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electron

e

Proton

p

Neutron

n

charge

(q)-1.6 x 10 -19 +1.6x10-19 0

Relative charge

-1 +1 0

mass(g) 9.1x10-28 1.67X10-24 1.67X10-24

mass(amu) 0.00055 u 1.007825 u 1.008665 u

Relative mass 0.0005

(0)

1.0007

(1)

1.0086

(1)

Mass of a Hydrogen atom = Mass 1 proton + mass of 1 electron

= 1.67X10-24 + 9.1x10-28 = 1.67x10-24g

or equal to, 0.000 000 000 000 000 000 000 0017 g

Mass of a methane molecule, CH4 =

0.000 000 000 000 000 000 000 022 4 g

2.24 x 10-23g

Learning outcomes: A student is able to:State the meaning of relative atomic mass based on carbon-12 scale,

State the meaning of relative molecular mass based on carbon-12 scale,

State why carbon-12 is used as a standard for determining relative atomic mass and relative molecular mass,

Calculate the relative molecular mass of substances

In practice, chemist do not use these actual masses of atoms and molecules in their calculations. They use relative masses instead.

When chemist first started comparing masses of different atoms, they compared each of them with the mass of one hydrogen atom.

Relative atomic mass = mass of one atom of an element

mass of one atom of hydrogen

Electric fieldReflection

chamber

to vacuum pump

Gas inlet

Ion source

Mass spectrometer

Y

H

a

b

HX

HX

H H

The mass of a X atom

= The mass of 2 hydrogen atoms

Relative atomic mass of X = 2 What does it mean?

The mass of a hydrogen atom

=2

The mass of one X atom

An X atom is 2 times heavier than a hydrogen atom

The mass of a lithium atom

= The mass of 7 hydrogen atoms

Relative atomic mass of lithium = 7

The mass of a hydrogen atom

= 7

The mass of a lithium atom

LiH H H

H H H HLi H H HH H H H

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C

H H H

H H H H

The mass of a carbon atom

= The mass of 12 hydrogen atoms or

12 x the mass of a Hydrogen atom

Relative atomic mass of carbon = 12

The mass of a hydrogen atom

=12

The mass of a carbon atom

C

HHH H H

H H H HH H H

The mass of one hydrogen atom = 1 of the mass one 12 C atom. 12

In 1991, chemists agreed to compare the masses of atoms with the mass of a carbon atom . Carbon consists of more than one isotope. Carbon-12 was chosen since the isotope has the highest relative abundance among carbon isotopes.

Relative atomic mass of an atom is defined by IUPAC as;

Relative atomic mass = mass of one atom of an element 1 x mass of one atom of 12C 12

If the atomic mass ratio of an atom of X to 12C = 0.75, then

Mass of one atom of X

Mass of one atom of 12C= 0.75

Mass of one atom X = 0.75 x mass of one atom of 12C

Relative atomic mass of X

= 0.75 x mass of one atom of 12C

1 X mass of one atom of 12C

12

= 9.0

126C

2412Mg

Relative atomic mass of Mg = 24

The mass of 1 magnesium atom

1 x the mass of 1 126C atom

12= 24

The mass of 1 magnesium atom = 24 x 1/12 x the mass of 1 126C atom

= 24x 1/12 x 12 = 24

126C24

12Mg 126C 12

6C

How many oxygen atoms have equal weight as 4 carbon atoms?

[ Relative atomic mass ; oxygen = 16 ; carbon = 12]

N x mass of 1 oxygen atom = 4 x mass of 1 carbon atom

N = 4 x 12 16 = 3

CC

CC

O

O

OC

C C

C O

O

O

How many times is the mass of 20 neon atoms is heavier than 10 carbon atoms?

20 x mass of 1 neon atom = 20 x 20 10 x mass of 1 carbon atoms 10 x 12

= 3.32

The sum of mass of 20 neon atoms is 3.32 times heavier than the sum of mass of 10 carbon atoms

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H H

O

Relative molecular mass of substance X

= The mass of a molecule of substance X1/12 x the mass of 1 atom of 12

6C

The mass of a water molecule is the sum of mass of all atoms of each element in a water molecule

The mass of a water molecule 1/12 x the mass of 1 12

6C atomRelative molecular mass of water =

1 1

16

Mass of a water molecule = 1u + 1u + 16 u = 18u

=18 u 1 x12 u 12

= 18

The relative molecular mass of a substance is the sum of relative atomic mass of all atoms of each element in the molecule

A water molecule is 18 times heavier than 1- twelfth of the mass of a Carbon – 12 atom.

Calculate the relative molecular mass of the following compounds

6(12)+12(1)+6(16) = 180C6H12O6Glucose

2(12)+4(1)+2(16) = 60CH3COOHEthanoic acid

2(1)+32+4(16) = 98H2SO4Sulphuric acid

12+2(16) = 44CO2Carbon dioxide

Relative molecular mass

Molecular formulaCompounds

For ionic compounds, the term relative molecular mass is replaced by relative formula mass

65+ 2[14+3(16)] =189Zn(NO3)2Zink nitrate

56 +3[16 + 1] =107Fe(OH)3Iron(III) hydroxide

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

= 250CuSO4.5H2OCopper sulphate

(hydrated)

2[14+4(1)] +32+4(16)

= 132(NH4)2 SO4Ammonium sulphate

2(23) +32+4(16)

= 142Na2SO4

Sodium sulphate

Relative formula mass Empherical formula Compound