Chapter 2.0
Titration
Including Mole Calculations &
Concentration Experiments
4
2
Aqueous ammonia is a solution of ammonia gas.
A student titrates two samples of aqueous ammonia, A and B, with 0.500 mol / dm3 hydrochloric acid.
The student does four titrations, two with A and two with B.
The student:
rinses and fills a burette with 0.500 mol / dm3 hydrochloric acid
uses a volumetric pipette to add 25.0 cm3 of A to a conical flask
adds five drops of methyl orange indicator to the conical flask
places the conical flask on a white tile
adds 0.500 mol / dm3 hydrochloric acid from the burette while swirling the contents
of the flask, adding drop by drop near the end-point, until the solution just changes
colour
Step 6. empties the conical flask and rinses it with distilled water
Step 7. repeats steps 2 to 6
Step 8. repeats steps 2 to 7 using aqueous ammonia B instead of aqueous ammonia A.
Step 1.
Step 2.
Step 3.
Step 4.
Step 5.
(a) Fig. 2.1 shows the burette readings for the two titrations with A.
Titration 1
Titration 2
0
14
14
29
1
15
15
30
2
16
16
31
initial
final
initial
final
Fig. 2.1
Record the burette readings in Table 2.1.
Complete Table 2.1 with the volume used in each titration.
Table 2.1
Titration 1
Titration 2
final burette reading / cm3
initial burette reading / cm3
volume used / cm3
[3]
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(b) Fig. 2.2 shows the burette readings for one of the titrations with B.
Titration 4
0
34
1
35
2
36
initial
final
Fig. 2.2
Record the burette readings for Titration 4 in Table 2.2.
Complete Table 2.2 with the volume used in each titration.
Table 2.2
Titration 3
final burette reading / cm3
34.9
initial burette reading / cm3
0.2
Titration 4
volume used / cm3
[1]
(c) Explain why the student uses a white tile in these titrations.
............................................................................................................................................. [1]
(d) Calculate the mean volume of 0.500 mol / dm3 hydrochloric acid needed to neutralise 25.0 cm3
of A and of B.
mean volume of acid needed for solution A ........................................................ cm3
mean volume of acid needed for solution B ........................................................ cm3
[1]
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(e) The equation for the reaction between hydrochloric acid and aqueous ammonia is shown.
HCl + NH3
NH4Cl
Use the mean volume of acid needed to neutralise 25.0 cm3 of A in (d) to calculate the
concentration of ammonia in A.
Give your answer to an appropriate number of significant figures.
concentration .......................................... mol / dm3 [2]
(f)
Calculate the volume of ammonia gas measured at room temperature and pressure, r.t.p.,
dissolved in 25.0 cm3 of A.
The volume of one mole of any gas at r.t.p. is 24 dm3.
volume .................................................. dm3 [2]
(g) Use your answers to (d) and (f) to calculate the volume of ammonia gas measured at r.t.p.
dissolved in 25.0 cm3 of B.
volume .................................................. dm3 [1]
(h) The student rinses the burette with 0.500 mol / dm3 hydrochloric acid immediately before it is
filled.
Explain why the student should not rinse the burette with distilled water immediately before it
is filled with 0.500 mol / dm3 hydrochloric acid.
...................................................................................................................................................
...................................................................................................................................................
............................................................................................................................................. [2]
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(i)
Suggest why universal indicator is not suitable for use in these titrations.
............................................................................................................................................. [1]
[Total: 14]
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9
5
Solution K is an aqueous solution of sodium hydroxide, NaOH.
A student determines the concentration of solution K using a method that involves titration.
The student measures 25.0 cm3 of solution K using a pipette.
The student makes up the solution to 250 cm3 with distilled water. This is solution L.
(a) Explain why a pipette is used instead of a measuring cylinder to measure 25.0 cm3 of solution K.
............................................................................................................................................. [1]
(b) Name the container in which solution L is made.
............................................................................................................................................. [1]
(c) A pipette is used to transfer 20.0 cm3 of solution L into a flask.
The pipette is washed out before measuring solution L.
Identify the liquid that is used to wash out the pipette.
............................................................................................................................................. [1]
(d) The student adds two drops of methyl orange to solution L in the flask and then places the
flask on a white tile.
