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AS Chemistry exam questions > Mock exams mistakes > Flashcards

Mock exams mistakes Flashcards

1
Q

Give an equation, showing structural formulas, for the conversion of chloroethene into poly(chlorothene)

A

H Cl —> [ H Cl ]
n C=C —> [C - C ]
H H —> [ H H ] n

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2
Q

State what you would observe if bromine water was added to poly(chloroethene). Explain this observation.

A

No reaction.
Polymer contains no double bonds / polymer is saturated.

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3
Q

Give the meaning of the term relative atomic mass.

A

The average / mean mass number of an atom (of an element) compared to 1/12 mass of one atom of C12

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4
Q

The sample of chromium is analysed in a TOF mass spectrometer.
Give two reasons why it is necessary to ionise the isotopes of chromium before they can be analysed in a TOF mass spectrometer.

A
  1. Ions will be accelerated by an electric field.
  2. Ions create a current when hitting the detector.
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5
Q

A 53Cr+ ion travels along a flight tube of length 1.25m. The ion has a constant kinetic energy of 1.102 x 10-13 J.
m = mass of ion / kg
V = speed of ion / m s-1.

Calculate the time, in s, for the 53Cr+ ion to travel down the flight tube to reach the detector.

The Avogadro constant, L = 6.022 x 10 23 mol-1. (5)

A

1 mass of ion = 8.8 x 10-26 Kg
(53 / (6.022 x10 23) ) / 1000
/1000 to get from g to Kg

2 v2 = 2KE / m
3 v = √2KE / m
v = √2 x 1.102 x 10-13 / 8.8 x 10-26 = 1.58 x 10 6 (ms-1)
4 v = d/t
5 t = 7.9 x 10-7

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6
Q

Magnesium is used in one of the stages in the extraction of titanium.
Give an equation for the reaction between titanium (IV) chloride and magnesium. State the role of magensium in this reaction.

titanium (IV) shows that it has a +4 charge.

A

EQUATION
TiCl4 + 2Mg -> 2MgCl2 + Ti

ROLE OF MAGNESIUM
Reducing agent (of Ti).

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7
Q

A mixture of magnesium oxide and magnesium hydroxide has a mass of 3200mg.
This mixture is reacted with carbon dioxide to form magnesium carbonate and water. The mass of water produced is 210mg.

Calculate the percentage by mass of magnesium oxide in this mixture.

x1000
->
g to mg
<-
/1000

A

1 moles of water in 210mg
210mg/1000 = 0.210g
mass / mr = 0.210 / 18
=0.0117 mol
equals to moles of MgOH produced in stage 1.

2 mass of Mg(OH)2
0.0117 x 58.3 = 0.680g

3 mass of MgO = 3.2 - 0.68
= 2.52g
3200mg/1000 = 3.2g

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8
Q

State the meaning of the term standard enthalpy of combustion.

A

The enthalpy change when one mole of a substance burns completely in oxygen 1
with all substances in their standard states (stated temp and 100kPa) 2

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9
Q

A student does an experiment to determine the enthalpy of combustion of propan-1-ol (CH3CH2CH2OH, Mr = 60.0)
Combustion of 0.497g of propan-1-ol increases the temperature of 150g of water from 21.2°C to 35.1°C.

Calculate a value, in kJ mol-1, for the enthalpy of combustion of propan-1-ol in this experiment. Specific heat capcaity of water is 4.18 JK-1 g-1.

A

1 q = m c ∆T
q = 150 x 4.18 x 13.9
q = 8715.3 J / 1000
q = 8.7153 kJ

2 n(propan-1-ol) = mass / mr
n = 0.497 / 60.0
n = 0.00828 mol

3
∆H = - (8.7153 / 0.00828)
∆H = - 1050 kJ mol-1

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10
Q

The enthalpy of combustion determined experimentally is less exothermic is less exothermic than that calculated using enthalpies of formation.

Give on possible reason for this, other than heat loss.

A

Incomplete combustion.

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11
Q

Propene reacts with concentrated sulfuric acid to form two isomers, E and F. Structure of E is shown.
CH3 CH CH3
|
O
|
O=S = O
|
OH

Name and outline a mechanism for the formation of E in this reaction.

A

NAME
1 electrophillic addition.

