3A Relative Atomic Mass and Re(1) f4

8/11/2019 3A Relative Atomic Mass and Re(1) f4

1/14

Relative atomic mass and relative molecular mass

The mole and the number of particles

The mole and the mass of substances

The mole and the volume of gas

Chemical formulae

Chemical equations

Chapter 3

Chemical formulae and

Equations


2/14

Relative atomic mass, A r

Relative atomic massis the average

mass of one atom of the element

when compared with 1/12 of the massof an atom of carbon-12.


3/14

Calculation of

Relative atomic Mass

Example 1Calculate how many times greater is the mass of acarbon atom compared to that of an helium atom ?

[Ar: He, 4 ; C, 12]


4/14

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

ARsulphur = 2 x ARoxygen

= 2 x 16

= 32

Calculation of



5/14

Example 3

How many magnesium atoms will have the same massas 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 atomsn x 24 = 2 x 108

n = 2 x 108

24

= 9

Calculation of



6/14

Relative Molecular Mass (Mr)

The relative molecular mass of a substance is the

average mass of a molecule of the substance

when compared with 1/12 of the mass of an atomof carbon-12

12carbonofatomanofmassthe12

1substanceofmoleculeone

ofmassaverage

Mr


7/14

Calculation of Mrand Fr

The term relative molecular mass, Mr can onlybe used for substances that are made up ofmolecules.

For ionic compound, the term relative formulamass, Fris used

Mror Fr can be calculated by adding up therelative atomic masses of all the atoms present ina molecule of the substance.


8/14

Calculation of Mror Fr

Example 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


9/14

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 Mror Fr


10/14

Example 3:

Calculate the relative molecular mass of carbondioxide.

[AR: C, 12 ; O,16 ]

Solution:

The formula of carbon dioxide is CO2MrCO2= C + 2O

= 12 + 2(16)

= 44

Calculation of M r or F r


11/14

Example 4:

Calculate the relative molecular mass of ethanol,C2H5OH.

[ AR: C, 12; H, 1; O,16 ]

Solution:

Mr of C2H5OH = 2C + 5H + O + H

= 2(12) + 5(1) + 16 + 1

= 46



12/14


Example 5:

Calculate the relative molecular mass of sodiumchloride

[ AR: Na,23 ; Cl,35.5 ]

Solution:The formula of sodium chloride is NaCl

AR of NaCl = Na + Cl

= 23 + 35.5

= 58.5


13/14


Example 6

Calculate the relative molecular mass of aluminiumsulphate.

[ AR: Al,27 ; S, 32 ; O,16 ]

Solution:

The formula of aluminium sulphate ialahAl2(SO4)3

Mr of Al2(SO4)3 = 2 Al + 3 SO4= 2(27) + 3[ 32 + 4(16) ]

= 342


14/14


Example 7:

Calculate the relative molecular mass of hydratedcopper(II)sulphate.

[ AR: Cu,64 ; S, 32 ; O,16 ]

Solution:

The formula of hydrated copper (II) sulphate isCuSO4.5H2O

Frof CuSO4. 5H2O = Cu + S + 4O + 5H2O

= 64 + 32 + 4(16) + 5[2(1) +16]

= 250

Documents

3A Relative Atomic Mass and Re(1) f4