The Mole

The Mole

Introduction to Moles

  • In chemistry, a mole is a standard unit for measuring the amount of a substance.
  • It is based on the number of atoms in 12 grams of carbon-12, which is approximately 6.022 x 10^23 atoms. This value is known as Avogadro’s number.
  • The molar mass of a substance is the mass of one mole of that substance. For elements, this matches the relative atomic mass in grams (g).

Using Moles in Chemical Equations

  • Stoichiometric coefficients in balanced chemical equations tell us about the molar ratios of reactants and products.
  • For example, in the equation 2H2 + O2 -> 2H2O, it requires 2 moles of hydrogen and 1 mole of oxygen to produce 2 moles of water.

Calculating Moles

  • The number of moles of a substance can be calculated using the equation: n = m/M, where n is the number of moles, m is the mass of substance in grams, and M is the molecular mass of the substance.
  • By rearranging this equation, you can calculate the mass of substance when you know the number of moles and the molecular mass: m = n x M.

Converting Between Moles and Particles

  • To convert between the number of moles and the number of particles (atoms, molecules, ions, etc.), use the equation: N = n x Avogadro’s number.
  • If you know the mass of a substance and want to find out how many particles of substance there are, remember to convert the mass to moles first.

Concentration and Moles

  • Another common concept in chemistry is concentration, which measures how much of a particular substance exists in a certain volume of solution.
  • The concentration of a solution in moles per litre (mol/L), also known as its molarity, is calculated using the equation: C = n/V, where C is the concentration, n is the number of moles, and V is the volume in litres.

Percentage Composition by Mass

  • The percentage composition of an element in a compound is calculated by dividing the total mass of the element in the formula by the molar mass of the compound and multiplying by 100%.

Limitations of the Mole Concept

  • Whilst the mole concept is integral to quantitative chemistry, it is a simplified model and doesn’t account for variations in the exact number of particles present due to isotopes.
  • Similarly, whilst it allows calculation of how much of a substance reacts or is produced, it doesn’t consider that reactions often don’t go to completion, or may have side-reactions that affect the quantities involved.