csonn t1 atoms, molecules and stoichiometry

Atoms, molecules and
stoichiometry

Atoms and elements
• An atom is the smallest particle into which an element can be
divided without losing its identity (the smallest indivisible particle of
an element).
• A molecule is a group of atoms (held together by covalent bonding)
which is capable of independent existence.
• Elements are pure substances that contain only one type of atoms.
• Ions are charged species formed as a result of loss of electrons
(cations) or gain of electrons (anions) thus having an overall
electrical charge.
• Isotopes are atoms of the same element that have the same
number of protons but different number of neutrons.

Atomic structure
mass of one proton = ?
mass of one neutron = ?
mass of one electron = ?
Atomic Structure
Details of the three Sub-atomic (fundamental) Particles
Particle Position Relative Mass Relative Charge
Proton Nucleus 1 +1
Neutron Nucleus 1 0
Electron Orbit 1/1800 -1
7
3
Li
Atomic Number
Atomic SymbolMass Number
The atomic number, Z, is the number of protons in the nucleus.
The mass number ,A, is the total number of protons and neutrons in the atom.
Number of neutrons = A - Z
Isotopes Isotopes are atoms with the same number of protons, but different numbers of neutrons.
Isotopes have similar chemical properties because they have the same electronic
structure. They may have slightly varying physical properties because they have
different masses.
THE MASS SPECTROMETER
Electrons
ionise the
Electric field
accelerates the
ions
The mass spectrometer
used to determine all th
isotopes present in a sa
There are var
models for at
structure
ionise the
sample
ions isotopes present in a sa
an element and to ther

Atomic structure
Particle Position Actual mass Relative mass Actual charge Relative charge
Proton Nucleus ? 1 ? +1
Neutron Nucleus ? 1 ? 0
Electron Orbit ? 1/1800 ? -1

Atomic structure - deflection
- A’Level physics
• The force on a charge q moving with a velocity v through a magnetic field B, is
F = Bqv
• Inside mass spectrometer, F = Bqv
• Where deflection occurs, the motion is assumed to be in circular motion.
• The centripetal force acting on an object moving in a circular motion is,
F = mv
2
/r
• Hence, equating the two,
F = Bqv = mv
2
/r
• r = radius of the ion path deflected = mv/Bq
• Electrons and protons have the same charge numerically but deflected in opposite directions.
• Protons is so much heavier than electrons, then r is smaller to maintain constant F - it get
deflected less.

Atomic structure - Q1
A B C D
1, 2 and 3
are
correct
1 and 2
only are
correct
2 and 3
only are
correct
1 only
is
correct
No other combination of statements is used as a correct response.
31 The isotope cobalt-60 ( Co60
27 ) is used to destroy cancer cells in the human body.
Which statements about an atom of cobalt-60 are correct?
1 It contains 33 neutrons.
2 Its nucleus has a relative charge of 27+.
3 It has a different number of neutrons from the atoms of other isotopes of cobalt.
32 The conversion of graphite has only a small positive value of DH.
C (graphite) ® C (diamond) DH = +2.1 kJ mol –1
However, the production of synthetic diamonds using this reaction is very difficult.
Which statements help to explain this?

© UCLES 2005 9701/01/M/J/05
ionisation
energy/kJmol−1 1st 2nd 3rd 4th 5th 6th
A 494 4560 6940 9540 13400 16600
B 736 1450 7740 10500 13600 18000
C 1090 2350 4610 6220 37800 47000
D 1400 2860 4590 7480 9400 53200
4 In which species are the numbers of electrons and neutrons equal?
A Be9
4 B F19
9 C 23
11Na+
D 18
8 O2–

A B C D
1, 2 and 3
are
correct
1 and 2
only are
correct
2 and 3
only are
correct
1 only
is
correct
No other combination of statements is used as a correct response.
31 Use of the Data Booklet is relevant to this question.
The technetium–99 isotope (99
Tc) is radioactive and has been found in lobsters and seaweed
adjacent to nuclear fuel reprocessing plants.
Which statements are correct about an atom of 99
Tc?
1 It has 13 more neutrons than protons.
2 It has 43 protons.
3 It has 99 nucleons.
32 Which of the following solids contain more than one type of chemical bond?
1 brass (an alloy of copper and zinc)
2 graphite

Relative masses
• Carbon-12 (12C) isotope adopted by IUPAC as
reference standard for relative atomic masses.
• On the carbon-12 scale, an atom of the isotope 12C
is assigned a mass value of 12 atomic mass units
(a.m.u.)
• Overall, it is a dimensionless quantity (no physical
units).

