chem

• Relative isotopic mass


  A measure of how heavy or light an isotope is compared to a single atom of carbon-12
• Most elements contain a mixture of isotopes
• The isotope masses are different because they have different numbers of neutrons
• Percentage abundance


  The relative amount of each isotope in the natural environment
• Avogadro's constant


  The chemist's counting unit, the number of particles in one mole, defined as 6.02 x 10^23
• Mole


  The amount of substance, given the symbol n and unit mol
• One mole of any substance contains 6.02 x 10^23 particles
• The average relative mass of an element, taking into account the relative abundances of its isotopes, is called the relative atomic mass (Ar)
• To calculate the average relative atomic mass (Ar)


  Multiply the relative isotopic mass of each isotope by its percentage abundance, then add up all the results
• The expression "one mole of oxygen" is ambiguous as it could refer to either the atom O or the molecule O2
• Empirical formula


  Indicates the number of each ion in one unit of an ionic compound
• In 1 mole of AlCl3, there is one mole of aluminium ions and 3 moles of chloride ions
• Mole


  Unit symbol n
• Avogadro's constant


  Symbol NA, value 6.02 x 10^23
• Number of particles (atoms, ions, or molecules)

  Symbol N
• Relationship between the 3 quantities


  n = N/NA
• Calculate number of molecules in 3.5 moles of water


  • N = n x NA = 3.5 x 6.02 x 10^23 = 2.1 x 10^24 molecules
• Calculate number of oxygen atoms in 1.5 mole of Fe₂(SO₄)₃

  n(O) = 12 x 1.5 = 18 mole

  N(O) = n(O) x NA = 18 x 6.02 x 10^23 = 1.08 x 10^25 oxygen atoms
• to calculate the average relative mass (Ar), the formula is:
• Ar=((relative isotope mass x % abundance) + (RIM x %A)) : 100