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DAY : WEDNESDAY
DATE : 10 MARCH 2010
TIME : 12.30 PM -1.30 PM
QUESTIONS: 25 MCQ AND 2 STRUCTURE
QUESTIONS
TOPIC: CHAPTER 1 UNTIL CHAPTER 3.2
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Relative atomic mass, ARelative atomic mass, Arr
Relative atomic massRelative atomic massis theis the averageaverage
mass of one atom of the elementmass of one atom of the element
when compared with 1/12 of thewhen compared with 1/12 of the
mass of an atom of carbon-12.mass of an atom of carbon-12.
12carbonofatomanofmassthe12
1elementanofatomoneofmassaveragetheAr
=
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Calculation of
Relative atomic Mass
Example 1Calculate how many times greater is the mass of a carbonatom compared to that of an helium atom ?[Ar : He, 4 ; C, 12]
times
heliumofAcarbonofA
atomheliumaofMass
atomcarbonaofMass
Solution
R
R
3
4
12
:
=
=
=
=
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Example 2
Mass of a sulphur atom is 2 times greater than
that of the mass of an oxygen atom. What is the
relative atomic mass for sulphur? [ AR : O,16]
Solution:
Mass of a sulphur atom = 2 x mass of an oxygen atom
AR sulphur = 2 x AR oxygen
= 2 x 16
= 32
Calculation of
Relative atomic Mass
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Example 3How many magnesium atoms will have the samemass as two silver atom?
[AR : Mg,24 ; Ag,108 ]
Solution:
Let the number of magnesium atom = n
Mass of n magnesium atom = mass of 2 silver atoms
n x 24 = 2 x 108n = 2 x 108
24
= 9
Calculation of
Relative atomic Mass
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Relative Molecular Mass (MRelative Molecular Mass (Mrr ))
The relative molecular mass of aThe relative molecular mass of a
substance is thesubstance is the average mass of aaverage mass of a
molecule of the substance whenmolecule of the substance when
compared with 1/12 of the mass of ancompared with 1/12 of the mass of anatom of carbon-12atom of carbon-12
12carbonofatomanofmassthe12
1
substanceofmoleculeoneofmassaverage
Mr
=
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Calculation of Mr and Fr
The term relative molecular mass, Mr canonly be used for substances that are made upofmolecules.
Forionic compound, the term relative formulamass, Fr is used
Mr or Frcan be calculated by adding up the
relative atomic masses of all the atomspresent in a molecule of the substance.
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Calculation of Mr or FrExample 1:
Calculate the molecular mass of water.
[ AR
: H,1 ; O, 16]
Solution:
The formula of water is H2O
MrH2O = 2H + O
= 2(1) + 16
= 18
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Example 2:Calculate the relative molecular mass of
ammonia.
[ AR: N,14 ; H,1 ]
Solution:
Formula of ammonia is NH3
Mrof NH3 = N + 3H
= 14 + 3(1)= 17
Calculation of Mr or Fr
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Example 3:
Calculate the relative molecular mass of
carbon dioxide.
[AR : C, 12 ; O,16 ]
Solution:
The formula of carbon dioxide is CO2
MrCO2 = C + 2O
= 12 + 2(16)
= 44
Calculation of Mr or Fr
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1.Calculate the relative molecular mass of
ethanol, C2H5OH.
[ AR: C, 12; H, 1; O,16 ]
2. Calculate the relative molecular mass of
sodium chloride
[ AR : Na,23 ; Cl,35.5 ]
3. Calculate the relative molecular mass of
aluminium sulphate.
[ AR : Al,27 ; S, 32 ; O,16 ]
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ANSWER:
1. 46
2. 58.5
3. 342
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