Relative masses - Important!
mass of one particle
relative mass of a particle =
1~ x mass of one atom of 12C isotope
The 12C scale is one on which an atom of ~2C isotope is assigned a mass of
exactly 12.000 units.
Carbon is chosen as the standard because it is a very common element which is
easy to store and transport (carbon being solid at room temperature).
The relative atomic mass, Ar, of an element is deﬁned as the ratio of the average
the mass of an atom of ~2C isotope,mass of one atom of the element to
expressed on the ~2C scale.
mass of one atom of an element
± x mass of one atom of 12 C isotope
1 2
OSt~p-by-Step
Advanced Guide - Chemistry

1.1 Atoms, Molecules & Stoichiometry
The relative isotopic mass, Ar, of a particular isotope is deﬁned as the ratio of the
i the mass of an atom of 12C isotope,mass of one atom of the isotope to ]2
expressed on the 12C scale.
mass of one atom of a certain isotope
x mass of one atom of 12C isotope
1 2
The relative molecular mass, Mr, of a substance is deﬁned as the ratio of the average
mass of one molecule of the substance to ~ the mass of an atom of
12C isotope, expressed on the 12C scale.
Mr = mass of one moleculeof the substance± x mass of one atom of ~2C isotope
1 2

The relative isotopic mass, Ar, of a particular isotope is defined as the ratio of the
i the mass of an atom of 12C isotope,mass of one atom of the isotope to ]2
expressed on the 12C scale.
mass of one atom of a certain isotope
x mass of one atom of 12C isotope
1 2
The relative molecular mass, Mr, of a substance is defined as the ratio of the average
mass of one molecule of the substance to ~ the mass of an atom of
12C isotope, expressed on the 12C scale.
Mr = mass of one moleculeof the substance± x mass of one atom of ~2C isotope
1 2
Mr can be simply taken to be the sum of the Ar of all the atoms shown in the formula
of the substance.
e.g. Mr ofCaCO3 = 40.1 + 12.0+ 3(16.0) = 100.1
[NB. Relative atomic masses, Ar, are given in the Periodic Table.]
The relative formula mass of an ionic compound, Mr, is defined as the ratio of the

1 2
Mr can be simply taken to be the sum of the Ar of all the atoms shown in the formula
of the substance.
e.g. Mr ofCaCO3 = 40.1 + 12.0+ 3(16.0) = 100.1
[NB. Relative atomic masses, Ar, are given in the Periodic Table.]
The relative formula mass of an ionic compound, Mr, is deﬁned as the ratio of the
the mass of an atom ofaverage mass of one formula unit of the compound to
12C isotope, expressed on the lZc scale.
Mr = mass of one formula unit of the compound_x x mass of. one atom of 12 C isotope
12
~" The term relative formula mass is used for all ionic and giant molecular
compounds, which do not form simple molecular entities.
~" Mr can be simply taken to be the sum of the Ar of all the atoms shown in the
formula unit of the compound; the charge on the ion is ignored.
e.g. Mr of CO32-= 12.0+3(16.0) = 60.0
[NB. A formula unit refers to the smallest group of atoms from which the formula

Isotopes
• Look for chlorine in the Periodic Table.
• No. of protons = ?
• No. of electrons = ?
• No. of neutrons = ?

Mass spectrometer
• Used to determine the masses of elements and
their isotopes
MASS SPECTROMETER
A mass spectrometer consists of ... an ion source, an analyser and a detector.
Atomic StructureF321
22
Ne
21
Ne
20
NeANALYSER
ELECTRON GUN
& ION SOURCE
DETECTOR

Mass spectrometer
Ion source
• gaseous atoms bombarded by electrons and ionised => 1+ charge ions formed
• resulting ions accelerated out of ion source by electric field
Analyser
• charged particles deflected by a magnetic or electric field
• radius of path depends on m/z ratio
• r = mv/Bq => lighter ions get deflected more
Detector
• mass spectra record the: mass/charge (m/z x-axis) and relative abundance (y-
axis)

Relative atomic masses
• From the Periodic Table, look up chlorine.
• How many protons, neutrons and electrons?
• 18.5 neutrons - does that make sense?
• Actually, 35.5 is the relative atomic mass, NOT
relative isotopic mass!

