Atomic masses

1
Atomic masses

• L.O.
• Define the terms relative isotopic mass and
• relative atomic mass, based on the 12C scale
• Calculate the relative atomic mass of an
• element given the relative abundances of its
• isotopes
• Use the terms relative molecular mass and
• relative formula mass and calculate values
• from relative atomic masses.

2
HW pre-work on mole Read worksheet and answer
Q1-2
3
Relative atomic mass is the mass of an atom of an
isotope compared with one-twelfth of the mass of
an atom of carbon-12
4
ISOTOPES - CALCULATIONS
There are two common isotopes of chlorine.
Calculate the average relative atomic mass of
chlorine atoms
Protons Neutrons
35 Cl 17 17 18 75
37 Cl 17 17 20 25
5
ISOTOPES - CALCULATIONS
There are two common isotopes of chlorine.
Calculate the average relative atomic mass of
chlorine atoms Out of every 100 atoms
75 are 35Cl and 25 are 37Cl
Average (75 x 35) (25 x 37)
35.5 100
Protons Neutrons
35 Cl 17 17 18 75
37 Cl 17 17 20 25
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1.1 Exercise 2
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Worked examples

• What is the relative atomic mass (Ar) of a
  mixture of 90 hydrogen-1 (1H) and 10 hydrogen-2
  (2H)?
• What is the relative atomic mass (Ar) of a
  mixture of 75 chlorine-35 (35Cl) and 25
  chlorine-37 (37Cl)?
• Calculate the Ar of
• 43 16O and 5718O
• 14 40Ca, 26 42Ca and 60 46Ca

8

• Relative molecular mass, Mr, is the weighted mean
  mass of a molecule compared with one-twelfth of
  the mass of an atom of carbon-12

9
Relative molecular mass, Mr

• This is the total mass of a given molecule
  calculated by adding together the relative atomic
  masses (Ar) of all of its atoms
• Worked examples
• Mr of H2O 2x 1 (H) 1x 16 (O) 18
• Mr of HCl 1x 1 (H) 1x 35.5 36.5

10
Work out the relative molecular masses of these
molecules

• CH4
• CO2
• HBr
• H2SO4
• CH2Cl2

11
Work out the relative molecular masses of these
molecules

• CH4
• CO2
• HBr
• H2SO4
• CH2Cl2

12
Relative formula mass

• Very large compounds such as NaCl and SiO2 are
  too big to work out the mass of the whole
  structure, so we only work out the basic unit
• Worked examples NaCl and SiO2

13

• Relative formula mass is the weighted mean mass
  of a formula unit compared with one-twelfth of
  the mass of an atom of carbon-12

14

• Jan 11, Q1

15
Atomic masses

• L.O.
• Define the terms relative isotopic mass and
• relative atomic mass, based on the 12C scale
• Calculate the relative atomic mass of an
• element given the relative abundances of its
• isotopes
• Use the terms relative molecular mass and
• relative formula mass and calculate values
• from relative atomic masses.