STOICHIOMETRY-CHEMICAL
CALCULATIONS
RELATIVE ATOMIC MASS
Relative atomic mass
• The actual mass of an atom is very small.Such small quantities are not
easy to work with and a scale called the relative atomic mass is used.
• in this scale an atom of carbon carbon is given a relative atomic
mass,Ar of 12.00 .
• All the other atoms of the other elements are given a relative atomic
mass compared to that of carbon.
• Relative atomic mass Ar,is the average mass of the isotope of an
element compared to 1/12 of the mass of an atom of 12C.
Reacting masses
• what mass of carbon dioxide would be produced if 6g of carbon was
completely burned in oxygen gas?
• C+ O2→CO2
• The molecular formula of a compound is defined as the number
and type of different atoms in one molecule.
• Relative molecular mass Mr is the sum of the relative atomic
masses.Relative formula mass Mr, is used for ionic compounds.
• Relative formula mass,Mr is used for ionic compounds.
calculating moles
• The mole,symbol mol is the unit of amount of substance ,1 mole
contains 6.02 × 1023 particles e.g atoms,ions,molecules.
• This number is called the Avogadro constant.
• For example the relative atomic mass of iron is 56,so 1 mole of
iron is 56 g.Therefore,56g of iron contains 6.02 × 1023 atoms.
• the Ar of aluminium is 27.In 27 g of aluminium it is found that
there are 6.02 × 1023 atoms.Therefore, 1 mole of aluminium
atoms.
• So we can calculate the mass of a substance present in any
number of moles using;
• Mass in g=number of moles × molar mass of the element.
Moles and compounds
• the molar mass of a compound is called its molar mass;it has the
units of g/mol.
• if you write the molar mass of a compound without any units then it
is the relative molecular mass Mr.
• So the relative molecular mass of water is 18.
Moles and gases
• measuring the volume rather than the mass.
• 1 mole of any gas occupies a volume of approximately 24dm3(
24litres) at room temperature and pressure (r.t.p)
• Number of moles of a
𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑔𝑎𝑠(𝑖𝑛 𝑑𝑚3 𝑎𝑡 𝑟.𝑡.𝑝)
gas=
24𝑑𝑚3
• Volume of gas(in dm3 at r.t.p)=Number of moles of gas × 24 dm3