Relative atomic masses - Q1
Magnesium has several naturally occurring isotopes.
(a) What is meant by the term isotope?
..........................................................................................................................................
..........................................................................................................................................
......................................................................................................................................[2]
(b) Complete the table below for two of the isotopes of magnesium.
isotope
number of
protons
number of
neutrons
number of
electrons
24Mg
26Mg
[2]
A sample of magnesium had the following isotopic composition:
24Mg, 78.60%; 25Mg, 10.11%; 26Mg, 11.29%.
(c) Calculate the relative atomic mass, Ar, of magnesium in the sample.
Express your answer to an appropriate number of significant figures.
[2]

A sample of chlorine containing isotopes of mass
numbers 35 and 37 was analysed in a mass-
spectrometer.
a) The relative abundance of 35Cl is 75.8% and of 37Cl is
24.2%. Calculate relative atomic mass of a chlorine atom.
b) How many peaks corresponding to Cl2
+ were
recorded? Identify them.

The isotopic composition of an element is indicated
below. What is the relative atomic mass of the
element?

Mole concept and the
Avogadro’s constant
One mole of a substance contains as many particles (or elementary entities) of that
substance as there are atoms of carbon in 12 grams (0.012 kg) of carbon-12.
What the particles are must be stated (e.g. atoms, molecules, ions, electrons).
[givenin"Signs,SymbolsandSystematics(TheASECompanionto16-19Science,2000)"]
o
t
The Avogadro constant, L, may be taken as the number of carbon atoms in 12 g of
carbon-12, and has been determined experimentally to have a value of about
6.02 × 1023 mol-~ (3 sf). Equivalently,
One mole of any substance is the amount of substance containing a number of
particles equal to the Avogadro constant (6.02 × 1023).
1 mol of 12C atoms - 6.02 × 1023 12C atoms
1 mol of H20 molecules - 6.02 × 1023 H20 molecules
1 mol of CO32- ions - 6.02 x 1023 CO32- ions
1 mol of electrons - 6.02 × 1023 electrons
A mole of any substance has a mass in grams numerically equal to it's Ar or Mr.
e.g. mass of 1 molof ~2C atoms = 12.0 g
mass of 1 mol of H20 molecules = [2(1.0) + 16.0] g = 18.0 g
mass of 1 mol of CO32- ions = [ 12.0 + 3(16.0)] g = 60.0 g

Avogadro’s constant - Q12
Section A
For each question there are four possible answers, A, B, C, and D. Choose the one you consider to
be correct.
1 Which of these samples of gas contains the same number of atoms as 1g of hydrogen
(Mr : H2, 2)?
A 22g of carbon dioxide (Mr : CO2, 44)
B 8g of methane (Mr : CH4, 16)
C 20g of neon (Mr : Ne, 20)
D 8g of ozone (Mr : O3, 48)
2 Self-igniting flares contain Mg3P2. With water this produces diphosphane, P2H4, which is
spontaneously flammable in air.
Which equation that includes the formation of diphosphane is balanced?
A Mg3P2 + 6H2O ® 3Mg(OH)2 + P2H4
B Mg3P2 + 6H2O ® 3Mg(OH)2 + P2H4 + H2

B 1.2 × 10
23
C 6.0 × 1023
D 3.6 × 1025
15. Avogadro’s Constant is the same as the
number of
A molecules in 16.0 g of oxygen
B atoms in 20.2 g of neon
C formula units in 20.0 g of sodium
hydroxide
D ions in 58.5 g of sodium chloride.

Nickel makes up 20% of the total mass of a coin. The
coin has a mass of 10.0 g.
How many nickel atoms are in the coin?

Analytical chemists can detect very small amounts of
amino acids, down to 3 x 10-21 mol.
How many molecules of an amino acid (Mr = 200)
would this be?

A fullerene molecule consists of 60 carbon atoms.
Approximately how many such molecules are present
in 12 g of this type of carbon?

The mass of 1 mole of sodium is 23 g. What is the
approximate mass of one sodium atom?