MECHANISM
2 arrow from double bond towards H atom of the H2SO4 molecule.
3 show breaking of the H-O bond in H2SO4.
4 structure of correct carbocation.
5 Show arrow from the lp of e- on negatively charged oxygen of HSO4- towards positively charged atom of their carbocation

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12
Q

State why the student should use sodium hydroxide solution rather than water for the final rinse of the burette.

A

The use of water would dilute NaOH.

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13
Q

Use your answer to part c (0.63%) to find the mass, in mg, of citric acid dissolved in 250cm3 of the solution.
RMM (Mr) of citric acid is 192.0.

A

1 moles of NaOH
23.95/1000 x 0.0500 = 0.001198
2 moles of acid in flask
m1 / 3 x 10 = 0.003992
(divide by 3 as there are 3NaOH)
3 mass acid =
0.003992 x 192.0 = 0.766g
0.766 x 1000 = 766mg

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14
Q

The total uncertainty when using the burette is ±0.15 cm3. This is the combination of uncertainties in the start reading, final reading and the determination of the end point.

Use your answer to part (c) to calculate the percentage uncertainty for the use of the burette in this experiment.

part c = 23.95

A

(uncertantity / mean titre) x 100

(0.15 / 23.95) x 100 = 0.63%

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15
Q

Calculate the percentage purity of this sample of citric acid.

The student dissolved 784mg of impure citric acid in water to prepare 250cm3 of solution in a volumetric flask.

A

(766 / 784) x 100 = 97.7 or 97.8 %

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16
Q

Trichlorofluoromethane (CCl3F) was developed as a refrigerant. The production and use of CCl3F is now restricted.

The equation for a process used to manufacture CCl3F is
SbF3Br2 + CCl4 → CCl3F +SbF2Br2Cl

Calculate the percentage atom economy for the production of CCl3F in this reaction. Give your answer to 3 significant figures.

A

1 12 + 3(35.5) + 19 / 121.8 + 3(19) + 2(79.9) + 12 + 4(35.5)

(137.5 / 492.6) x 100 2 = 27.9%

17
Q

An alternative synthesis of CCl3F is the free-radical substitution reaction between fluoromethane (CH3F) and chlorine.
An intermediate in this alternative synthesis is dichlorofluoromethane (CHCl2F)

Give equations to represent the two propagation steps in the conversion of CHCl2F into CCl3F

A

Propagation step 1
CHCl2F + *Cl -> CHCl2F + *CCl2F + HCl
Propagation step 2
*CCl2F + Cl2 -> CCl3F + *Cl

18
Q

Analysis of the products of this reaction shows the formation of a compound with the empirical formula CCl2F

Give an equation to represent a termination step forming this compound. Show the structural formula of the product in the equation.

A

2*CCl2F -> CCl2FCCl2F

19
Q

A gas cylinder contains 5.0 kg of propane.
How many propane molecules are in the cylinder?
The Avogadro constant, L = 6.022 × 1023 mol–1

A
6.8 × 1022

B
7.2 × 1022

C
6.8 × 1025

D
7.2 × 1025

A

C

20
Q

Which equation represents an initiation step?
A
CH3CH2*CHBr + Br2 -> CH3CH2CHBr2 + *Br

B
O3 + *Cl -> O2 + *ClO

C
RCH2CH2 + H2C=CH2 -> RCH2CH2CH2CH2

D
CH3CFCl2 -> CH3*CFCl + *Cl

A

D

21
Q

Which alcohol can be oxidised by acidified potassium dichromate(VI) but cannot be dehydrated by heating with concentrated sulfuric acid?
A
2,3-dimethylbutan-2-ol

B
2,2-dimethylpropan-1-ol

C
2-methylpropan-2-ol

D
pentan-3-ol

A

B

22
Q

Two sealed flasks with the same volume are left side by side. Flask A contains 4.0 × 10−3 mol of methane. Flask B contains 340 mg of a different gas. Both gases are at the same temperature and pressure. Which gas could be in Flask B?
A
CH2Cl2

B
HBr

C
Kr

D
PF3

A

A

23
Q

An atom has all its electrons in their lowest energy levels. Which atom contains only two unpaired electrons?
A
Helium

B
Beryllium

C
Oxygen

D
Iron

A

C

24
Q

In which pair is the first ionisation energy of atom Y greater than that of atom X?
Electron configuration of atom X
Electron configuration of atom Y