The student fills a burette with 0.100 mol / dm3 sulfuric acid, H2SO4(aq).
The H2SO4(aq) is added to the flask until there is a colour change.
(i)
Explain why the conical flask is placed on a white tile.
..................................................................................................................................... [1]
(ii)
State the colour change of the methyl orange indicator at the end-point.
The colour changes from ............................................. to .......................................... . [1]
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(e) The student does three titrations. The diagrams below show parts of the burette with the
liquid levels both at the beginning and at the end of each titration.
first titration
second titration
third titration
0
22
23
46
9
32
23
24
47
10
33
24
25
48
11
34
1
2
Use the diagrams to complete the table.
titration number
1
2
3
final burette reading
/ cm3
initial burette reading
/ cm3
volume of H2SO4(aq) added
/ cm3
best titration results
(3)
Tick (3) the best titration results in the table.
Use the best titration results to calculate the average volume of H2SO4(aq) added.
................................................... cm3 [4]
(f)
Calculate the number of moles of H2SO4 in the average volume of 0.100 mol / dm3 H2SO4(aq)
added in (e).
................................................... mol [1]
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(g) The equation for the reaction of sodium hydroxide with sulfuric acid is shown.
2NaOH + H2SO4
Na2SO4 + 2H2O
Use this equation to calculate the number of moles of NaOH in 20.0 cm3 of solution L.
................................................... mol [1]
(h) Calculate the number of moles of NaOH in 250 cm3 of solution L.
................................................... mol [1]
(i)
Deduce the number of moles of NaOH in 25.0 cm3 of solution K.
................................................... mol [1]
(j)
Calculate the concentration of solution K in mol / dm3.
........................................... mol / dm3 [1]
(k) A different student does the same experiment using 20 drops of methyl orange instead of
2 drops of methyl orange.
Methyl orange is acidic.
State if the average titration volume of H2SO4(aq) is smaller, larger, or unchanged
when 20 drops of methyl orange are used.
Explain your answer.
...................................................................................................................................................
...................................................................................................................................................
............................................................................................................................................. [2]
[Total: 16]
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7
5
Solution K is dilute sulfuric acid, H2SO4.
A student determines the concentration of solution K using a method that involves titration.
The student measures 20.0 cm3 of solution K using a pipette.
The student makes up the solution to 250 cm3 with distilled water. This is solution L.
(a) Name another piece of apparatus that could be used instead of a pipette to accurately
measure 20.0 cm3 of solution K.
............................................................................................................................................. [1]
(b) Name the container in which solution L is made.
............................................................................................................................................. [1]
(c) A pipette is used to transfer 25.0 cm3 of solution L into a conical flask.
Name the other piece of apparatus that is used with the pipette.
............................................................................................................................................. [1]
(d) The student adds three drops of methyl orange to solution L in the conical flask and then
places the flask on a white tile.
The student fills a burette with 0.100 mol / dm3 potassium hydroxide, KOH(aq).
The KOH(aq) is added to the flask until there is a colour change.
(i)
State which liquid should be used to wash out the burette before filling the burette with
KOH(aq) for use in the titration.
..................................................................................................................................... [1]
(ii)
Explain why the conical flask is placed on a white tile.
..................................................................................................................................... [1]
(iii)
State the colour change of the methyl orange indicator at the end-point.
The colour changes from ........................................... to ........................................... . [1]
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(e) The student does three titrations. The diagrams below show parts of the burette with the
liquid levels both at the beginning and at the end of each titration.
first titration
second titration
45
0
third titration
25
24
11
32
12
33
13
34
46
1
26
25
47
2
27
26
Use the diagrams to complete the following table.
titration number
1
2
3
final burette reading
/ cm3
initial burette reading
/ cm3
volume of KOH(aq)
added / cm3
best titration results
(3)
Tick (3) the best titration results in the table.
Use the best titration results to calculate the average volume of KOH(aq) used.
......................................................... cm3
[4]
(f)
Calculate the number of moles of KOH in the average volume of 0.100 mol / dm3 of KOH(aq)
used in (e).
................................................... mol [1]
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(g) The equation for the reaction of potassium hydroxide with sulfuric acid is shown.
