Download calculations-questions-part

Name: _______________________________________________
Calculations revision qs
part 3
Date:
Time: 1 hour 12 minutes
Total marks available: 72
Total marks achieved: ______
Q1. A solution of nitric acid, HNO3, of concentration 100 g dm–3, can be used to artificially age wood.
A sample of nitric acid, thought to be suitable for this use, was diluted by pipetting 10.00 cm3 of this acid into
a 250 cm3 volumetric flask, adding deionised water and making the solution up to the mark. The solution
was thoroughly mixed.
A titration was carried out using this diluted solution of nitric acid. The burette was filled with 0.0800 mol dm–
3
sodium hydroxide solution and 25.00 cm3 of the diluted nitric acid was pipetted into each of three conical
flasks. The following results were obtained.
The equation for the reaction is
HNO3 + NaOH → NaNO3 + H2O
(a) Select the appropriate titres and calculate the mean titre in cm3.
(1)
(b) Calculate the concentration of the undiluted nitric acid in g dm–3. Give your answer to one decimal
place. Deduce whether this nitric acid is suitable for use in artificially ageing wood.
(5)
(Total for question = 6 marks)
Q2.
*(a) Define the term enthalpy change of neutralization.
(2)
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
(b) The enthalpy change of the neutralization reaction between hydrochloric acid, HCl(aq), and sodium
hydroxide, NaOH(aq), can be determined by the following procedure.
Procedure:
 50.0 cm3 of 2.00 mol dm−3 hydrochloric acid is transferred to a polystyrene cup and its temperature
recorded
 50.0 cm3 of 2.00 mol dm−3 sodium hydroxide solution is placed in another polystyrene cup and its
temperature recorded
 The two solutions are mixed, with stirring, and the maximum temperature is recorded
Results:
Initial temperature of both the HCl(aq) and NaOH(aq)
= 19.0 °C
Maximum temperature reached after mixing
= 32.5 °C
Assumption:
 The specific heat capacity of all aqueous solutions is 4.18 J g−1 °C−1
 The density of all aqueous solutions is 1.00 g cm−3
(i) Calculate the heat energy released (in joules) on mixing the hydrochloric acid and the sodium hydroxide
solutions.
Use the expression
energy released (J) = mass of solution × 4.18 × temperature change
(2)
(ii) Calculate the number of moles of hydrochloric acid used in the experiment.
(1)
(iii) Give the ionic equation, including state symbols, for the reaction between hydrochloric acid and
sodium hydroxide solution.
(1)
(iv) Use your answers to (b)(i), (ii) and (iii) to calculate the enthalpy change of
neutralization for the above reaction.
Include a sign and units in your answer.
(3)
(v) Explain why the enthalpy change of neutralization for the reaction between dilute nitric acid, HNO3(aq),
and potassium hydroxide solution, KOH(aq), is predicted to be the same as the enthalpy change of
neutralization for the reaction carried out in part (b).
(1)
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
(Total for Question = 10 marks)
Q3.
Complexes involve ligands forming dative covalent bonds with a central cation; the number of dative
covalent bonds is the coordination number of the cation in the complex. Complexes may be positive,
negative or neutral. The ligands may be neutral or negatively charged, but are never positively charged. The
ligands must have a lone pair of electrons. In some complexes the central atom is neutral, as is the case
with nickel carbonyl which is used in the Mond process for the purification of nickel:
The formation of complexes is typical of transition metals but other elements also form complexes. Zinc,
which is a d block element but not a transition metal, and aluminium, which is a Group 3 element, both form
complexes. Transition metal complexes are usually coloured, but if the interaction between the ligand and
the central ion is very strong, then the resulting complex may well be colourless. This is the case with the
complex [FeF6]3−.
The ligand in a complex affects its redox properties and this is apparent from considering the relevant
standard electrode potentials. For example, with copper(II) species:
Complexes are industrially important, for example in the purification of nickel mentioned above and in the
extraction of gold which involves the complex [Au(CN)2]−.
