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Chemical Formulae
And
Equations
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A. Relative Atomic Mass and Relative
Molecular Mass Based on the theory of particles:
particles are very small and discrete. A single atom
is too small and light and cannot be weigheddirectly
Thus, the mass of an atom is obtained bycomparing it with another atom which is taken as a
standard.
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3 types of scale to determine the mass of theparticles
a) Compared with a hydrogen-1 scaleb) Compared with an oxygen-16
c) Compared with carbon-12 (modern comparisonUNTILL TODAY)
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Relative atomic mass, RAM Meaning;
The average mass of one atom of the element when
compared with 1/12 of the mass of an atom ofcarbon-12.
Relative Atomic Mass, RAM
= Average mass of one atom of the element
1/12 x the mass of an atom of carbon-12
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Example:
RAM of magnesium
= 24 = 241/12 x 12
= magnesium is 24 times larger than carbon-12
** THE VALUE OF NUCLEON NUMBER IN THEPERIODIC TABLE OF ELEMENT
= RELATIVE ATOMIC MASS, RAM
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Relative molecular mass, RMM Meaning;
The average mass of one molecule when comparedwith 1/12 of the mass of an atom of carbon-12.
Relative Molecular Mass, RMM
= Average mass of one molecule
1/12 x the mass of an atom of carbon-12
Calculate RMM/RFM by adding up the relativeatomic mass of all the atoms that present in the
molecule/ionic compound
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B. The Mole and the Number of
Particles The number of particles in matter is measured inmole.
Definition:The amount of substance that contains as manyparticles as the number of atoms in exactly 12 g ofcarbon-12
Symbol of mole: mol
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How many atoms are there in 12 g of carbon-12?
= 6.02 1023
The value of 6.02 1023 is called theAvogadroconstant or Avogadro number
Avogadro constant, NAThe number of particles in one mole of a substance
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Point to note:
One mole of any substance contains 6.02 1023 particles
1 mol of atomic substance contains 6.02 1023
atoms 1 mol of molecular substance contains 6.02 1023
molecules
1 mol of ionic substance contains 6.02 1023 ions
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Relationship between the number of moles and thenumber of particles
Number of moles,
(mol)
Number of particles
(atoms, molecules, ions)
NA
NA
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Number ofparticles
Moles NA
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Example 1:A closed glass bottle contains 0.5 mol of oxygen gas, O2(a) How many oxygen molecules, O2 are there in the
bottle?(b) How many oxygen atoms are there in the bottle?
[Avogadro constant: 6.02 1023 mol-1]
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a) The number of oxygen molecules, O2= 0.5 mol 6.02 1023 mol-1
= 3.01 1023
molecules
b) The number of oxygen atoms
= 0.5 mol 6.02 1023 mol-1 2
= 6.02 1023 atoms
Number ofparticles
Moles NA
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Example 2:Find the number of moles of 9.03 1023 molecules in asample containing molecules of carbon dioxide, CO2
[Avogadro constant: 6.02 10
23
mol
-1
]
The number of moles carbon dioxide
= 9.03 1023
6.02 1023 mol-1
= 1.5 mol
Number ofparticles
Moles NA
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C. The Mole and the Mass of
Substances The mass of one mole of any substance is called molar
mass
Units: g mol
-1
The molar mass of substances are numerically equalto relative mass
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ElementRelative
massMass of
1 molMolarmass
Helium 4 4 4 g mol-1
Sodium 23 23 23 g mol-1
Water, H2O 2(1) + 16 = 18 18 18 g mol-1
Ammomia, NH3 14 + 3(1) = 17 17 17 g mol-1
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Relationship between the number of moles and themass of a substance
Number of moles,
(mol)
Mass
(g)
molar mass
molar mass
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Mass
(g)
MolesRAM /RMM /
RFM
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Example 1:What is the mass of
(a) 0.1 mol of magnesium?
(b) 2.408 1023
atoms of magnesium?[Relative atomic mass: Mg=24; Avogadro constant: 6.02 1023 mol-1]
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(a) Molar mass of Mg = 24 g mol-1
Mass of Mg = 0.1 mol 24 g mol-1
= 2.4 g
Mass (g)
Moles
RAM /RMM /
RFM
(b) The number of moles Mg atoms
= 2.408 1023
6.02 1023
mol-1
= 0.4 mol
Mass of Mg atoms
= 0.4 mol 24 g mol-1
= 9.6 g
Number ofparticles
Moles NA
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How many moles of molecules are there in 16 g of sulphurdioxide gas, SO2?
