Introduction to stoichiometry - 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 symbol / formula) 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 lesson on Introduction to chemical reactions that a chemical equation tells you the fixed ratio of reactants the reaction uses, and products that the reaction produces.
\circ In the made up example below, we are being told that to produce 2 molecules of C and 1 molecule of D, the reaction requires 1 molecule of A and 2 molecules of B 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 fact!

• A mole is just a number of atoms – 6.02×10236.02 \times 10^{23} (six hundred and two billion trillion!) but 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}).
\circ Take carbon dioxide, CO2_2 as an example. CO2_2 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_2 would be half a mole of CO2_2 (as 22g is half of 44g).
\circ 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.
  • Intro Lesson
    Introduction to Stoichiometry
  • 1.
    Find the ratio of molecules used and produced in a process: the reaction stoichiometry.
    2H2+_2+O2_2 →2H2_2O
  • 2.
    Find the reaction stoichiometry and use it to predict expected mass of products.
    CH4+_4 + 2O2_2 →CO2+_2 + 2H2_2O
  • 3.
    Find the reaction stoichiometry and use it to predict the mass of chemicals used in a reaction.
    HCI+ + NaOH→NaCl+ + H2_2O
  • 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
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Introduction to stoichiometry

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