Empirical & molecular
formulae
. . . . 111 Atoms, Molecules & Stoichiometry
~,,,, ,. _, ......
4, " Deﬁnition .Of th terms empirical ahd mrlecidar fo~lae~ : i i?~ ;: ? '-:':i" ~;:~ : ;i.:i:-; ;;: !":i)5 i: . i~ ,'- ~ :.
~ Caicu!atiOn. of empirical and molecular formulae from combustion data or
2-..compositionbymass,. ..... .... , . :~:. .-:~.~J.....'.-. :i-~.. :~:: .... -...:-~: ..;
~ . Moi¢~ular formula de~:ived from empirica!fo_rm.u_!a._ _qnﬁ re..!ative m._o_le~la.rZ.~...~i, . . i
[The empirical formula of a compound is the simplest formula which shows the ratio
of the atoms of the different elements in the compound.
]The molecular formula of a compound is one which shows the actual number of
[atoms of each element present in one molecule of a compound.
The molecular formula can be obtained if the empirical formula and Mr are known.
Em
Fo
Mo
Fo
Steps to follow to calculate empirical formula:
1. Change percentage composition to mass, m (in g).
m
2. Divide the mass (m) of each element by its respective relative n =
atomic mass (AO to get the number of moles of each.
3. Divide the number of moles of each element by the smallest number of moles.
This usually gives whole number ratios of moles.
Com
b

Empirical formula - Q1
Which compound is the only gas at room temperature and pressure?
A CH3CH2CH2NH2 Mr = 59.0
B CH3CH2CH2OH Mr = 60.0
C CH2OHCH2OH Mr = 62.0
D CH3CH2Cl Mr = 64.5
Which formula represents the empirical formula of a compound?
A CH4O B C2H4 C C6H12 D H2O2
A washing powder contains sodium hydrogencarbonate, NaHCO3, as one of t
titration, a solution containing 1.00g of washing powder requires 7.15cm3
sulfuric acid for complete reaction. The sodium hydrogencarbonate is the o
reacts with the acid.

Empirical formulae - Q2
In leaded petrol, there is an additive composed of
lead, carbon and hydrogen only. This compound
contains 29.7 % carbon and 6.19 % hydrogen by
mass.
What is the value of x in the empirical formula
PbC8Hx?

Molecular formulae - Q1
5
12 Camphor is a white solid which was used to make the early plastic celluloid. Camphor contains
the same percentage by mass of hydrogen and oxygen.
What is the molecular formula of camphor?
A C10H6O6 B C10H8O C C10H16O D C10H10O2
13 Why is the first ionisation energy of phosphorus greater than the first ionisation energy of silicon?
A A phosphorus atom has one more proton in its nucleus.
B The atomic radius of a phosphorus atom is greater.
C The outer electron in a phosphorus atom is more shielded.
D The outer electron in a phosphorus atom is paired.

Molecular formulae - Q2
– 3 – N06/4/CHEMI/SPM/ENG/TZ0/XX+
1. The empirical formula of a compound is C H O2 4 . Which molecular formulas are possible for this
compound?
I. CH COOH3
II. CH CH CH COOH3 2 2
III. CH COOCH CH3 2 3
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
2. Calcium carbonate decomposes on heating as shown below.

Empirical & molecular
formulae
9
A7 Phosphorus is a non-metal.
This diagram shows the structure of one molecule of phosphorus(III) oxide.
(a) (i) Give the molecular formula of phosphorus(III) oxide.
...................................................................................................................................
(ii) Give the empirical formula of phosphorus(III) oxide.
...............................................................................................................................[2]
(b) Explain why phosphorus(III) oxide has the properties given below.
O
P
For
Examiner’s
Use

Mole concept - using mass
• For pure substances (elements, compounds) with
given mass,
mole = mass/molar mass [UNITS?]
• Also note that
density = mass/volume [UNITS?]

Mole concept - using
concentration
• Solutions refer to substances dissolved in water.
• What is the unit of concentration? deﬁnition?
• Given concentration and volume,
mole = concentration (mole/dm3) * volume (dm3)
mole = mass concentration (g/dm3) / Mr (g/mole)

Mole concept - using molar
volume of gases
• What is molar volume? unit?
• Conditions?
• Room temperature and pressure (r.t.p.)
=> 25°C (298K), 1 atmosphere, molar volume = 24.0 dm
3
/mole
• Standard temperature and pressure (s.t.p.)
=> 0°C (273K), 1 atmosphere, molar volume = 22.4 dm
3
/mole
• Given volume of gases, at r.t.p.
• mole = volume (dm
3
) / 24 (dm
3
/mole)
• mole = volume (cm
3
) / 24 000 (cm
3
/mole)

Mole concept - Q1
What is the number of molecules in 500 cm3 of
oxygen under room conditions?

