Chemistry Solutions 9701/11/O/N/14

Ref: 9701/11/O/N/14 #4
Two glass vessels M and N are connected by a closed valve.

M contains helium at 20 degree Celsius at a pressure of 1 x 10^5 Pa. N has been evacuated, and has three times the volume of M. In an experiment, the valve is opened and the temperature of the whole apparatus is raised to 100 degree Celsius.

What is the final pressure in the system?

A. 3.18 x 10^4 Pa

B. 4.24 x 10^4 Pa

C. 1.24 x 10^5 Pa

D. 5.09 x 10^5 Pa

Solution

A 3.18 x 10^4 Pa

Let the volume of M be V and the volume of N be 3 V. Initially, the gas helium is contained inside M only. When the valve is opened, the gas spreads to N as well, hence the total volume containing the gas increases from V to 4V. Pressure is inversely proportional to volume. Hence the new pressure in the system is calculated using the formula P1V1=P2V2.

(1 x 10^5)(V)=P2(4V)





By cancelling out V on both sides, the new pressure, P2 when the valve is opened is 25000 Pa. The temperature of the apparatus is raised from 20 to 100 degree Celsius. The new pressure is calculated from the formula P2/T1=P3/T2. Remember to convert degree Celsius to Kelvin by adding 273 for this equation.

25000/293=P3/373

Hence, P3 is 3.18 x 10^4 Pa. The answer is A.

Ref: 9701/11/O/N/14 #6



Aluminium carbide, Al4Cl3, reacts readily with aqueous sodium hydroxide. The two products of the reaction are NaAlO2 and a hydrocarbon. Water molecules are also involved as reactants.

What is the formula of the hydrocarbon?

A. CH4

B. C2H6

C. C3H6

D. C6H12

Solution

A CH4

To solve this question, work on finding the balanced equation for this reaction. The equation is

Al4C3+4NaOH+4H2O--->4NaAlO2+3CH4

So the answer is CH4.

Ref: 9701/11/O/N/14 #15

Use of the Data Booklet is relevant to this question.

A sample of potassium oxide, K2O is dissolved in 250 cm3 of distilled water. 25.0cm3 of this solution is titrated against sulfuric acid of concentration 2.00 moldm-3. 15.0cm3 of this sulfuric acid is needed for complete neutralisation.

Which mass of potassium oxide was originally dissolved in 250cm3 of distilled water?

A. 2.83g

B. 28.3g

C. 47.1g

D. 56.6g

Solution

B 28.3g

The balanced equation for this reaction is K2O+H2SO4---> K2SO4+H2O. The number of moles of sulfuric acid needed for neutralisation is 15x2/1000=0.03 mol. K2O and H2SO4 react in a 1:1 mole ratio, so 25 cm3 of the solution containing dissolved K2O contains 0.03 mol of K20. The number of moles in 250 cm3 of the solution 250/25x 0.03=0.3 mol. The Mr of K2O is 94.2. Hence the mass of K20 originally dissolved=94.2 x 0.3=28.26g.





Ref: 9701/11/O/N/14 #23

Considering only structural isomers, what is the number of alcohols of each type with the formula C5H12O?

Solution

C

Since the question is only asking for structural isomers, we do not need to consider stereoisomers.

Primary

Pentan-1-ol

2-methylbutan-1-ol

3-methylbutan-1-ol

2,2-dimethylpropan-1-ol

Secondary

Pentan-2-ol

Pentan-3-ol

3-methylbutan-2-ol

Tertiary

2-methylbutan-2-ol

Ref: 9701/11/O/N/14 #25

In the hydrolysis of bromoethane by aqueous sodium hydroxide, what is the nature of the attacking group and of the leaving group?

Solution

D

The reaction between bromoethane and sodium hydroxide is nucleophilic substitution. It begins with the nucleophilic attack of OH- from NaOH on the slightly positive carbon atom in the C-Br bond. The leaving group is the Br- ion, which is a species with a lone pair of electrons, thus both of the attacking and leaving groups are nucleophiles.

Ref: 9701/11/O/N/14 #30

B-carotene is responsible for the orange colour of carrots.

B-carotene is oxidised by hot, concentrated, acidified KMnO4.

When an individual molecule of B-carotene is oxidised in this way, many product molecules are formed.

How many of these product molecules contain a ketone functional group?

A. 4

B. 6

C. 9

D.11

Solution

B 6

We need to focus on the carbon atoms in double bonds which have two carbon atoms attached to each of them. In the molecule, there are 8 carbon atoms in double bonds which are able to form ketone functional groups once oxidised by KMnO4. When these double bonds are broken, 6 product molecules containing ketone functional groups will be produced. 4 of these contain one ketone group, whereas the other two (in boxes) contain 2 ketone groups each.

Ref: 9701/11/O/N/14 #32

Use of the  Data Booklet is relevant to this question.

The bond energy of the Br-O bond is 235 kJmol-1.

Which reactions are exothermic?

1 OH+HBr---> H2+BrO

2 OH+HBr---> H2O+ Br

3 H+HBr--->H2+Br2

Solution

C 2 and 3 only

For reaction 1, an O-H bond and a H-Br bond are broken. A H-H bond and Br-O bond are formed. Bond breaking is always endothermic and bond formation is always exothermic. Hence the enthalpy change for the reaction is 460+366-436-235=+155kJmol-1. So the reaction is endothermic.

For reaction 2, a H-Br bond is broken and an O-H bond is formed. Enthalpy change= 366-460=-94kJmol-1. The reaction is exothermic.

For reaction 3, a H-Br bond is broken and a H-H bond is formed. Enthalpy change= 366-436= -70kJmol-1. The reaction is exothermic.

Ref: 9701/11/O/N/14 #36

Which statements about calcium oxide are correct?

1 It reacts with cold water

2 It is produced when calcium nitrate is heated

3 It can be reduced by heating with magnesium

Solution

B 1 and 2 only

Calcium oxide reacts with cold water to produce calcium hydroxide and is produced when calcium nitrate decomposes. Statement 3 is incorrect as calcium is more electropositive than magnesium and is a better reducing agent since it is lower down the group (greater electropositivity), hence calcium oxide cannot be reduced by magnesium.

Ref: 9701/11/O/N/14 #40

A reaction pathway diagram is shown.

Which reactions would have this profile?



1 (CH3)3CBr+NaOH--->(CH3)3COH+NaBr

2 CH3CH2Br + NaOH---> CH3CH2OH+NaBr

3 (CH3)3CCH2CH2Cl+2NH3---> (CH3)3CCH2CH2NH2+NH4Cl

Solution

D 1 only

The reaction pathway diagram shows that for a tertiary halogenoalkane which undergoes the SN1 mechanism for nucleophilic substitution reactions.

Only 1 is correct as (CH3)3CBr is a tertiary halogenoalkane. The halogenoalkanes in 2 and 3 are primary halogenoalkanes which undergo the SN2 mechanism for nucleophilic substitution.