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Stoichiometry

Chemistry: IB Diploma Higher Level, topic 5.3.1

Stoichiometry

Stoichiometry is used to find the relative amounts of reactants or products involved in chemical reactions.

Stoichiometric factors

Stoichiometric factors

  • Atoms must be conserved in any chemical reaction.
  • We can use this together with the coefficients in a balanced equation to derive stoichiometric factors.
  • Stoichiometric factors are ratios of the amount of a particular species in the reaction compared to the amount of another species.
  • Typically stoichiometric factors are calculated using the number of molecules or number of moles.
Ammonia

Ammonia

  • The production of ammonia is given by the following balanced equation:
  • NH2(g)+3H2(g)→2NH3(g)NH_2(g) + 3H_2(g)\rightarrow2NH_3(g)
  • The stoichiometric factor for ammonia molecules produced from hydrogen molecules is:
  • 2  NH3 molecules3  H2 molecules=2 mol  NH33 mol  H2=23\frac{2\;NH_3\text{ molecules}}{3\;H_2\text{ molecules}} = \frac{2\text{ mol}\;NH_3}{3\text{ mol}\;H_2} = \frac{2}{3}
Ideal gas law

Ideal gas law

  • We can work out the volume of reactants and products using the stoichiometric factors in conjunction with the ideal gas law.
  • Volume = number of moles ÷ molarity
  • Once we know the volume, we can do calculations using the ideal gas law PV=nRTPV = nRT to find the properties of reactants and products.

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