3.1.2 Amount of Substance
Relative Atomic Mass and Relative Molecular Mass
The relative atomic mass Ar is the weighted average mass of an atom of an element relative to 1/12 of the
mass of an atom of carbon-12.
The relative molecular mass Mr is the weighted average mass of a molecule relative to 1/12 of the mass of
an atom of carbon-12.
The term relative formula mass is used for ionic compounds because they do not exist as molecules.
However, this uses the same symbol Mr.
Avogadro’s Constant and the Mole
A mole is 6.022 x 1023 of something - this is Avagadro’s Constant, and the number of atoms in 12g of
carbon-12. The amount of a substance that contains 6.022 x 1023 particles is called a mole. The Ar of a
element in grams contains one mole of atoms.
𝑚𝑎𝑠𝑠 (𝑔)
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠 (𝑛) =
𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠
Concentration
The concentration of a solution tells us how much solute is dissolved in a known volume of solution.
Concentrations are measured in mols per decimetre3 (mol dm-3).
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠
𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 =
𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
Ideal Gases
The ideal gas equation is found by combining multiple gas laws. No gases obey it exactly, but at room
temperature and pressure it is a valid estimation.
𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 × 𝑣𝑜𝑙𝑢𝑚𝑒 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠 × 𝑔𝑎𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 × 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒
• Pressure, P in Pascals (Pa)
• Volume, V in metres cubed (m3)
• Number of Moles, n
• Gas Constant, R, given as 8.31 J K-1 mol-1
• Temperature, T in Kelvin
The equation can be rearranged to find any variable.
Empirical and Molecular Formulae
The empirical formula represents the simplest number ratio of atoms in a molecule.
The molecular formula represents the actual number of atoms in a molecule.
Balanced Equations
Chemical equations must contain the same number of atoms on each side, and should also show the state
of the reactant or product.
Ionic equations are simplified equations which show the combinations of oppositely charged ions during a
reaction. For example, the reaction between hydrochloric acid and sodium hydroxide is shown below:
HCl + NaOH → NaCl + H2O
The ions present are:
HCl NaOH NaCl
Relative Atomic Mass and Relative Molecular Mass
The relative atomic mass Ar is the weighted average mass of an atom of an element relative to 1/12 of the
mass of an atom of carbon-12.
The relative molecular mass Mr is the weighted average mass of a molecule relative to 1/12 of the mass of
an atom of carbon-12.
The term relative formula mass is used for ionic compounds because they do not exist as molecules.
However, this uses the same symbol Mr.
Avogadro’s Constant and the Mole
A mole is 6.022 x 1023 of something - this is Avagadro’s Constant, and the number of atoms in 12g of
carbon-12. The amount of a substance that contains 6.022 x 1023 particles is called a mole. The Ar of a
element in grams contains one mole of atoms.
𝑚𝑎𝑠𝑠 (𝑔)
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠 (𝑛) =
𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠
Concentration
The concentration of a solution tells us how much solute is dissolved in a known volume of solution.
Concentrations are measured in mols per decimetre3 (mol dm-3).
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠
𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 =
𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
Ideal Gases
The ideal gas equation is found by combining multiple gas laws. No gases obey it exactly, but at room
temperature and pressure it is a valid estimation.
𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 × 𝑣𝑜𝑙𝑢𝑚𝑒 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠 × 𝑔𝑎𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 × 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒
• Pressure, P in Pascals (Pa)
• Volume, V in metres cubed (m3)
• Number of Moles, n
• Gas Constant, R, given as 8.31 J K-1 mol-1
• Temperature, T in Kelvin
The equation can be rearranged to find any variable.
Empirical and Molecular Formulae
The empirical formula represents the simplest number ratio of atoms in a molecule.
The molecular formula represents the actual number of atoms in a molecule.
Balanced Equations
Chemical equations must contain the same number of atoms on each side, and should also show the state
of the reactant or product.
Ionic equations are simplified equations which show the combinations of oppositely charged ions during a
reaction. For example, the reaction between hydrochloric acid and sodium hydroxide is shown below:
HCl + NaOH → NaCl + H2O
The ions present are:
HCl NaOH NaCl
