The Mole

• Mole


  A fixed number (6.0 x 10^23) used to measure the amount of atomic-sized particles
• Measuring amounts in chemistry

  • How much ammonia, NH3, and nitric acid, HNO3, are required to produce 1kg of the fertilizer, ammonium nitrate, NH4NO3?
• Relative atomic mass (Ar)


  A number that compares the average mass of an atom with the mass of the C-12 atom, taken to be 12 units
• All known elements have tabulated values for relative atomic masses which are based on the weighted mean of their isotopes
• Relative molecular mass (RMM)


  The mass of a molecule of a covalent compound
• Relative formula mass (RFM)


  The mass of a formula unit of an ionic compound
• Determining RMM or RFM


  Add the Ar of all the elements present in the compound
• The RMM and the RFM DO NOT have units
• Molar mass (M)


  The mass of one mole of a substance
• Molar mass


  • Unit is grams per mole, i.e. g mol-1
• Molar mass of Carbon

  12 g mol-1
• The molar mass of different elements will not be the same even though they are all one mole
• This is due to the differences in the mass of each element
• How to calculate the molar mass of other elements

  1. Using the relative atomic mass (Ar) of the element

  2. Multiply the Ar by 1 mole to get the molar mass in g mol-1
• Molar mass of a compound


  The relative formula mass or relative molecular mass amount in grams per mole
• Types of molecules


  • Molecules of elements as found in most non-metallic elements
  • Molecules of compounds formed by covalent bonding
• Molar mass


  The mass of one mole of a substance
• Calculating moles of molecules

  1. Calculate relative molecular mass

  2. Use relative molecular mass to solve problems
• Example 3: Calculating moles of molecules


  • How many moles of molecules are there in 88g of carbon dioxide?
  • CO2 molar mass = 44 (12 + 2 x 16) g mol-1
  • 44g = 1 mol
  • 88g = 2 mol of molecules
• Moles of ionic substances

  • There are no molecules present in ionic substances, they are composed of ions
  • Relative formula mass is calculated in the same way as relative molecular mass
  • If the salt contains water of crystallization, this must also be added in the mass to find the molar mass of the compound
• Finding moles of specific ions in ionic compounds


  1 mole (NH4)3PO4 = 3 mol NH4+ and 1 mol PO43-
• Avogadro's number


  The number of particles (atoms, molecules or ions) in one mole of any substance
• Example 5: Calculating number of atoms

  • Calculate the number of Sulphur atoms in 2 moles of SO2
  • There are 2 moles of S atoms in 2 moles of SO2
  • 1 mole of S atoms = 6.0 x 10^23 atoms
  • 2 moles = 2 x 6.0 x 10^23 = 1.2 x 10^24 atoms
• Example 6: Calculating number of ions

  • Calculate the number of chloride ions in 190g of MgCl2
  • Molar mass of MgCl2 = 24 + 2 x 35.5 = 95 g mol-1
  • 190 g = 2 mol
  • 1 mol MgCl2 contains 2 mol Cl- ions
  • 2 mol contain 4 mol Cl- ions
  • No. of Cl- ions = 4 x 6.0 x 10^23 Cl- ions
• Example 7: Calculating mass of element

  • What mass of Sulphur contains the same number of atoms as 12 g of magnesium?
  • 24 g = 1 mol Mg = 6.0 x 10^23 atoms
  • 12 g = 0.5 mol = 3.0 x 10^23 atoms
  • 6.0 x 10^23 atoms of S = 32 g
  • Hence 3.0 x 10^23 atoms = 16 g
• Empirical formula


  Gives the simplest whole number mole ratio between the atoms or ions in the compound
• Molecular formula


  Gives the actual number of moles of atoms of each element present in one mole of the compound
• • Ionic compounds are always represented by empirical formulae
  • Covalent compounds are always represented by molecular formulae
• Determining empirical formula


  1. Write down the mass or convert percentage composition to actual masses

  2. Find the number of moles by dividing the mass by the appropriate molar mass (RAM)

  3. Find the ratio of the moles of the elements present in the compound by dividing through by the smallest number
• Finding molecular formula from empirical formula

  1. Calculate the ratio of the empirical formula mass to the molar mass

  2. Multiply the empirical formula by this factor to determine the molecular formula
• Percentage composition by mass

  The mass composition of a compound gives the percentage by mass of each element present in the compound
• Calculating percentage composition


  1. Calculate the mass of 1 mol of the compound

  2. Calculate the mass which each element contributes to 1 mol of the compound