2KOH + H2SO4
K2SO4 + 2H2O
Use this equation to calculate the number of moles of H2SO4 in 25.0 cm3 of solution L.
................................................... mol [1]
(h) Calculate the number of moles of H2SO4 in 250 cm3 of solution L.
................................................... mol [1]
(i)
Deduce the number of moles of H2SO4 in 20.0 cm3 of solution K.
................................................... mol [1]
(j)
Calculate the concentration of solution K in mol / dm3.
........................................... mol / dm3 [1]
(k) A different student does the same experiment using 30 drops of methyl orange instead of
3 drops of methyl orange.
Methyl orange is acidic.
State if the average titration volume of KOH(aq) is smaller, larger, or unchanged when
30 drops of methyl orange are used.
Explain your answer.
...................................................................................................................................................
...................................................................................................................................................
............................................................................................................................................. [2]
[Total: 17]
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3
2
A student investigated the reaction between two different solutions of aqueous sodium carbonate,
solution K and solution L, and dilute hydrochloric acid using two different indicators.
Two experiments were done.
Experiment 1
● A burette was rinsed with water and then with the dilute hydrochloric acid.
● The burette was filled with dilute hydrochloric acid. Some of the dilute hydrochloric acid was
run out of the burette so that the level of the dilute hydrochloric acid was on the burette scale.
● Using a measuring cylinder, 25 cm3 of solution K was poured into a conical flask.
● Five drops of methyl orange indicator and five drops of thymolphthalein indicator were added
to the conical flask.
● The conical flask was placed on a white tile.
● Dilute hydrochloric acid was added slowly from the burette to the conical flask, while the flask
was swirled, until the solution turned yellow. This is the first colour change.
● More dilute hydrochloric acid from the burette was added to the conical flask, while swirling the
flask, until the solution changed colour again. This is the second colour change.
(a) Use the burette diagrams to complete the table for Experiment 1.
1
11
22
2
12
23
3
13
24
initial burette reading
burette reading at
first colour change
burette reading at
second colour change
Experiment 1
burette reading at first colour change / cm3
final burette reading at second colour change / cm3
initial burette reading / cm3
volume of dilute hydrochloric acid added for first
colour change / cm3
total volume of dilute hydrochloric acid added for
second colour change / cm3
[3]
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(b) Experiment 2
● The conical flask was emptied and rinsed with distilled water.
● Experiment 1 was repeated using solution L instead of solution K.
Use the burette diagrams to complete the table for Experiment 2.
3
19
35
4
20
36
5
21
37
initial burette reading
burette reading at
first colour change
burette reading at
second colour change
Experiment 2
burette reading at first colour change / cm3
final burette reading at second colour change / cm3
initial burette reading / cm3
volume of dilute hydrochloric acid added for first
colour change / cm3
total volume of dilute hydrochloric acid added for
second colour change / cm3
[3]
(c) State the colour change observed at the end-point when dilute hydrochloric acid is added to
methyl orange in an alkaline solution.
from ................................................................ to ................................................................ [1]
(d) For Experiment 1, compare the volume of dilute hydrochloric acid needed for the first colour
change with the volume of dilute hydrochloric acid for the second colour change.
....................................................................................................................................................
.............................................................................................................................................. [2]
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(e) Compare the concentration of solution K used in Experiment 1 to the concentration of solution L
used in Experiment 2.
Explain your answer.
....................................................................................................................................................
....................................................................................................................................................
.............................................................................................................................................. [3]
(f) (i)
Deduce the volume of dilute hydrochloric acid needed for the second colour change when
Experiment 2 is repeated using 50 cm3 of solution L.
....................................................................................................................................... [2]
(ii)
State why using 50 cm3 of solution L would cause a problem.
.............................................................................................................................................
....................................................................................................................................... [1]
(g) State the advantage of using a pipette instead of the measuring cylinder in these experiments.
.............................................................................................................................................. [1]
(h) Explain why the conical flask was swirled as the dilute hydrochloric acid was added from the
burette.
....................................................................................................................................................
.............................................................................................................................................. [1]
(i) At the start of Experiment 1, the burette was rinsed with water and then with dilute
hydrochloric acid.
At the start of Experiment 2, the conical flask was rinsed with water but not with solution L.