A
EC of atom X EC of atom Y
1s22s2 1s22s22p1

B
EC of atom X EC of atom Y
1s22s22p3 1s22s22p4

C
EC of atom X EC of atom Y
1s22s22p 1s22s22p6

D
EC of atom X EC of atom Y
1s22s22p6 1s22s22p63s1

A

C

25
Q

5.0 g of an oxide contains 4.0 g of molybdenum.
What is the empirical formula of this oxide?
A
MoO2

Page 21 of 43

B
MoO5

C
Mo2O3

D
Mo3O2

A

C

26
Q

When hexadecane (C16H34) is heated to a high temperature, one molecule of hexadecane decomposes to form an alkane containing eight carbon atoms and two different unsaturated compounds. Which equation could represent this reaction?
A
C16H34 → C8H16 + C5H12 + C3H6

B
C16H34 → C8H18 + C6H10 + C2H6

C
C16H34 → C8H18 + 2 C2H4 + C4H8

D
C16H34 → C8H18 + C6H14 + C2H2

A

C

27
Q

Which compound has the highest boiling point?
A
CH3COCH2CH3

B
CH3CH2CH2CH2OH

C
CH3CH2CH2CHO

D
CH3CH2CH2CH2CH3

A

B

28
Q

Methanol is made in this equilibrium reaction, using a catalyst.
CO(g) + 2 H2(g) ⇌ CH3OH(g) ΔH = – 91 kJ mol–1
The reaction reaches equilibrium in a container of fixed volume.
Which is the expression for Kc for this equilibrium?
A
Kc = [CH3OH] / [CO] + [H2]2

B
Kc = [CH3OH] / [CO] + [H2]2

C
Kc = [CO] + [H2]2 / [CH3OH]

D
Kc = [CO] + [H2]2 / [CH3OH]

A

B

29
Q

Which bond has the most unsymmetrical electron distribution?
A
H–O

B
H–S

C
H–N

D
H–P

A

A

30
Q

Which element in Period 3 has the highest melting point?

A
Aluminium

B
Silicon

C
Sodium

D
Sulfur

A

B

31
Q

Which alkene shows E–Z isomerism?

A
2,3-dimethylbut-2-ene

B
4-methylpent-2-ene

C
methylpropene

A

B

32
Q

Which reaction has an enthalpy change equal to the standard enthalpy of formation of lithium fluoride?

A
Li(g) + F2(g) → LiF(s)

B
Li+(g) + F–(g) → LiF(s)

C
Li+(aq) + F–(aq) → LiF(s)

D
Li(s) + F2(g) → LiF(s)

A

D

33
Q

The temperature changed from 21.8 °C to 19.2 °C during a calorimetry experiment.
The uncertainty of each reading of the thermometer is ±0.1 °C
What is the percentage uncertainty in the temperature change?

A
0.5%

B
1.0%

C
3.8%

D
7.7%

A

D

34
Q

Which compound contains a co-ordinate bond?
A
HF

B
NH3

C
CHCl3

D
NH4Cl

A

D

35
Q

Which represents the correct order of increasing radius of the ions?

A
F– O2– Li+ Be2+

B
Li+ Be2+ O2– F–

C
Be2+ Li+ F– O2–

D
O2– F– Li+ Be2+

A

C

36
Q

When heated, a sample of potassium chlorate(V) (KClO3) produced 67.2 cm3 of oxygen, measured at 298 K and 110 kPa

2 KClO3(s) → 2 KCl(s) + 3 O2(g)
What is the amount, in moles, of potassium chlorate(V) that has decomposed?
The gas constant,
R = 8.31 J K–1 mol-1

A
9.95 × 10–4

B
1.99 × 10–3

C
2.99 × 10–3

D
4.48 × 10–3

A

B

37
Q

Which has a bond angle of 109.5°?
A
C (diamond)

B
C (graphite)

C
NH2–

D
NH3

A

A

38
Q

In the test for a halide ion in aqueous solution, dilute nitric acid is added before the addition of silver nitrate solution.
Why is nitric acid added?

A
It increases the concentration of nitrate ions.

B
It prevents the precipitation of silver compounds other than halides.

C
It prevents the silver nitrate being precipitated.

D
It provides the acidic solution required for precipitation.

A

B

AS Chemistry exam questions (2 decks)

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