Candidates should be able to:
• carry out quantitative experiments with appropriate care and precision; • make and record measurements reliably and accurately;
• perform calculations, based on their practical work;
• use units accurately;
• use appropriate numbers of significant figures consistent with their least accurate
measurement;
• construct and interpret appropriate graphs from data collected or provided;
• reach a valid conclusion based upon the data obtained from experiments.
Unit F323
For each candidate, the following apparatus may be required to complete the assessed Tasks:
• Pipettes (10 cm3 and 25 cm3
• Pipette fillers
• One burette (50 cm3
• One volumetric flask (250 cm3,150 cm3 or 100 cm3
• Two conical flasks (250 cm3
• One wash bottle
• Four measuring cylinders (250 cm3
, 100 cm3
, 50 cm3
and 10 cm3
• Evaporating basin (at least 30 cm3
• Beakers (400 cm3
, 250 cm3
and 100 cm3
• Plastic cup for use as a calorimeter
• Thermometer (–10 to +110 o
C) or equivalent
• Stop clocks/watches reading to 1 s or better.
• Pipeclay triangle
• Porcelain crucible + lid
• Test-tubes and boiling tubes
• Test-tube holders
• Dropping pipettes
• Two stands and clamps
• Bunsen burner
• Balance reading to at least two decimal places
• Glass rods
• Heat proof mat
• Tripod and gauze
In addition to the apparatus indicated in Unit F323 above, the following may also be required.
• Melting point apparatus or oil bath/thiele tubes
• Apparatus for filtration under reduced pressure
• Melting point tubes
Appendix 1
Measurements
Useful terms
Accuracy is a measure of the closeness of agreement between an individual test result and the accepted reference value. If a test result is accurate, it is in close agreement with the accepted reference value.
Error (of measurement) is the difference between an individual measurement and the true value (or accepted reference value) of the quantity being measured.
Precision is the closeness of agreement between independent measurements obtained under the same conditions. It depends only on the distribution of random errors (i.e. the spread of
measurements) and does not relate to the true value.
Uncertainty is an estimate attached to a measurement which characterises the range of values within which the true value is asserted to lie. This is normally expressed as a range of values such as 44.0 ± 0.4.
Reliability is the opposite of uncertainty, i.e. if the uncertainty is great; the measurement is not
very reliable.
How accurate are measurements?
When using a digital measuring device (such as a modern top pan balance or ammeter),
• record all the digits shown.
When using a non-digital device (such as a ruler or a burette),
• record all the figures that are known for certain plus one that is estimated.
As a general rule, the uncertainty is often taken to be half a division on either side of the smallest unit on the scale you are using. However, the accuracy of measurements does also depend on the quality of the apparatus used, such as a balance, thermometer or glassware.
For example, a 100 cm3 measuring cylinder is graduated in divisions every 1 cm3
.
• A Class A measuring cylinder has a maximum error of half a division or 0.5 cm3
• A Class B measuring cylinder has a maximum error of a whole division or 1 cm3
.
Because of this variability, assessed Tasks will state the maximum error in any measurement that is being made.
Examples of maximum errors
When glassware is manufactured there will always be a maximum error. This is usually marked on the glassware.
Some examples are shown below.
Note that the actual maximum error on a particular item of glassware may differ from the values given below.
Volumetric or standard flask (Class B)
• A 250 cm3 volumetric flask has a maximum error of 0.2 cm3 or 0.08%. © OCR 2010 29
GCE Chemistry A
Pipette (Class B)
• A 25 cm3 pipette has a maximum error of 0.06 cm3 or 0.24%.
Burette (Class B)
• A pipette has a maximum error of 0.05 cm3 in each measurement.
Some examples
The significance of the maximum error in a measurement depends upon how large a quantity is
being measured. It is useful to quantify this error as a percentage error.
Percentage error = maximum error quantity measured ×100%
For example, a two-decimal place balance may have a maximum error of 0.005 g.
