CHEMISTRY
Titration calculations
HIGHER This whole page covers Higher material
Calculating a concentration
A titration should be repeated to
culate
identify any anomalous values. Cal
the volume of solution added from
rt
the burette by subtracting the sta
h
reading from the end reading in eac
run. Any anomalous volumes should
an
be left out when calculating the me
volume added.
In a titration, you will know the concentration
and volume of one of the reagents. You will
only know the volume of the other reagent –
its concentration will have to be calculated
using the mean volume from the titration. The
worked example below shows you the steps
needed to do this.
25.00 cm3 of sodium hydroxide solution was pipetted into a conical
flask. It was titrated against 0.10 mol dm–3 hydrochloric acid.
The mean volume of acid needed was 24.00 cm3.
(a) Write the balanced equation for the reaction.
HCl + NaOH → NaCl + H2O
(b) Calculate the concentration of sodium hydroxide used
in the titration above.
Divide by 1000 to convert
3
from cm3 to dm .
Notice that you do not need to
use any relative atomic masses
or relative formula masses in
these calculations.
Volume of HCl = 24.00 cm3 ÷ 1000 = 0.024 dm3
Number of moles of HCl = concentration × volume
= 0.10 mol dm–3 × 0.024 dm3 = 0.0024 mol
From the equation, 1 mol of HCl reacts with 1 mol of NaOH, so there will be 0.0024 mol of NaOH
Volume of NaOH = 25.00 cm3 ÷ 1000 = 0.025 dm3
Concentration of NaOH = number of moles ÷ volume
= 0.0024 mol ÷ 0.025 dm3 = 0.096 mol dm–3
1. 25.00 cm3 of potassium hydroxide, KOH,
solution was pipetted into a conical flask.
target
It was titrated against 0.20 mol dm–3 nitric
acid, HNO3.
(a) A group of students carried out the
experiment three times and the volume
of acid used was 25.5cm3, 29.0 cm3
and 27.5 cm3. Calculate the mean
volume of the acid used.
(2 marks)
(b) With another group of students the
mean volume of the acid used was
28.00 cm3. Calculate the concentration
of potassium hydroxide solution.
(3 marks)
A*-C
A titration calculation like this one is usually
worth three marks in the examination. In a recent
examination, fewer than one in ten of students
gained full marks and over half only gained one
mark. Remember that your working out often earns
marks, even if the final answer is wrong.
Students have struggled with this topic
in recent exams - be prepared!
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CHEMISTRY
More calculations
from equations
HIGHER This whole page covers Higher material
Calculating a volume
You can use mole calculations to predict the volume of one reagent that will neutralise a
measured volume of another reagent. To do this, you need to know the concentrations of
both reactants.
(a) Write the balanced equation for the reaction
between sulfuric acid and sodium hydroxide.
H2SO4 + 2NaOH → Na2SO4 + 2H2O
with 2 mol
Notice that 1 mol of H2SO4 reacts
situation
of NaOH. This is different than the
n 1 mol of
when hydrochloric acid is used, whe
H:
HCl reacts with just 1 mol of NaO
HCl + NaOH → NaCl + H2O
(b) What volume of 1.0 mol dm–3 sulfuric acid will be needed to neutralise 25.00 cm3 of 0.8 mol dm–3
sodium hydroxide solution?
vert
Divide by 1000 to con
3
Volume of NaOH = 25.00 cm3 ÷ 1000 = 0.025 dm3
from cm3 to dm .
Number of moles of NaOH = concentration × volume
= 0.8 mol dm–3 × 0.025 dm3 = 0.02 mol
From the equation, 1 mol of H2SO4 reacts
with 2 mol of NaOH, so there will be
0.01 mol of H2SO4
acid and
The ‘mole ratio’ between sulfuric
this is why
–
1:2
sodium hydroxide solution is
is half
the number of moles of sulfuric acid
ide.
rox
hyd
the number of moles of sodium
Volume of H2SO4 = number of moles ÷ concentration
= 0.01 mol ÷ 1.0 mol dm–3 = 0.01 dm3 (or 10 cm3)
Multiply by 1000 to
3
3
convert from dm to cm .
1. Calculate the volume of 0.5 mol dm–3
hydrochloric acid needed to neutralise
target
25.00 cm3 of 0.20 mol dm–3 sodium
hydroxide solution.
(3 marks)
C-A*
3. Calculate the volume of 0.25 mol dm–3
sulfuric acid needed to neutralise 20.00
target
cm3 of 0.10 mol dm–3 sodium hydroxide
solution.
(3 marks)
C-A*
2. Calculate the volume of 0.10 mol dm–3
sodium hydroxide solution needed to
neutralise 20.00 cm3 of 0.25 mol dm–3
hydrochloric acid.
(3 marks)
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CHEMISTRY
Chemistry extended writing 1
When answering extended writing questions you should try to:
✓ apply your scientific knowledge
✓ present your answer in a logical and organised way
✓ write full sentences, and make sure that your spelling, punctuation and grammar are
as good as you can make them.
Ammonium bromide, NH4Br, is a white solid.
Describe chemical tests that you could carry on an unknown solid to identify if it is ammonium bromide.
You should include balanced chemical equations for any tests that you describe.
(6 marks)
Sample answer 1
You can do a flame test for the ammonium ion and the flame goes yellow. To test for
bromide ions, you add silver nitrate. This makes a sort of creamy-white precipitate of
silver bromide.
This is a basic answer. The flame test is not correct – ammonium ions do not give a
flame colour. The test for the bromide ion is correct, although it does not give the other
reagent added. However, it does identify the substance that makes the cream precipitate.
Sample answer 2
Ammonium ion: add sodium hydroxide solution. This makes ammonia gas, which has a nasty
smell. It’s also alkaline.
Bromide ion: add nitric acid and then silver nitrate solution. This makes a creamy
precipitate. The equation is NH4Br (aq) + AgNO3 (aq) → AgBr (s) + NH4NO3 (aq)
This is a good answer. The description of the bromide ion test is very good: the chemical
substances suggested for the test are correct and ‘creamy’ is OK although ‘cream’
is more usual. The equation is also correct, including the state symbols. The test for
the ammonium ion is mostly correct, but there are some important details missing. The
solution usually needs warming, and there should be a test given for ammonia gas (using
moist red litmus paper). These details would make the answer excellent – if you added the
equation for the reaction of ammonium bromide with sodium hydroxide to make ammonia
gas, the examiner would be very impressed indeed!
1. A chemist wants to make a sample of the soluble
salt potassium chloride. He has been given
a solution of potassium hydroxide and some
hydrochloric acid.
Describe how he would make pure, dry crystals of
potassium chloride.
(6 marks)
Remember that the method you
need here is titration. You need
to say how this method works.
Remember that the final crystals
made need to be dry and pure.
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