Mole concept - Q2
Section A
For each question there are four possible answers, A, B, C, and D. Choose the one you consider to
be correct.
Titanium(IV) oxide, TiO2, is brilliantly white and much of the oxide produced is used in the
manufacture of paint.
What is the maximum amount of TiO2 obtainable from 19.0 tonnes of the ore ilmenite, FeTiO3?
A 10.0 tonnes B 12.7 tonnes C 14.0 tonnes D 17.7 tonnes
2 Carbon disulphide vapour burns in oxygen according to the following equation.
CS2(g) + 3O2(g) → CO2(g) + 2SO2(g)
A sample of 10 cm3
of carbon disulphide was burned in 50 cm3
of oxygen. After measuring the
volume of gas remaining, the product was treated with an excess of aqueous sodium hydroxide
and the volume of gas measured again. All measurements were made at the same temperature
and pressure, under such conditions that carbon disulphide was gaseous.
What were the measured volumes?
volume of gas
after burning/cm3
volume of gas after
adding NaOH(aq)/cm3
A 30 0

Mole concept - Q3
What could be the identities of X, Y and Z?
X Y Z
A Br2 Al Si
B Br2 Na Mg
C I2 Mg Na
D I2 Si K
Use of the Data Booklet is relevant to this question.
Lead(IV) chloride will oxidise bromide ions to bromine. The Pb4+
ions are reduced to Pb2+
ions in
this reaction.
If 6.980 g of lead(IV) chloride is added to an excess of sodium bromide solution, what mass of
bromine would be produced?
A 0.799g B 1.598g C 3.196g D 6.392g
Which element has an equal number of electron pairs and of unpaired electrons within orbitals of
principal quantum number 2?
A beryllium
B carbon

Percentage by mass - Q1
© UCLES 2013 9701/13/M/J/13
A CH4O B C2H4 C C6H12 D H2O2
A washing powder contains sodium hydrogencarbonate, NaHCO3, as one of the ingredients. In a
titration, a solution containing 1.00g of washing powder requires 7.15cm3
of 0.100moldm–3
sulfuric acid for complete reaction. The sodium hydrogencarbonate is the only ingredient that
reacts with the acid.
What is the percentage by mass of sodium hydrogencarbonate in the washing powder?
A 3.0 B 6.0 C 12.0 D 24.0

Percentage by mass - Q2
2.0 g of an iron wire was dissolved in excess dilute
sulphuric acid and the solution was made up to 200
cm3. 25.0 cm3 of this solution needed 26.45 cm3 of a
0.0188 mol dm-3 potassium manganate (VII) solution
for oxidation.
Calculate the percentage of iron in the iron wire.

Limiting and excess reactant
• Excess reactant - not completely used up at the end
of reaction
• Limiting reagent - completely used up at the end of
the reaction - it determines the yield of the reaction

Percentage yield
• The percentage yield is a measure of the efﬁciency
of the reaction.
• % yield = actual yield/theoretical yield x 100%
• actual yield uses mole value for limiting reactant

- Q1
[X012/12/02] Page three
3. A positively charged particle with electron
arrangement 2, 8 could be
A a neon atom
B a fluoride ion
C a sodium atom
D an aluminium ion.
B an increase in the enthalpy change
C a decrease in the activation energy
D more molecules per second forming an
activated complex.
7. Calcium carbonate reacts with nitric acid as follows.
CaCO3(s) + 2HNO3(aq) → Ca(NO3)2(aq) + H2O(ℓ) + CO2(g)
0·05 mol of calcium carbonate was added to a solution containing 0·08 mol of nitric acid.
Which of the following statements is true?
A 0·05 mol of carbon dioxide is produced.
B 0·08 mol of calcium nitrate is produced.
C Calcium carbonate is in excess by 0·01 mol.
D Nitric acid is in excess by 0·03 mol.
[Turn over

- Q1

- Q2
29 The product of the reaction between propanone and hydrogen cyanide is hydrolysed under acidic
conditions.
What is the formula of the final product?
A CH3CH(OH)CO2H
B CH3CH2CH2CO2H
C (CH3)2CHCONH2
D (CH3)2C(OH)CO2H
Ethyl ethanoate can be obtained from ethanoic acid and ethanol by the following reaction.
CH3CH2OH + CH3CO2H CH3CO2CH2CH3 + H2O
Ethanol (30 g) and ethanoic acid (30 g) are heated under reflux together, and 22 g of ethyl
ethanoate are obtained.
What is the yield of the ester?
A 25% B 38% C 50% D 77%

- Q2

Common reactions
• Acid-base reactions
• Redox reactions
• Precipitation reactions

Balancing equations - Q1
Ammonium sulphate in nitrogeneous fertiliser in the
soil can be slowly oxidised by air producing sulphuric
acid, nitric acid and water.

