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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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