WCH06 01 Que 20160517
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Chemistry
Advanced
Unit 6: Chemistry Laboratory Skills II
Instructions
• Use black ink or ball-point pen.
• centre
Fill in the boxes at the top of this page with your name,
number and candidate number.
• Answer allthequestions.
Answer
• – there may bequestions in the spaces provided
more space than you need.
Information
• The total mark for this paper is 50.
• – use this asfora guide
The marks each question are shown in brackets
as to how much time to spend on each question.
• ideas,
You will be assessed on your ability to organise and present information,
descriptions and arguments clearly and logically, including your use of
grammar, punctuation and spelling.
• A Periodic Table is printed on the back cover of this paper.
Advice
• Keep
Read each question carefully before you start to answer it.
• Try toananswer
eye on the time.
• Check your answers
every question.
• if you have time at the end.
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Answer ALL the questions. Write your answers in the spaces provided.
Test Observations
(i) Write the formula of the complex ion that is present at the end of Test A.
(1)
(ii) Write the formula of the complex ion that is present at the end of Test B.
(1)
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Test Observations
(i) Write an ionic equation, including state symbols, for the formation of the
green precipitate in Test C.
(1)
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(ii) Give the oxidation numbers, including signs, of the d-block element in Test C.
(2)
Before the addition of hydrogen peroxide ..............................................................
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(ii) Deduce the number of moles of each of the other components in the
equilibrium mixture.
(3)
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(iii) Give the expression for the equilibrium constant, Kc, for the reaction
CH3COOC2H5(l) + H2O(l) U C2H5OH(l) + CH3COOH(l)
(1)
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(iv) The equilibrium constant, Kc, is defined in terms of the concentrations in the
equilibrium mixture. However, in this case, Kc can be calculated using moles
rather than concentrations.
Kc = ........................................ . . . . . . . . . . . . . . . . . . . . . .
(v) Explain why it is possible, in this case, to calculate Kc using moles rather than
concentrations.
(1)
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(b) The experiment was repeated by a student whose value for Kc differed from the
value calculated in (a)(iv).
The student made several suggestions to explain this.
State and explain how, if at all, each suggestion would affect the Kc value
obtained by the student, compared with that from (a)(iv).
(i) Suggestion 1 – The concentration of the sodium hydroxide solution used by
the student was less than 0.500 mol dm−3.
(2)
How the student’s value of Kc compared with that from (a)(iv) ........................................................................................ . . . . . . . . . . . . . . . . . . . .
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(ii) Suggestion 2 – The mixture had been left in a warm part of the laboratory and
the equilibrium had been reached at a temperature above room temperature.
(2)
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How the student’s value of Kc compared with that from (a)(iv) ........................................................................................ . . . . . . . . . . . . . . . . . . . .
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(c) A second student repeated the original experiment using exactly the same
method. However, when using the pipette and the burette, the readings were
taken from the top of the meniscus.
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(i) How, if at all, will this affect the volume of the equilibrium mixture delivered
from the pipette? Justify your answer.
(1)
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(ii) How, if at all, will this affect the volume of sodium hydroxide solution
delivered from the burette? Justify your answer.
(1)
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(iii) One of the student’s titres was 42.60 cm3 of sodium hydroxide solution.
The burette has a maximum uncertainty of ±0.05 cm3 for each reading.
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(a) Student A followed the method exactly and obtained 0.021 mol of benzoic acid crystals.
Calculate the percentage yield of the benzoic acid obtained by Student A.
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(d) Student D decided to miss out Steps 4 and 5. This student’s mass of crystals was
greater than the mass of crystals obtained by Student A.
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The benzoic acid obtained by Student A had fewer impurities than the
benzoic acid obtained by Student D.
Name the procedure described in step 4 and 5, and explain why the benzoic acid
prepared by Student A was purer than that obtained by Student D.
(1)
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(e) Student E decided not to carry out Steps 2, 3, 4 and 5. Instead, Student E
evaporated off the water after Step 1 and found that 4.6 g of crystals had been
formed, which was more than the mass obtained by Student A.
One reason why Student E obtained a greater mass of crystals than Student A was
that the product was not benzoic acid.
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Give the structural formula of the organic product that Student E obtained.
(1)
(f ) Student F ran out of time and filtered the solution in Step 5 before it had cooled
properly. A low yield was obtained.
Explain why Student F’s failure to cool fully the solution resulted in a low yield.
(1)
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NO2
NO2
P Q R
(g) A student measured the melting temperature range of one of the separated
products (Sample 1).
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(i) Why are the melting temperatures of Sample 1 and Sample 2 different?
(1)
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(ii) Suggest which isomer is present in these samples. Justify your answer.
(2)
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(h) (i) Can the number of peaks in the low resolution proton nmr spectroscopy be
used to distinguish between the three isomers P, Q and R?
Justify your answer.
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(ii) In a mass spectrum, which m/e value could be used to confirm that only
mono-nitration occurred in this reaction?
(1)
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4 Vanadium is a transition metal and forms ions in which it has oxidation numbers +2,
+3, +4 and +5.
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(b) The student then reduces the vanadium(V) oxide to vanadium(III) oxide, V2O3, by
passing hydrogen over the heated oxide.
Identify the hazard associated with this procedure.
(1)
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(c) The student reacts 1.498g of vanadium(III) oxide, V2O3(s), completely with an
excess of sulfuric acid, H2SO4(aq), to make 250cm3 of a solution, T, containing
aqueous vanadium(III) ions, V3+(aq).
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(i) Calculate the concentration, in mol dm–3, of vanadium(III) ions, V3+(aq), in the
solution T.
(2)
(1)
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(d) The second student decides to make a solution containing VO2+(aq) ions, in which
vanadium has an oxidation number of +4.
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(ii) When the student bubbles sulfur dioxide into a solution containing acidified
VO+2(aq) ions, VO2+(aq) ions are obtained, rather than V3+(aq) ions.
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Suggest why VO2+(aq) ions are produced, rather than the V3+(aq) ions
predicted in (d)(i).
(1)
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(iii) Construct the overall equation for the reduction of acidified VO+2(aq) ions to
VO2+(aq) ions by aqueous sulfur dioxide, SO2(aq).
State symbols are not required.
(1)
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Part 2
Having made solutions of VO2+(aq) and V3+(aq) ions, the students decide to measure
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