Complex formation is used in estimating the concentrations of metal ions in solution. Examples of this are
the use of EDTA in titration and dimethylglyoxime in gravimetric analysis. Complexes are also used in
qualitative analysis, for example, in Tollens' reagent. This reagent is usually prepared by the addition of
sodium hydroxide solution to an aqueous solution of silver nitrate followed by aqueous ammonia. The
resulting solution contains diamminesilver(I) ions.
(a) The hydrated copper(II) ion is classed as a complex but the hydrated sodium ion is not. By considering
the interaction between the water molecules and the sodium ion, explain why the hydrated sodium ion is not
considered to be a complex.
(2)
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
(b) Suggest how the aluminium ion is able to form dative covalent bonds in its complexes such as [AlF6]3−.
(1)
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
*(c) (i) Transition metal complexes are usually coloured. Explain how the colours occur.
(4)
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
(ii) Why are zinc complexes such as [Zn(NH3)4]2+ colourless?
(1)
.............................................................................................................................................
.............................................................................................................................................
(iii) Suggest why the strong interactions between the F− ligands and Fe3+ ion result in the complex
[FeF6]3− being colourless.
(2)
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
(d) Use the standard electrode potentials given in the passage to predict whether it is feasible for
thiosulfate ions, S2O32−, to reduce copper(II) ions to copper. Consider the case when the copper(II) ions are
dissolved in water, and when they are dissolved in aqueous ammonia. Give chemical equations to support
your answer.
(3)
The relevant redox reaction for thiosulfate ions is
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
(e) The proportion of nickel in nisil, an alloy of nickel and silicon used in thermocouples, may be
determined by the following gravimetric analysis.
Finely ground nisil is dissolved in concentrated hydrochloric acid and, after neutralization, excess
dimethylglyoxime dissolved in ethanol is added to the solution formed. The resulting red precipitate is
filtered, washed, dried and weighed. The equation for the formation of the precipitate is
(i) State the coordination number of the nickel in the complex ion.
(1)
.............................................................................................................................................
(ii) In an experiment, 1.02 g of nisil formed 4.82 g of the complex. Calculate the percentage by mass of
nickel in the alloy. The molar mass of the complex is 288.7 g mol−1.
(2)
(Total for question = 16 marks)
Q4. Phosphorus(V) chloride, PCl5, reacts with water according to the equation
PCl5(s) + 4H2O(l) → H3PO4(aq) + 5HCl(aq)
If 1.04 g of phosphorus pentachloride (molar mass = 208 g mol−1) is reacted completely with water and the
solution made up to 1 dm3, the concentration of the hydrochloric acid in mol dm−3 is
A
0.001
C
0.025
B
0.005
D
0.250
(Total for Question = 1 mark)
Q5. This question is about the preparation of the alum, potassium aluminium sulfate,
KAl(SO4)2 12H2O. It is a double salt consisting of potassium ions, aluminium ions and sulfate ions, and
water of crystallization.
(a) The first step of the preparation involves adding an excess of aluminium foil to 10 cm3 of 2 mol dm−3
potassium hydroxide to form potassium aluminate.
The equation for this reaction is
(i) Write a balanced ionic equation for this reaction.
(1)
(ii) Calculate the number of moles of potassium hydroxide used.
(1)
(iii) Hence state the number of moles of aluminium that react with the potassium
hydroxide.
(1)
(iv) Use your answer to (iii) to calculate the mass of aluminium that reacts with the
potassium hydroxide. Use the Periodic Table as a source of data.
(1)
(v) Calculate the total mass of aluminium added to the potassium hydroxide if a
10% excess of aluminium is required.
(1)
(vi) Identify two hazards in this first step of the preparation.
(2)
Hazard 1
.............................................................................................................................................
.............................................................................................................................................
Hazard 2
.............................................................................................................................................