For a mass measurement of 2.56 g,
• percentage error =
0.005
2.56
×100% = 0.20%
• For a mass measurement of 0.12 g, the percentage error is much greater:
Percentage error =
0.005
0.12
×100% = 4.2%
Multiple measurements
For multiple measurements using the same two-decimal place balance, there will be a maximum error of 0.005 g for each measurement.
For two mass measurements that give a resultant mass by difference, there are two maximum
errors:
Percentage error = 2× maximum error in each measurment
quantity measured
×100%
For example, using the same two-decimal place balance,
Mass of crucible + crystals before heat = 23.45 g maximum error = 0.005 g
Mass of crucible + crystals after heat = 23.21 g maximum error = 0.005 g
Mass lost = 0.23 g maximum overall error = 2 x 0.005 g
There is a negligible percentage error in each mass measurement but the overall error in mass
loss is much greater:
Percentage error in mass loss = 2× 0.005
0.23
×100% = 4.3%
Reading burettes
A burette is graduated in divisions every 0.1 cm3
.
A burette is a non-digital device, so we record all figures that are known for certain plus one that is estimated.
Using the half-division rule, the estimation is one of 0.05 cm3
. We therefore record burette measurements to two decimal places with the last figure either ‘0’ or ‘5’.
The maximum error in each measurement = 0.05 cm3
The overall maximum error in any volume measured always comes from two measurements, so
the overall maximum error = 2 x 0.05 cm3 = 0.1 cm3
.
In a titration, a burette will typically deliver about 25 cm3 so the percentage error is small.
• Percentage error = 2× 0.05
25.00
×100% = 0.4%
For small volumes, the percentage error becomes more significant
For delivery of 2.50 cm3
,
• percentage error = 2× 0.05
2.50
×100% = 4%
Recording volumes during titrations
As shown above, each burette measurements should be recorded to two decimal places with the last figure either ‘0’ or ‘5’.
During a titration, it is expected that students will record both initial and final burette readings from which a titre is calculated by difference. It is usual practice to record titration results in a table of the type shown below.
trial 1 2 3
final burette reading / cm3
initial burette reading / cm3
titre / cm3
titres used to calculate mean (tick)
mean titre / cm3
When recording the titre, it is normal practice to use two decimal places. This is what will be expected within the assessment Tasks.
Mean titres
When recording a mean titre, is usual practice to take an average of the concordant titres, i.e.
those that agree to within 0.10 cm3
. Where this is not possible, the two titres that have the closest agreement should be used.
For example, three recorded titres are 25.80 cm3 , 25.30 cm3 and 25.20 cm3
.
The mean titre is the average of the 2nd and 3rd titres which agree to within 0.1 cm3
.
• The mean titre is 25.30+ 25.20
2
cm3
= 25.25 cm3
The overall maximum error is 2 x 0.05 = 0.1 cm3
.
There is a case for arguing that the accumulated errors indicate that one decimal place is more
appropriate but this should not be used. The maximum error is the worst-case scenario and it is
likely that the actual titre will in reality be more accurate than one decimal place.
A student obtaining concordant titres within 0.05 cm3 of one another may encounter a problem
when calculating the mean titre. For example, a student may obtain three recorded titres of 25.80 cm3, 25.25 cm3 and 25.20 cm3
.
The mean titre is 25.25+ 25.20 divided by 2 cm3 = 25.225 cm3
This mean titre has a value that is more accurate than the burette can measure. The value of
25.225 cm3 should more correctly be ‘rounded’ to 25.25 cm3
.
It would seem very unfair not to credit a mean titre of 25.225 cm3 in this case, especially as this student has carried out the titration better than the first student.
What is acceptable in assessed Tasks?
As there are clearly problems with both the accumulated errors argument (leading to a one-decimal place titre) and titres that differ by 0.05 cm3 (leading to a three-decimal place mean titre), the Mark Schemes of assessment Tasks will allow some licence for what is acceptable in the calculation of a mean titre.
