Evaluation of procedures:
Precision
The precision of each of the measurements you made is indicated by
the % uncertainty calculated.
Accuracy
The accuracy of an overall result (concentration of acid) means how
close it is to the ‘true’ value.
It is possible to make very precise measurements but still end up with
an inaccurate result because of a weakness in the procedure.
You repeat measurements and if they are close then they are said to
be reliable. A single measurement could be an outlier.
This is a stage in the procedure which could have led to an inaccuracy
in the overall result (concentration of acid).
Reliable
Limitation
Points to bring out:
 Do go over the definitions and include why we evaluate both measurements
and procedures. Refer back to enthalpy of combustion in DF and the huge
loss of heat to the surroundings with the apparatus used.
 Ask students to make a list of aspects of the procedure they were careful
about to ensure the overall result (concentration of acid) was as accurate as
possible. Students could fill in the relevant page in the Notes here.
 Talk about washing apparatus. Discuss the conical flask and if reused, what
should / should it not be washed out with.
 Although the new assessments are less demanding in terms of evaluation,
we are still training for Y13 coursework and you might talk about how some
of the limitations might affect the overall result (concentration of the acid).
E.g. If we had overshot the mark on the volumetric flask by not using a
dropping pipette here or we hadn’t washed the burette out with the solution
to be used after the distilled water. Time is tight however.
 You could look at the quantitative effect of not being careful at the endpoint
refered to in the EL activity. Burette (class B) – one drop from a burette has
a volume of approximately 0.05 cm3. All burette readings should include 2
decimal places in which the second figure is either 0 or 5. An error of one
drop in a volume of 25.00 cm3 gives a percentage error of 0.2% for each
reading.
 You could try to pull it all together – measurements and procedures. I think
the most likely source of error is in judging the end point.
A little book of tips for titrations.
 Recording results
 Calculating the average titre
 Evaluation of results
 Evaluation of procedures
Recording results and calculating the average/mean titre:
Titration
Final burette reading/cm3
Initial burette reading/cm3
Titre/cm3
Rough
22.00
0.00
22.00 T
1
43.85
22.00
21.85 √
2
22.95
1.00
21.95 √
3
4
5
Average titre = 21.85 + 21.95 / 2 = 21.90 (quote to 2 decimal places)
Titration
Final burette reading/cm3
Initial burette reading/cm3
Titre/cm3
Rough
22.00
0.00
22.00 T
1
43.85
22.00
21.85 √
2
23.15
1.00
22.15 √
3
44.10
22.15
21.95 √
4
5
Average titre = 21.85 + 22.15 + 21.95 / 3 = 21.98 (there is a rule that you use any titres
within a 0.3cm3 range of the concordant titres to calculate the average)
What is the average titre?
 21.10 + 21.50 + 21.20?
Points to bring out:
 Use a ruler
 Include units
 Readings to 2 d.p. and second number is 0 or 5 – don’t put 0 or 0.0!
 Go through their examples from Act ES5.1 as well as the ones above
 Show working for average see below from Derek Denby
 Use T and √ (THE LATER IF INCLUDED IN CALCULATION OF AVERAGE TITRE)
 Average now to 2 d.p and the second number after the decimal place can be anything.
If a student gets 21.20, 21.50, they need to do another titration because there results are not within 0.1. If they then
get 21.3 they can stop as they have two results within 0.1. They should calculate an average of all 3 results because
they are within 0.3.
If they get 21.20. 21.50 and 21.10 they should stop because they now have two results within 0.1. They should use
only 21.20 and 21.10 to calculate the average because 21.50 is beyond the 0.3 range.
If there were a case where there is more than one way of calculating an average I guess I would suggest that the
student chooses the smallest range.

Evaluation has two aspect – evaluation of measurements and
evaluation of procedures (same as GCSE now)
Evaluation of results:
We look at how precise the measurements are. We look at the uncertainty associated with a
measurement (sometimes called the precision error). Then we calculate the percentage
uncertainty (sometimes called the percentage error).
Percentage uncertainty (percentage error) = uncertainty (precision error) x 100
reading
For graduated equipment the uncertainty / precision error is taken to be half a division on
either side of the smallest unit on the scale you are using. For volumetric (or ungraduated)
measuring equipment you use the precision error associated with it.
Using the balance:
The uncertainty associated with a two decimal place balance reading = ±0.005g
The percentage uncertainty associated with a reading of 119.06g = 0.005 x 100 = 0.004%
119.06
The uncertainty associated with the difference between two balance readings = 2 x 0.005 =
±0.010g
The percentage uncertainty in weighing out a mass of 2.56g = 0.010 x 100 = 0.4%
2.56
Using the 250cm3 volumetric flask:
The uncertainty associated with a volumetric flask reading = ±0.2cm 3
The percentage uncertainty associated with volumetric flask reading = 0.2 x 100 = 0.08%
250
Using the 25cm3 pipette:
The uncertainty associated with a pipette reading = ±0.06cm 3
The percentage uncertainty associated with pipette reading = 0.06
25
x 100 = 0.2%
Using the burette:
The uncertainty associated with a burette readings = ±0.05cm 3
The percentage uncertainty associated with a reading of 25.85cm 3 = 0.05 x 100 = 0.2%
25.85
The uncertainty associated with the difference between two burette readings (a titre)
= 2 x 0.05 = ±0.10cm3
The percentage uncertainty in measuring out a volume of 25.85cm 3 = 0.10 x 100 = 0.4%
25.85
Points to bring out:
 Alternative terminology – need to know both
 Go through how we get double the uncertainty when we have a difference
 The uncertainty to use will be given
 Compare the burette and pipette with the relevant measuring cylinder
 Percentage error will be least for a large reading – what would you do if your titre was
2.00 cm3? Why not have a huge titre of 100 cm3?
 Look at the errors for students own measurements as asked at the end of the activity
 Quote percentage error to 1 s.f – idea that you use the same number of s.f. as the
number with the least s.f.
 May want to have a reminder of s.f. and d.p.??? May be more later
 Make the point that all of these measurements have a low precision error.
 You could mention that in Y13, students often add the precision errors together …