Chemistry Solutions 9701/13/O/N/14

Ref: 9701/13/O/N/14 #8
Use of the Data Booklet is relevant to this question.
Ferrochrome is an alloy of iron and chromium. Ferrochrome can be dissolved in dilute sulfuric acid to produce a mixture of FeSO4 and Cr2(SO4)3. The FeSO4 reacts with K2Cr2O7 in acid solution according to the following reaction.

14H+ +6Fe2+ +Cr2O72---->2Cr3+ +6Fe3+ +7H2O

When 1.00 g of ferrochrome is dissolved in dilute sulfuric acid, and the resulting solution titrated, 13.1cm3 of 0.100moldm-3 K2Cr2O7 is required for complete reaction.

What is the percentage by mass of Fe in the sample of ferrochrome?

A. 1.22

B. 4.39

C. 12.2

D. 43.9

Solution

D 43.

The number of moles of K2Cr2O7 required for the reaction is 13.1x0.1/1000=1.31 x 10^-3 mol. From the equation, 1 mol of K2Cr2O7 reacts with 6 mols of Fe. Hence the number of moles of Fe=7.86x10^-3. The mass of Fe present in 1.00 g of ferrochrome= 7.86x10^-3 x 55.8=0.4386g. Hence the percentage by mass of Fe in ferrochrome is 0.4386/1.00 x 100%=43.9%.

Ref: 9701/13/O/N/14 #14
X, Y and Z are compounds of three elements in Period 3. Their electrical conductivities are shown in the table.

What could be compounds X, Y and Z?

Solution
C

For this type of question, look to SiO2 for reference.
SiO2 cannot conduct electricity in any state. Z is very likely to be SiO2. Let's take a look at all the choices for Z.
NaF is ionic, so it can conduct electricity in both molten and aqueous state.
For now, A can be eliminated.
SiCl4 is a covalent compound, which hydrolyses in water to form H+ and Cl-, so it can conduct electricity when added to water, but does not conduct electricity in its molten state due to its covalent nature.
Referring to the properties of SiCl4 and all the choices left for X, we can eliminate D.
Al2O3 is an ionic compound with a lot of covalent character, so it should be able to conduct electricity in both molten and aqueous state.
Y cannot be Al2O3.
The best answer would have to be C.

Ref: 9701/13/O/N/14 #19
A student examines two semi-precious stones; one is agate, SiO2, and the other is calcite, CaCO3.

How could they be distinguished?

A. Add a fixed amount of cold aqueous sodium hydroxide to each separately and measure any temperature change.

B. Heat each separately over a gentle Bunsen flame and note which one melts first.

C. Shake each separately with dilute hydrochloric acid and test any gas formed.

D. Shake each separately with distilled water and add a few drops of Universal indicator.


Solution
C Shake each separately with dilute hydrochloric acid and test any gas formed.
A is incorrect as SiO2 only reacts with hot, concentrated NaOH and CaCO3 does not react with NaOH at all. Hence there will be no reaction between cold NaOH with both compounds.
B is incorrect as SiO2 has a giant molecular structure and CaCO3 has a high thermal stability. Hence both will not melt over a gentle Bunsen flame.
C is correct as SiO2 will not react with HCl (it is an acidic oxide). CaCO3 will react with HCl to produce CO2 gas. Hence HCl can be used to distinguish between these two compounds.
D is incorrect as both SiO2 and CaCO3 are insoluble in water.




Ref: 9701/13/O/N/14 #20
In this question, structural isomerism and stereoisomerism should be considered.
How many isomeric aldehydes have the formula C5H10O?
A. 3
B. 4
C. 5
D. 6


Solution
C 5
Pentanal
2-methylbutanal (2)
3-methylbutanal
2,2-dimethylpropanal
* 2-methylbutanal has a chiral 2nd carbon atom, hence it displays optical isomerism.


Ref: 9701/13/O/N/14 #25
The compound known as 'gamma-linolenic acid' is found in the seeds of the evening primrose plant.

How many cis-trans isomers are there with this structural formula?
A. 3
B. 6
C. 8
D. 12


Solution
C 8
The number of cis-trans isomers can be determined using the formula 2^n, where n represents the number of alkene groups/C=C bonds in the molecule. Hence for this question it is 2^3=8.


Ref: 9701/13/O/N/14 #27
The compound 2-ethyl-3-methylbutanoic acid is used as a flavouring in some food.

Which compound will produce 2-ethyl-3-methylbutanoic acid when heated under reflux with sodium cyanide in ethanol, followed by acid hydrolysis of the reaction product?

Solution
B
When halogenoalkanes are heated under reflux with ethanolic sodium cyanide, there is an addition of a C atom across the chain when the halogen atom is substituted by CN. 2-ethyl-3-methylbutanoic acid has a total of 7 carbon atoms, hence the halogenoalkane used to synthesise it should have 6 C atoms. Option D can be eliminated as it has 7 C atoms.

Ref: 9701/13/O/N/14 #29
The diagram shows the structure of the naturally-occurring molecule cholesterol.

Student X stated that the seventeen carbon atoms in the four rings all lie in the same plane.
Student Y stated that this molecule displays cis-trans isomerism at the C=C double bond.
Which of the students are correct?

A. both X and Y
B. neither X nor Y
C. X only
D. Y only

Solution
B. neither X nor Y
Student X's statement is wrong because not all carbon atoms have sp2 hybridised orbitals (those which do possess trigonal planar structure). Some of the carbon atoms have sp3 hybridised orbitals, and so possess the tetrahedral structure, which means that not all carbon atoms lie in the same plane. (Concept of "same plane" = imagine a flat piece of paper where all the carbon atoms are lying on the piece of paper, none of them are above or below the piece of paper.)
Student Y's statement is wrong because the C=C double bond is part of a ring structure. Cis-trans isomerism cannot exist when both the carbon atoms in the double bond belong to a ring structure due to restriction of rotation.

Ref: 9701/13/O/N/14 #30
Methyl butanoate, C5H10O2, is an ester used in the food industry to give products the flavour of apples.

Including methyl butanoate, how many structural isomers are there of C5H10O2 that are esters?
A. 6
B. 8
C. 9
D. 10

Solution
C. 9
Please refer to the attached diagram for the 9 possible structural isomers.

Ref: 9701/13/O/N/14 #33
Which types of intermolecular forces can exist between adjacent urea molecules?

1 hydrogen bonding
2 permanent dipole-dipole forces
3 instantaneous dipole-induced dipole forces


Solution
A 1, 2 and 3
Hydrogen bonding exists when the hydrogen atoms, attached to the very electronegative N atom in one molecule, accepts lone pairs of electrons from the O or N atoms of other molecules.
The urea molecule is polar due to the polar C=O bond where there is the displacement of the bond pair of valence electrons closer to the more electronegative O atom. Hence the urea molecules can form permanent dipoles with one another.

Ref: 9701/13/O/N/14 #37
Which statements about the photochemical chlorination of ethane are correct?

1. A propagation step in the mechanism is C2H5 + Cl2 -> C2H5Cl + Cl
2. Butane is present in the products.
3. The initiation step is the heterolytic fission of chlorine.

Solution
B. 1 and 2 only
Statements 1 is one of the propagation steps in the mechanism.
Statement 2: Butane is formed according to the following equation: C2H5 + C2H5 -> C4H10. 
Statement 3 is wrong because of the word "heterolytic". Fission of chlorine to form free radicals is homolytic. Heterolytic = to form nucleophiles and electrophiles.