What is Stoichiometry?

Stoichiometry is a chemical division that evaluates the quantitative content of the chemical process with the help of the reacting molecules or products taking part in the reaction. The term “stoicheion” in “Stoichiometry”, is an old Greek term for "element" whereas “metron” means "measure".

A chemical equation that is completely balanced is similar to the cooking recipe. It demonstrates which reactants (ingredients) will give us the desired products. It will also give us a numerical estimation between reactants and products, for instance, the number of cups of a particular ingredient should be added to obtain a single set of products. Such numerical descriptions are called stoichiometry of reactions.

The law of stoichiometry has the following two characteristics:

  • Stoichiometry can provide you with the concentration of the reacting elements as well as the products created in a chemical process.
  • Stoichiometry is based on the conservation law of masses that says matter cannot be formed or damaged it can only be rearranged. The mass of reactants and products should be equal in any chemical reaction. 
"An image showing stoichiometry using oxygen and hydrogen molecules."

Stoichiometric Coefficient

In any equalized chemical equation, the number of components on either side of the equation will be the same. In any chemical reaction the number that is expressed in before molecules, ions, and atoms such that it balances out the number of components present on either section of the equation is called the Stoichiometric coefficient. The stoichiometric coefficients can be a fraction or a whole number and it is useful in determining the mole ratio among the reactants and products.

"An image showing stoichiometry coefficient"

For example, in the following balance chemical equation:

2K(s) +2HCl (aq) →2KCl (aq) +H2 (g)

Here we can conclude that 2 moles of HCl reacts with 2 moles of Na(s) to give 2 moles KCl along with 1 mole of H 2 . Now if only we are familiar with number of K moles to start out with, we will be able to employ the ratio of 2 moles of KCl to 2 moles of K to conclude how many moles of KCl will be produced or else we can also utilize the ratio of 1 mole of H 2 and 2 moles of K to convert to KCl This is referred to as the coefficient factor. Here the chemical equation in a balanced state helps us to transform all the data regarding one reactant or product into numerical facts of another component existing in the reaction.

Mole Ratios  

In a balanced equation the coefficients indicate the correlations between the reactant’s and product’s moles. This coefficient can also be used to write mole ratios. In a balanced chemical equation, a mole ratio is a ratio among moles of two different substances.

Consider the following example to understand the mole ratio.

2H 2 O l   O g  + 2H 2 l

Now for determining number of mole ratios for a particular chemical reaction, all you need to do is check the coefficient of each element involved.

For example, in the above chemical equation the number of participating elements will be 3 (H 2 O 2, O 2 and H 2 O) in the ratio of 2, 1 and 2 respectively so the mole ratio will be 2:1:2.

Balanced Chemical Equations and Mole Ratios

The mole ratio is a common type of stoichiometric interaction that links the quantities in the moles of two chemically responsive substances. In a balanced chemical equation, we would be able to calculate the mole ratio for any two substances by considering the coefficients for each of the elements participating in the chemical reaction.

Fe 2 O 3 (s) + 2Al(s)   2Fe (l) + Al 2 O 3 (s)

Here we can see that one mole of Fe 2 O interacts with 2 moles of Al to give 2 moles of Fe and one mole of Al 2 O 3 .

Limiting Reactant

A chemical reaction might have one of the reactants present in excess amount and the chemical reaction may end as soon as one of the reactant ends leaving surplus of the other reactant unused. The reactant which is consumed fully during a chemical process is known as the limiting reactant.

Consider the example of making an egg sandwich where one egg and two bread slices will give you one egg sandwich.

Applying the stoichiometric analysis to the above recipe you can observe the ratio to be 2:1 for two slices of bread and one egg. Consider you are provided with a bread packet that has 28 bread slices and 11 eggs you will be able to prepare only 11 egg sandwiches from the given recipe. Here as you can observe the number of egg sandwiches that can be made is restricted to 11 by using all the eggs whereas the bread slices would be in excess and unused.

Consider the same concept in terms of a chemical reaction

  N 2  + 3H 2NH 3

Here in a balanced equation, we can see that one mole of nitrogen requires three moles of hydrogen to form a product hence here hydrogen acts as a limiting reactant which need two extra moles to react.

"An image showing limiting reactants using oxygen and hydrogen molecules"

Percentage Yield

The product quantity generated at the end of a balanced chemical equation performed in a specific environment is known as the theoretical reaction yield. Practically, it is observed that the product quantity obtained also known as the actual generation is sometimes a reduced amount than its theoretical outcome for a variety of causes. Inherently, certain reactions are unproductive, followed by side processes that produce supplementary products. Certain products are difficult to obtain without certain amount of loss and so the actual yield is decreased by less than flawless recovery. Few others are formed naturally incomplete.

Percentage yield can be expressed in the following form:

Percent yield=(actual amount/theoretical amount)×100%

The yield can be expressed in terms of masses, volumes in case of gases or by molar amounts but all the products should be given in the same molecular units so that they can be cancelled out and percentage yield can be calculated.

Applications of Stoichiometry

Chemical stoichiometry can be used to calculate an empirical formula by specifying which elements are present in the molecule and in what ratio.

In industry, stoichiometry is most commonly used to calculate the amount of resources needed to manufacture the demanded quantity of goods in a certain chemical equation. Stoichiometric estimation allows company technologists to determine the quantity of products they would receive from a given process it can also help to evaluate if the commodity is economical to manufacture or not. For instance, addition of SnF2 stannous fluoride in toothpaste to prevent tooth decay also applies stoichiometry as the numbers of products that will be formed from the reactants are all based on the ratios of the moles of the reacting elements.

Common Mistakes

Care should be taken while balancing the equation or else the mole calculation will be incorrect.

Practice Problems

Question: Calculate molar mass of H 2 O


Molar mass of H 2 O = 2  × molar mass of hydrogen  + 1 × molar mass of oxygen                                 = 2×  1.007 g/mol  + 1 ×  15.99 g/mol                                 = 18.015 g/mol

Context and Applications   

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for 

  • Bachelors in Chemistry
  • Masters in Chemistry

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