Student Number: _________________________
NORTH SYDNEY GIRLS HIGH SCHOOL
2019
HSC
Assessment Task 2
Chemistry
General Instructions
Total Marks – 35
•
Reading Time – 5 minutes
Weighting – 25 %
•
Working Time – 55 min
•
Write using black or blue pen
•
Draw diagrams using pencil
•
Board-approved calculators may be used
•
Write your student number at the top of
this booklet
•
Write your answers in the space
provided.
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Question 1 (5 marks)
The strength of an acid or base is dependent on its ability to ionise in solution. Strong acids ionise
completely and strong bases dissociate completely, while weak acids and weak bases only partially
ionise. The less the ionisation, the weaker the acid/base.
Formula
Per cent ionisation/dissociation
of 0.1 molL-1 at 25°C
Hydrochloric acid
HCl(aq)
100
Citric Acid
C6H8O7(aq)
8.6
Ethanoic acid
CH3COOH(aq)
1.3
Sodium hydroxide solution
NaOH(aq)
100
Full Name
A student conducted an experiment to identify 4 unknown solutions in unlabelled containers. Each
unknown solution was one of the following:
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•
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0.1 molL-1 hydrochloric acid
0.1 molL-1 citric acid
0.1 molL-1 ethanoic acid
0.1 molL-1 sodium hydroxide solution
The student used thymol blue to identify the different solutions. In aqueous solution thymol blue is:
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•
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red/pink below pH 1.2
yellow between 2.8 and 8.0 and
blue in solutions with a pH higher than 9.6.
The students results were:
Unknown Solution
A
B
C
D
Colour with thymol blue
Orange
Yellow
Red
Blue
(a) Using this information, identify each unknown solution A-D
A: ___________________________________
B: ___________________________________
C: ___________________________________
D: ___________________________________
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(b) Calculate the pH of a 0.1 molL-1 ethanoic acid solution if only 1.3% of the molecules have
ionised. Show all working.
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Question 2 (4 marks)
The student conducted an experiment to determine the concentration of some hydrochloric acid.
The student made a 0.100 molL-1 sodium carbonate primary standard solution. Using a 25.0 mL
aliquot of this solution and a suitable indicator, she titrated the acid to end point four times. Her
results are in the table below.
Titre
Test run
1
2
3
Volume titrated to
endpoint (mL)
24.7
23.5
23.4
23.4
(a) Write a formula equation for the reaction that occurred.
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(b) Use the above data to determine the concentration of the hydrochloric acid solution.
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Question 3 (10 marks)
In solution, pale yellow-coloured Fe3+ (aq) and colourless SCN- (aq) form an equilibrium with
FeSCN2+ (aq). FeSCN2+ (aq) is red in colour.
ΔH is negative
Fe3+ (aq) + SCN- (aq) ⇌ FeSCN2+ (aq)
red colour
(a) A student investigates this reaction using separate samples of an equilibrium mixture in
which significant quantities of Fe3+, SCN- and FeSCN2+ are present. In each case changes
are made as indicated in the table below.
Complete the table by placing ticks in the appropriate boxes to indicate the effect of each
change on
i.
The intensity of the red colour of the solution
ii.
The concentration of Fe3+(aq)
once the new equilibrium has been established.
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i. Colour at new
Change to the equilibrium
ii. [Fe3+] at new
equilibrium compared with equilibrium compared with
initial equilibrium
less red
more red
Sample 1
1 drop of concentrated Ag+ (aq) is
added which forms a AgSCN
precipitate
Sample 2
1 drop of a concentrated solution
of Fe3+(aq) is added
Sample 3
1 drop of a concentrated solution
of HPO42- (aq) is added which
forms colourless FeHPO4+(aq)
Sample 4
Addition of a large volume of
water
Question 3 continues on the next page.
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initial equilibrium
Decreased
increased
(b) The reaction is exothermic. The graph below represents the initial concentrations of the ions
at equilibrium. Sketch the changes that would be expected to occur to these concentrations if
the temperature of the equilibrium was increased to a new, constant value.
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2
Question 4 (4 marks)
The diagram below represents a particular acid-base titration.
(a) Identify what type of titration this is (e.g. weak acid/weak base).
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(b) Identify the pH of the equivalence point.
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(c) Which solution would have been in the burette for this titration? Justify your answer.
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2
Question 5 (3 marks)
A solution of 600 mL of KOH of pOH 5 was left to evaporate until the final volume was 30 mL.
(a) Calculate the pOH of the final solution.
2
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(b) Calculate the pH of the final solution.
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Question 6 (4 marks)
Consider the following weak acids and their Ka values
Acid
HA
H2B
HC
H3D
Ka (25°C) for loss of first proton
1.6 x 10-5
4 x 10-7
1 x 10-4
8 x 10-3
(a) Identify the strongest acid.
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(b) Calculate the pH of 0.1 molL-1 HA solution.
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3
Question 7 (5 marks)
Dinitrogen tetroxide, N2O4 (g), dissociates to form nitrogen dioxide, NO2 (g), according to the
equation
N2O4 (g) ⇌ 2NO2 (g)
0.45 moles of N2O4 gas is placed in an empty 1.00 L vessel at 100°C. When the system reaches
equilibrium, there is 0.36 moles of NO2 gas present in the vessel.
(a) Calculate the equilibrium constant for this reaction at 100°C.
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(b) At 25°C, the equilibrium constant for this reaction is 0.144. Is this reaction endothermic or
exothermic? Justify your answer.
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END OF TEST
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