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Chemistry: Atoms, Molecules and Stoichiometry by Mr Ong SH
CHEMISTRY DEPARTMENT
Atoms, Molecules and Stoichiometry
Summary Notes
Relative masses
Definitions
Calculation (no units, ratio)
Relative Isotopic
Mass (Ar)
The relative isotopic mass, Ar, of a particular
isotope is defined as the ratio of the mass of one
1
atom of the isotope to 12
the mass of an atom of 12C
Ar =
mass of one atom of a certain isotope
1
×mass of one atom of 12 C isotope
12
isotope, expressed on the 12C scale.
Relative Atomic
Mass (Ar)
The relative atomic mass, Ar, of an element is
defined as the ratio of the average mass of one atom
1
of the element to 12
the mass of an atom of 12C
Ar =
isotope, expressed on the 12C scale.
Relative
Molecular Mass
(Mr)
The relative molecular mass, Mr, of a substance is
defined as the ratio of the average mass of one
1
molecule of the substance to 12
the mass of an atom
Mr =
of 12C isotope, expressed on the 12C scale.
mass of one atom of an element
1
×mass of one atom of
12
12
C isotope
mass of one molecule of the substance
1
×mass of one atom of 12 C isotope
12
MOLAR MASS:
No. of moles of substance 
Mass of substance
Molar mass of substance
Molar mass of element  Relative atomic mass in g mol -1
Molar mass of compound  Relative molecular mass or Relative formula mass in g mol -1
MOLAR VOLUME (for gases only)
 1 mol of any gas occupies 22.4 dm3 at standard temperature pressure (s.t.p.)
or
24.0 dm3 at room temperature pressure (r.t.p.).
This is known as the molar volume.
IMPORTANT EQUATIONS FOR MOLE CALCULATIONS:
No. of moles of particles
No.of particles

23
6.02  10
For gases
At r.t.p. (25°C and 1 atm)
3
No. of moles of gas 
Volume of gas (in dm )
24 .0 dm
MOLE
No. of moles of X
Mass of X (in g)

Ar or M r of X
3
At s.t.p. (0°C and 1 atm)
3
No. of moles of gas 
Volume of gas (in dm )
22 .4 dm
3
Chemistry: Atoms, Molecules and Stoichiometry by Mr Ong SH
Please contact Mr Ong for complete summary notes @9863 9633
In aqueous solutions
No. of moles of X
-3
3
 Concentration (in mol dm )  Volume (dm )
-3
-3
Concentration (in g dm )  Concentration (in mol dm )  ( M r or Ar )
Dilution concept : C1V1 = C2V2