• For example, the mean of two titres of 25.25 cm3 and 25.20 cm3 will be allowed as 25.2,
25.20, 25.25 or 25.225 cm3
.
• These values are not all equally valid but the policy will be to give the student the benefit of
the doubt so long as the mean has been calculated from the appropriate values.
How many significant figures should be used?
The result of a calculation that involves measured quantities cannot be more certain than the least certain of the information that is used. So the result should contain the same number of significant figures as the measurement that has the smallest number of significant figures.
A common mistake by students is to simply copy down the final answer from the display of a
calculator. This often has far more significant figures than the measurements justify.
Rounding off
When rounding off a number that has more significant figures than are justified (as in the example above), if the last figure is between 5 and 9 inclusive round up; if it is between 0 and 4 inclusive round down.
For example, the number 350.99 rounded to:
4 sig fig is 351.0
3 sig fig is 351
2 sig fig is 350
1 sig fig is 400
Notice that when rounding you only look at the one figure beyond the number of figures to which
you are rounding, i.e. to round to three sig fig you only look at the fourth figure.
How do we know the number of significant figures?
In the example above, 351 has been rounded to the 2 sig fig value of 350.
However, if seen in isolation, it would be impossible to know whether the final zero in 350 is
significant (and the value to 3 sig figs) or insignificant (and the value to 2 sig figs).
In such cases, standard form should be used and is unambiguous:
• 3.5 x 102
is to 2 sig figs
• 3.50 x 102
is to 3 sig figs
When to round off
It is important to be careful when rounding off in a calculation with two or more steps.
• Rounding off should be left until the very end of the calculation.
• Rounding off after each step, and using this rounded figure as the starting figure for the
next step, is likely to make a difference to the final answer. This introduces a rounding
error.
Students often introduce rounding errors in multi-step calculations.
Example
When 6.074 g of a carbonate is reacted with 50.0 cm3
of 2.0 mol dm–3 HCl(aq) (which is an
excess), a temperature rise of 5.5 °C is obtained. The specific heat capacity of the solution is 4.18
J g–1 K–1
,
The heat produced = 50.0 × 4.18 x 5.5 for which a calculator gives 1149.5 J = 1.1495 kJ
Since the least certain measurement (the temperature rise) is only to 2 significant figures
the answer should also be quoted to 2 significant figures.
Therefore, the heat produced = 1.1 kJ
• It should be noted however, that if this figure is to be used subsequently to calculate the
enthalpy change per mole then the rounding off should not be applied until the final answer
has been obtained.
For example, if the carbonate has a molar mass of 84.3 g mol–1, the enthalpy change per mole of
carbonate can be calculated from the value above.
Using the calculator value of 1.1495 kJ for the heat produced,
• enthalpy per mole = 15.95371255 kJ mol–1
.
• rounding to 2 sig figs gives 16 kJ mol–1
Using the rounded value of 1.1 kJ for the heat produced,
• enthalpy per mole = 15.26671057 kJ mol–1
.
• rounding to 2 sig figs gives 15 kJ mol–1 and we have a ‘rounding error’.
Errors in procedure
The accuracy of a final result also depends on the procedure used. For example, in an enthalpy
experiment, the measurement of a temperature change may be precise but there may be large
heat losses to the surroundings which affect the accuracy of overall result.
Anomalous readings
Where an experiment uses repeated measurements of the same quantity, such as repeated
titration readings, anomalous readings should be identified. If a titre is clearly outside the range of all other readings, it can be judged as being anomalous and should be ignored when the mean titre is calculated.
Similarly, if a plotted graph reveals that a value is anomalous, then it should be ignored.
References
http://www.ocr.org.uk/Images/70646-practical-skills-handbook.pdf
The Royal Society of Chemistry has produced several very helpful documents on measurements and errors, see:
www.rsc.org/education/teachers/learnnet/pdf/learnnet/RSCmeasurements_teacher.pdf
www.rsc.org/pdf/amc/brief13.pdf
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