[Relative atomic mass: O=16, S=32]
Example 2:
RMM of SO2= 32 + 2(16) = 64
Molar mass of SO2 = 64 g mol-1
The number of moles= 16 g
64 g mol-1
= 0.25 mol
Mass (g)
Moles
RAM /RMM /
RFM
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D. The Mole and the Volume of Gas The volume occupied by one mole of the gas is called
molar volume
One mole of any gas always has the same volumeunder the same temperature and pressure
The molar volume of any gas is22.4 dm3 at STP or
24 dm3 at room condition
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Relationship between the number of moles and thevolume of gas
Number of moles,
(mol)
Volume of gas
(dm3
)
molar volume
molar volume
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Volume
(dm3)
Moles
22.4 dm3
(STP) /
24 dm3
(roomcondition)
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Example 1:What is the volume of 1.2 mol of ammonia gas, NH3 atSTP?
[Molar volume: 22.4 dm3
mol-1
at STP] Volume(dm3)
Moles
22.4 dm3
(STP) /24 dm3 (RC)
The volume of ammonia gas, NH3
= 1.2 mol 22.4 dm3
mol-1
= 26.88 dm3
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How many moles of ammonia gas, NH3 are present in600 cm3 of the gas measured at room conditions?
[Molar volume: 24 dm3
mol-1
at room condition]
Example 2:
Volume(dm3)
Moles
22.4 dm3
(STP) /24 dm3 (RC)
The number of moles of ammonia gas, NH3
= 600 cm3
1000= 0.6 dm3
= 0.6 dm3
24 dm3 mol-1
= 0.025 mol
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Relationship between the number of moles, number of
particles, mass and the volume of gas
Number of moles,(mol)
Mass(g)
molar volume molar volume
Number ofparticles
Volume of gas(dm3)
NA
NA molar mass
molar mass
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E. Chemical FormulaeA chemical formulae
A representation of a chemical substance using lettersfor atom and subscript numbers to show the numbersof each type of atoms that are present in the substance
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H2
Symbol ofhydrogen atom
Shows that thereare two hydrogen
atom in a hydrogengas, H2molecule
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H2O
Symbol ofhydrogen atom
Shows that thereare two hydrogenatom in a water
molecule
Symbol ofoxygen atom
Shows that thereare one oxygenatom in a water
molecule
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Compound can be represented by two types:
1. Empirical formula
2. Molecular formula
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Empirical formula Meaning
Formula that show the simplest whole number ratio ofatoms of each element in the compound
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ExampleA sample of aluminium oxide contains 1.08 g ofaluminium and 0.96 g of oxygen. What is the empiricalformula of this compound?
[Relative atomic mass: O = 16; Al = 27]
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Element Al O
Mass of element
(g)Number of mole
(mol)
Ratio of molesSimplest ratio
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Experiment question:Describe how you can carry out an experiment todetermine the empirical formula of magnesium oxide.Your description should include
Procedure of experiment
Tabulation of result
Calculation of the results obtained
[Relative atomic mass: O = 16; Mg = 24]
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Procedure:1. Clean (5-15 cm) magnesium ribbon with
sandpaper and coil it
2. Weigh an empty crucible with its lid3. Place the magnesium in the crucible and weigh
again
4. Record the reading
5. Heat the crucible strongly
6. Open and close the lid very quickly
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7. When burning is complete, stop the heating
8. Let the crucible cool and then weigh it again
9. The heating, cooling and weighing process isrepeated until a constant mass is recorded
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Result:Description Mass (g)
Crucible + lid x
Crucible + lid + Mg y
Crucible +lid + MgO z
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Calculation:Mg O
Mass (g) y-x z-yNo. of mole (y-x)/24 (z-y)/16
Ratio 1 1
Empirical formula = MgO
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Discussion1. H2 gas must be flowed through the apparatus toremove all the air
2. H2
gas must be flowed throughout the experiment toprevent the air from outside mixing with the H2 gas
3. H2 gas flowed through the apparatus during cooling toprevent copper being oxidised by air into copper(II)
oxide4. Repeat heating, cooling & weighing process to ensureall the copper(II) oxide changed into copper
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5. This method is to determine empirical formula ofoxide of metals which are less reactive than H2 in thereactivity series
6. Other example: Lead(II) oxide, Iron(II) oxide
7. Anhydrous calcium chloride to dry the H2 gas
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Molecular formula Meaning
Formula that show the actual number of atoms of eachelement that are present in a molecule of thecompound
Molecular formula = (Empirical formula)n
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Example:(CH3)n = 30n [12 + 3(1) ] = 30
15n = 30n = 30/15
= 2
Molecular formula = (CH3)2= C2H6
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Ionic formulaeIonic
compounds
Positive ions
(cation)
Negative ions
(anion)
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Formulae of cations & anionsCation Formula Anion Formula
Sodium ion Na+ Chloride ion Cl-
Potassium ion K+ Bromide ion Br-
Zinc ion Zn2+ Iodide ion I-
Magnesium ion Mg2+ Oxide ion O2-
Calcium ion Ca2+ Hydroxide ion OH-
Aluminium ion Al3+ Sulphate ion SO42-
Iron(II) ion Fe2+ Carbonate ion CO32-
Iron(III) ion Fe3+ Nitrate ion NO3-
Copper(II) ion Cu2+ Phosphate ion PO43-
Ammonium ion NH4+