.............................................................................................................................................
(b) The second step of the reaction is the addition of a slight excess of 1 mol dm−3 sulfuric acid.
(i) Balance the following equation for the reaction
(1)
(ii) Calculate the volume of the 1 mol dm−3 sulfuric acid that reacts with the potassium aluminate.
(1)
(iii) State how you would show that the acid had been added in excess.
(2)
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
*(iv) State and explain the steps necessary to obtain pure, dry crystals from the mixture.
(4)
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
(v) Suggest the colour of the crystals.
(1)
.............................................................................................................................................
(vi) Suggest the formula of another metal ion which could form an alum, in combination with
potassium and sulfate ions.
(1)
(Total for question = 17 marks)
Q6. A sample of sodium chlorate(V), NaClO3, was heated and 120 cm3 of oxygen gas was collected.
2NaClO3(s) → 2NaCl(s) + 3O2(g)
Calculate the number of moles of sodium chlorate(V) that were decomposed in the above reaction.
[Molar volume of a gas under the conditions of the experiment = 24000 cm3 mol−1]
A
2.50 × 10−3
C
5.00 × 10−3
B
3.33 × 10−3
D
7.50 × 10−3
(Total for Question = 1 mark)
Q7. The equation for the complete combustion of ethane is
2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(l)
What volume of oxygen, measured at room temperature and pressure, is needed to completely burn 0.1
mol of ethane?
[The volume of 1 mol of any gas measured at room temperature and pressure is 24 dm 3]
A
2.4 dm3
C
8.4 dm3
B
4.8 dm3
D
16.8 dm3
(Total for Question = 1 mark)
Q8. Potassium chlorate(V), KClO3, decomposes on heating as follows.
2KClO3(s) → 2KCl(s) + 3O2(g)
What is the maximum volume of oxygen, measured in dm3 at room temperature and pressure, which could
be obtained by heating 0.50 mol potassium chlorate(V)?
[Molar volume of a gas = 24 dm3 mol−1 at room temperature and pressure.]
A
8
C
36
B
18
D
72
(Total for question = 1 mark)
Q9. What is the number of atoms in 2.8 g of ethene, C2H4?
DATA
 The molar mass of C2H4 is 28 g mol−1
 The Avogadro constant is 6.0 × 1023 mol−1
A
1.0 × 1022
C
1.2 × 1023
B
6.0 × 1022
D
3.6 × 1023
(Total for question = 1 mark)
Q10. Ethanedioic acid has two carboxylic acid groups.
(a) Ethanedioic acid, H2C2O4, can be prepared from ethane-1,2-diol.
Give the reagents and condition required for this reaction.
(2)
Reagents
.............................................................................................................................................
Condition
.............................................................................................................................................
(b) The formula for ethanedioic acid crystals is H2C2O4.nH2O.
To determine the number of moles of water of crystallisation, n, in 1 mol of ethanedioic acid crystals, a
student carried out the following procedure.

Prepare 250.0cm3 of a solution containing a known mass of about 1g of ethanedioic acid crystals.

Titrate 25.0cm3 portions of the ethanedioic acid solution with 0.103 mol dm−3 sodium hydroxide
solution, using phenolphthalein as indicator.
The student obtained these results:
mass of ethanedioic acid crystals = 1.09g
mean titre = 16.20cm3
The equation for the reaction is
H2C2O4 + 2NaOH → Na2C2O4 + 2H2O
(i)
Describe how the student should prepare the 250.0 cm3 of ethanedioic acid solution.
(4)
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
(ii) Give the colour change at the end-point in this titration.
(1)
From ........................................ to ........................................
(iii) Calculate a value of n in the formula H2C2O4.nH2O from these data.
(5)
(iv) The student thought that the ethanedioic acid crystals used may have been slightly damp.
Explain the effect of using damp crystals on the titre and on the value of n.
(2)
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
(Total for question = 14 marks)