What is Stoichiometry?

Mary McMahon
Mary McMahon

Stoichiometry is the mathematics behind the science of chemistry. Using known physical laws such as the law of conservation of mass and the law of definite proportions, stoichiometry is used to gather information about the amounts of various elements used in a chemical reaction, and whether they took the form of gases, solids, or liquids. Using mathematics, someone can determine the quantities of particular elements needed to create a desired reaction, or the quantities used in the generation of a reaction which has already occurred.

Stoichiometry is the mathematics behind the science of chemistry.
Stoichiometry is the mathematics behind the science of chemistry.

Beginning chemistry classes typically include an introduction to stoichiometry, in which students are asked to balance chemical equations on their homework in and class. Stoichiometry relies on the fact that elements behave in predictable ways, and that matter cannot be created or destroyed. Therefore, when elements are combined to create a reaction, something known and specific will happen, and the outcome of the reaction can be predicted on the basis of the elements and quantities involved.

Chemists look at both the reactants and their products, which can include a new chemical compound along with remainders. In a simple example of stoichiometry, oxygen and hydrogen gases combine to create water, but only in set amounts. Given a set quantity of water, a chemist can determine how much oxygen and water went into the reaction that created the water, using stoichiometry and a knowledge of basic chemistry. Chemists working in the lab can also determine the amount of various products needed to create a particular reaction before they embark on an experiment.

One of the underpinnings of stoichiometric calculations is that amounts have to remain constant. One cannot pull elements out of thin air, and therefore an equation will be unbalanced if one side contains more than the other. Chemists use a unit of measurement called the mole to measure mass in stoichiometry, and when they balance equations, they make sure that both sides are equal, even if the chemical reaction produced a new chemical compound.

While stoichiometric calculations may seem like the purview of scientists alone, people actually utilize stoichiometry every day, although they may not be aware of it. Bakers, for example, rely heavily on the principles of chemistry and the need for balancing ingredients, and they use the known properties of the “elements” they work with to create a balanced equation: a recipe which will have a positive outcome. Stoichiometry is also used in a number of industries, ranging from oil refining to hair dye production.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


You will use a lot of stoichiometry in chemistry.


@valleyfiah- When I took my college chemistry courses, I used the mastering chemistry site. You will have to either to buy a code or enroll in a class, but it seemed to help quite a bit. It had all kinds of resources including a stoichiometry practice quiz. I also know that most advanced chemistry textbooks also have supplementary workbooks that will help improve your chemistry knowledge.

The one for my textbook was about $40 extra and it helped me work through solutions. The workbook gave sample problems that were identical to the problems in the book, with only the numbers being different. This still forces you to do the work, but it allows you to make sure you are not forgetting a step. By the end of my first semester, I was a pro.


Can anyone recommend any stoichiometry practice websites? I am struggling in chemistry and it is mostly because of the stoichiometry. I understand the concepts of chemistry the professor teaches, but when it comes time to perform calculations I spend a lot of time on them, and often get confused. I know all I need is practice so I would appreciate it if anyone can recommend a good practice site or workbook.


I found out how important stoichiometry is after I took an energy class. Not only is stoichiometry appropriate for calculations in chemistry, it is useful in doing conversions between energy units. Energy units are so confusing because there are two sets of numbers (SI and British).

The principles of stoichiometry really helped me understand how to do the different equations related to energy efficiency, collector area, and power. For anyone going into an energy class, I would highly recommend a general chemistry course or two to build a solid mathematical foundation in science calculations.

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