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the elemental composition of a sample.

Stoichiometry and Balanced Equations

In stoichiometry, balanced equations make it possible to compare different elements through the stoichiometric factor discussed earlier. This is the mole ratio between two factors in a chemical reaction found through the ratio of stoichiometric coefficients. Here is a real world example to show how stoichiometric factors are useful.


There are 12 party invitations and 20 stamps. Each party invitation needs 2 stamps to be sent. How many party invitations can be sent?


The equation for this can be written as

\[I + 2S \rightarrow IS_2\]


  • \(I\) represents invitations,
  • \(S\) represents stamps, and
  • \(IS_2\) represents the sent party invitations consisting of one invitation and two stamps.

Based on this, we have the ratio of 2 stamps for 1 sent invite, based on the balanced equation. 


Invitations                                    Stamps                           Party Invitations Sent

In this example are all the reactants (stamps and invitations) used up? No, and this is normally the case with chemical reactions. There is often excess of one of the reactants. The limiting reagent, the one that runs out first, prevents the reaction from continuing and determines the maximum amount of product that can be formed.


What is the limiting reagent in this example? 


Stamps, because there was only enough to send out invitations, whereas there were enough invitations for 12 complete party invitations. Aside from just looking at the problem, the problem can be solved using stoichiometric factors.

12 I x (1IS2/1I) = 12 IS2 possible

20 S x (1IS2/2S) = 10 IS2 possible

When there is no limiting reagent because the ratio of all the reactants caused them to run out at the same time, it is known as stoichiometric proportions.

Types of Reactions

There are 6 basic types of reactions.

  • Combustion: Combustion is the formation of CO2 and H2O from the reaction of a chemical and O2
  • Combination (synthesis): Combination is the addition of 2 or more simple reactants to form a complex product.
  • Decomposition: Decomposition is when complex reactants are broken down into simpler products.
  • Single Displacement: Single displacement is when an element from on reactant switches with an element of the other to form two new reactants.
  • Double Displacement: Double displacement is when two elements from on reactants switched with two elements of the other to form two new reactants.
  • Acid-Base: Acid- base reactions are when two reactants form salts and water.

Molar Mass