Balancing equations - Q2
The petrol additive tetraethyl-lead(IV), Pb(C2H5)4, is
now banned in many countries. When it is completely
burned in air, lead(II) oxide, CO2 and H2O are formed.

Conservation of mass
During any physical or chemical change, the total
mass of the products remains equal to the total mass
of the reactants (in a closed system - no products
escape off)

Conservation of mass - Q1
Given this reaction,
HgO (s) —> Hg (l) + ½O2 (g)
If you start with 100 g of HgO and you end up with
92.6 g of Hg, what is the mass of oxygen formed?

Calcium sulphate can exist in the hydrated form of
gypsum, CaSO4.2H2O. When it is heated, gypsum
loses some water to form plaster of Paris, which is
used to make plaster casts.
If 100 g of gypsum loses 15.6 g of water, ﬁnd the
formula of plaster of Paris.

A 5.00 g sample of an anhydrous Group II metal
nitrate loses 3.29 g in mass on strong heating.
Identify the Group II metal present.
N.B. Group II metal nitrate decomposes to give
metal oxide, nitrogen dioxide and oxygen.

Hydrocarbon combustion
• What is hydrocarbon?
• What are the products of complete combustion?

Complete combustion of
hydrocarbons
1.1 Atoms, Molecules & Stoichiometry
The molecular formula of hydrocarbons can be determined by combustion in excess
oxygen (to form carbon dioxide and steam). A gaseous hydrocarbon, CxHy, explodes
with excess 02 according to the general equation.
Y Y H 2 0
C x H y + ( x + - ~ ) 0 2 - ~ x C O 2 + ~ -
Molecular formula is a multiple of empirical formula, and can be determined given
both the empirical formula and the mass of one mole of a compound (or Mr value).
e.g. To calculate the molecular formula given the following combustion data:
(i) 10 cm3 of a gaseous hydrocarbon C required 20 cm3 of oxygen for complete
combustion. 10 cm3 of carbon dioxide was produced.

Hydrocarbon combustion
• This is the general equation for combustion of hydrocarbon
only, (remember by heart)
CxHy + (x+y/4)O2 —> xCO2 + y/2H2O
• With many of these calculation questions, make the
following assumptions,
a) vol. of gases = no. of mole of gases
b) vol. of liquid water = 0

Hydrocarbon combustion -
Q1
0.200 mole of a hydrocarbon undergo complete
combustion to give 35.2 g of carbon dioxide and 14.4
g of water as the only products.
What is the molecular formula of the hydrocarbon?

Q2
10 cm3 of gaseous hydrocarbon required 20 cm3 of
oxygen gas for complete combustion under standard
temperature and pressure (STP). 10 cm3 of carbon
dioxide was produced.
What is the molecular formula of the hydrocarbon and
hence it’s Mr?

Q3
What volume of oxygen, at room temperature and
pressure, is needed for complete combustion of 0.1
mol of ethanol?

Q4
A pure hydrocarbon is used in bottled gas for cooking
and heating. When 10 cm3 of the hydrocarbon is
burned in 70 cm3 of oxygen (an excess), the ﬁnal
gaseous mixture contains 30 cm3 of carbon dioxide
and 20 cm3 of unreacted oxygen.
All gaseous volumes were measured under
identical conditions.
What is the formula of the hydrocarbon?

Q5
Carbon disulphide vapour burns in oxygen according to the following
equation.
CS2 (g) + 3O2 (g) —> CO2 (g) + 2SO2 (g)
A sample of 10 cm
3
of carbon disulphide was burned in 50 cm
3
of o x y g e n .
After measuring the volume of gas remaining, the product was treated with an
excess of aqueous sodium hydroxide and the volume of gas measure again.
All measurements were made at the same temperature and pressure, under
such conditions that carbon disulphide was gaseous.
What were the measured volumes of gas, in cm
3
,
(a) after burning, and
(b) after adding NaOH (aq)?

Q6
20 cm3 of a gaseous hydrocarbon was burned in 150
cm3 of O2 at standard temperature and pressure
(STP).
The gaseous products have a total volume of 130
cm3. When this product is passed over soda lime, the
volume of the product is decreased to 90 cm3 (Soda
lime is used to remove CO2).
What is the molecular formula of the hydrocarbon?

Isotopes - Q2
8
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
b) Naturally occurring boron consists of the two isotopes, 10
B and 11
B. The relative atomic
mass of boron is 10.81. Determine the percentage abundance of these isotopes.
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csonn t1 atoms, molecules and stoichiometry

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