(i)
Explain why the conical flask was rinsed with water.
.............................................................................................................................................
....................................................................................................................................... [1]
(ii)
Explain why the conical flask was not rinsed with solution L in Experiment 2.
.............................................................................................................................................
....................................................................................................................................... [1]
[Total: 19]
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4
2
A student investigated the reaction between aqueous sodium hydroxide and two different solutions
of dilute hydrochloric acid with different concentrations, labelled Q and R, using two different
indicators.
Three experiments were done.
(a) Experiment 1
● A burette was filled with dilute hydrochloric acid Q. Some of the dilute hydrochloric acid
was run out of the burette so that the level of the dilute hydrochloric acid was on the
burette scale.
● Using a measuring cylinder, 25 cm3 of aqueous sodium hydroxide was poured into a
conical flask.
● Five drops of methyl orange indicator were added to the conical flask.
● The conical flask was placed on a white tile.
● Dilute hydrochloric acid was added slowly from the burette to the conical flask, while the
flask was swirled, until the solution just changed colour.
Use the burette diagrams to complete the table for Experiment 1.
2
20
3
21
4
22
initial reading
final reading
Experiment 1
final burette reading / cm3
initial burette reading / cm3
volume of dilute hydrochloric acid Q added / cm3
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Experiment 2
● The conical flask was emptied and rinsed with distilled water.
● The burette was rinsed with distilled water and then with dilute hydrochloric acid R.
● Experiment 1 was repeated using dilute hydrochloric acid R instead of dilute
hydrochloric acid Q.
Use the burette diagrams to complete the table for Experiment 2.
8
43
9
44
10
45
initial reading
final reading
Experiment 2
final burette reading / cm3
initial burette reading / cm3
volume of dilute hydrochloric acid R added / cm3
Experiment 3
● The conical flask was emptied and rinsed with distilled water.
● Experiment 2 was repeated using thymolphthalein indicator instead of methyl orange
indicator.
Use the burette diagrams to complete the table for Experiment 3.
6
42
7
43
8
44
initial reading
final reading
Experiment 3
final burette reading / cm3
initial burette reading / cm3
volume of dilute hydrochloric acid R added / cm3
[5]
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(b) Determine the simplest whole number ratio of the volumes of dilute hydrochloric acid R used
in Experiment 2 and Experiment 3.
.............................................................................................................................................. [1]
(c) Deduce the volume of dilute hydrochloric acid Q needed when Experiment 1 is repeated using
thymolphthalein indicator instead of methyl orange indicator.
volume of hydrochloric acid Q = .............................. [2]
(d) Compare the concentration of dilute hydrochloric acid Q used in Experiment 1 to the
concentration of dilute hydrochloric acid R used in Experiment 2.
Explain your answer.
....................................................................................................................................................
....................................................................................................................................................
....................................................................................................................................................
.............................................................................................................................................. [3]
(e) State how the results change, if at all, if the aqueous sodium hydroxide is warmed before
adding the dilute hydrochloric acid.
Give a reason for your answer.
effect on results .........................................................................................................................
reason ........................................................................................................................................
[2]
(f) State the advantage of using a pipette instead of the measuring cylinder in these experiments.
.............................................................................................................................................. [1]
(g) Explain why a white tile is used in these experiments.
.............................................................................................................................................. [1]
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(h) At the start of Experiment 2 the burette was rinsed with distilled water and then with dilute
hydrochloric acid R.
(i)
State what was removed from the burette when it was rinsed with distilled water.
....................................................................................................................................... [1]
(ii)
State what was removed from the burette when it was rinsed with dilute hydrochloric acid R.
.............................................................................................................................................
....................................................................................................................................... [1]
(iii)
Explain why the burette does not need to be rinsed at the start of Experiment 3.
....................................................................................................................................... [1]
(i) After the burette was filled with dilute hydrochloric acid at the start of Experiment 1, some of
the acid was run out of the burette.
One reason for running the acid out of the burette is to make sure the level of the hydrochloric acid
is on the scale.
Give one other reason why it is important to run some acid out of the burette after it has been
filled for the first time in an experiment.
....................................................................................................................................................
.............................................................................................................................................. [1]
[Total: 19]
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