Introduction to stoichiometry

Introduction to stoichiometry


In this lesson, we will learn:
  • How to analyze a balanced chemical equation.
  • What a "mole" is and how it is important to understanding chemical reactions.
  • How to predict amounts of product made in a reaction with a given amount of reactant.
  • How to apply the moles concept with conversion factors to find the mass of reactants and products involved in reactions.


  • The coefficients (the number before the chemical formulae) tell you how many molecules of that chemical are used, as a ratio, in the reaction. If no number is there, it's one molecule.

  • Recall from Introduction to chemical reactions that a chemical equation tells you the fixed ratio of reactants the reaction uses, and products that the reaction produces.
    • In the example below, we are being told that to produce 2 molecules of C and 1 molecule of D, the reaction needs 1 molecule of A and 2 molecules of B in that ratio. This is like making a mixed drink which might tell you “to make 1 drink serving, mix one part drink with two parts water”. If you want more, you have to add more of both the drink mix and the water in that ratio.

    • A + 2B \, \, 2C + D

  • The actual amounts of reactants used or products made in a particular reaction can be changed, but the ratio between them cannot – changing plans to make twice as much of a product will need twice as much reactants. There is no way around this!

  • A mole is just a fixed number of atoms – 6.02x1023 (six hundred and two billion trillion!), that is it. It is just a number in exactly the same way that a dozen or a pair is a number.
    It is used in chemistry because the mass of atoms and molecules are measured in grams per mole (written gmol1^{-1}).
    • Take carbon dioxide, CO2 as an example. CO2 has a molecular mass of 44 grams per mole. That means that 1 mole (six hundred billion trillion molecules) of carbon dioxide has a mass of 44 grams. Similarly, a sample of 22 grams of CO2 would be half a mole of CO2 (as 22g is half of 44g).
    • The relative mass, measured in grams per mole is vitally important for chemists because atoms and molecules are too small to be counted individually but we can measure the mass of substances very easily! With the mass and grams per mole known, we can work out the number of moles – how many molecules are in the sample of the substance we are using.

  • Use the chemical equation to find the ratio of reactant moles to product moles (the reaction stoichiometry) and then find their relative masses. Now, you can predict the mass of any reactants required or products expected for any reaction scale.
  • Introduction
    Introduction to Stoichiometry
    What is Stoichiometry?

    What information does a chemical equation tell us?

    What is a mole in chemistry?

  • 1.
    Find the ratio of molecules used and produced in a process: the reaction stoichiometry.
    2H2+_2+O2_2 →2H2_2O
    How many molecules of H2_2 will react with 6 molecules of O2_2?

    How many molecules of H2_2O would this produce?

    What is the mass of 6 moles of O2_2?

  • 2.
    Find the reaction stoichiometry and use it to predict expected mass of products.
    CH4+_4 + 2O2_2 →CO2+_2 + 2H2_2O
    How many moles of H2_2O were produced if 2.5 moles of CO2_2 were produced?

    What is the mass of 2.5 moles of CO2_2?

    In a repeat experiment, 132 grams of CO2_2 were produced. What was the mass of water also produced?

  • 3.
    Find the reaction stoichiometry and use it to predict the mass of chemicals used in a reaction.
    HCI+ + NaOH→NaCl+ + H2_2O
    How many moles of HCI are required to react with 4.75 moles of NaOH?

    What would be the mass of 4.5 moles of NaOH?

    In a repeat experiment, 54.75g HCI (dissolved in solution) reacted completely with some NaOH added. What was the mass of NaOH used in this experiment?

  • 4.
    Find the reaction stoichiometry and use it to predict the mass of chemicals used and produced in a reaction.
    2C2_2H6+_6 + 7O2_2 →4CO2+_2 +6H2_2O
    How many moles of O2_2 would react with 1 mole of C2_2H6_6?

    If 15 moles of H2_2O were produced in this experiment, how many moles of C2_2H6_6 were used?

    What is the mass of this amount of C2_2H6_6 used?

    A total of 0.19 mol of reactants and products combined were involved in this reaction. What is the number of moles of H2_2O produced?