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Stoichiometry Definition: Solved Examples & Calculation

Jul 8, 2022


Stoichiometry is the quantitative analysis of the reactants required or the products formed. Chemical reactions act on the chemical changes undergone by various compounds and elements.

A balanced chemical equation imparts a lot of information. The coefficients stipulate the molar ratios and the discrete number of particles participating in a specific reaction. Stoichiometry is the quantitative analysis of the reactants required or the products formed.


Although it sounds complicated while the vocabulary of the word, the concept of stoichiometry is very simple and easy to understand. This article will help you in learning more about stoichiometry in Chemistry.

What is Stoichiometry?

The word ‘stoichiometry‘ comes from the Greek words ‘stoicheion’ (meaning element) and ‘metron’ (meaning measure). Hence, it means measurement of the element.


Stoichiometry denotes the quantitative connection between a chemical reaction’s products and the reactants. Stoichiometry is a branch of chemistry that applies the laws of definite proportions and the conservation of mass and energy to chemical activity.

A well-balanced chemical reaction and the coefficients of reactants and products of a reaction are all part of the stoichiometry of a reaction. In short, stoichiometry means is the measurement of small parts in a reaction.


Definition of Stoichiometry 

The balanced chemical equation represents the stoichiometry definition for a reaction. It represents the quantities or amounts of products and reactants involved in the chemical equation.

In chemistry, stoichiometry definition is given as the quantitative connection between two or more substances, especially in chemical or physical change processes.


Stoichiometry definition can be the definite proportions in which elements or compounds react with one another. The rules followed in calculating the stoichiometric relationships are based on the laws of conservation of mass and energy and the law of combining weights or volumes.

To understand more about the stoichiometry definition, let us take the following stoichiometry example:


N2 (g) + 3H2(g) → 2NH3(g)

In this reaction, Nitrogen (N2) and Hydrogen (H2) react to form Ammonia (NH3).


1 molecule + 3 molecules = 2 molecules

So the stoichiometric coefficients in this reaction indicate that one molecule of N2 combines with three molecules of H to form two molecules of NH3.


Coefficient of Stoichiometric 

The stoichiometric coefficient is the counting or number of molecules that participate in the reaction. In other words, the stoichiometric coefficients in a chemical equation can be acknowledged as the number of moles of all substances (products and reactants). Let’s take some instances to understand the stoichiometric coefficient.

For a balanced equation for this reaction

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)

Coefficient 2 for H2O and O2 are called stoichiometric coefficients. Similarly, the stoichiometric coefficients for CH4 and CO2 are 1. These stoichiometric numbers represent the number of molecules and moles participating in or forming the reaction.

Stoichiometric coefficients can be whole numbers or fractions. These coefficients help to establish the mole ratio between reactants and products.

Stoichiometry Formulas

Using stoichiometric calculations, you can determine the amount of one or more reactant(s) required to produce a particular product (and vice-versa). The substance present in a stated volume of a solution is indicated in several ways, such as mass percent, molarity, mole fraction, and molality. Sometimes normality is also used.

Stoichiometric formulas help you calculate a solution’s concentration or the amount of substance present in its given volume. These can be expressed in the following ways:

  1. Weight per cent (w/w %) or Mass per cent

It is obtained by using the following relation: 

Mass percent = (Mass of solute / Mass of solution) x 100

  1. Molarity

 It is interpreted as the number of moles of the solute in 1 litre of the solution and denoted as M.

Molarity, M = (Number of moles of solute) / (Volume of solution in litre)

  1. Mole fraction

 If a substance ‘X’ liquefies in substance ‘Y’ and their number of moles are nX and nY, respectively, the mole fractions of X and Y will be:

Mole fraction of X = (Number of moles of X)/ (Number of moles of solution) = nX/ (nX + nY)

Mole fraction of Y = (Number of moles of Y)/ (Number of moles of solution) = nY/ (nX + nY)

  1. Molality

It is interpreted as the number of moles of solute present in 1 kg of solvent. It is denoted by m.

Molality, m = Number of moles of solute/ Mass of solvent in kg

Stoichiometry, in Chemistry, is a type of Mathematics that lets you determine and calculate the number of chemicals. There are many quantitative units used in stoichiometry. A few of them are known to everyone, like liters and grams.

One unit, the mole, is generally only used in a chemistry set. A mole is a computing unit, like a score or a dozen. A mole is approximately 6.022 x 1023 units. It is the number of atoms in certainly 12 grams of carbon-12. You can apply this value to any atom, molecule, or particle.

Examples of Solved Stoichiometry Formulas for Stoichiometry 

Writing the chemical equation for the reaction and then balancing it to perform stoichiometric calculations is vital. The balanced chemical equation gives the stoichiometric coefficients, which can determine the proportion in terms of moles. Some stoichiometry examples are

Example 1: A solution is prepared by adding 4g of substance X to 16 g of water. Calculate the mass percent of the solute.

Answer: Here, solute = substance X

Using stoichiometry formulas,

Mass percent = (Mass of solute/ Mass of solution) x 100

Mass percent = [4/ (4 + 16)] x 100 = 20%

Example 2: Find the molarity of NaOH solution when it is prepared by diffusing its 4g in water and forming 250 mL of the solution.

Answer: Here, the mass of NaOH = 4g and the molar mass of NaOH = 40 g

Number of moles of NaOH = 4/ 40 = 0.1

Now using stoichiometry formulas,

Molarity, M = (Number of moles of solute)/ (Volume of solution in liter)

Molarity, M = 0.1/ 0.250 = 0.4

Therefore, the molarity of NaOH = O.4M

Example 3: The density of the 3M solutions of NaCl is 1.25 g mL-1. Calculate the molality of the solution.

Answer: Here, molarity = 3M

The mass of NaCl in 1L solution = 3 x 58.5 = 175.5 g

Density = 1.25 g mL-1

So, the mass of 1L solution = 1000 x 1.25 = 1250 g

The mass of water in solution = 1250 – 175.5 = 1074.5 g

Using stoichiometry formulas,

Molality, m = Number of moles of solute/ Mass of solvent in kg

Molality, m = 3/ 1074.5 = 2.79 m

Stoichiometry Calculator

The online stoichiometry calculator can help you to get stoichiometric coefficients. With the help of a stoichiometry calculator, you can balance a given chemical equation and compute amounts of the reaction’s products and reactants, both in moles and grams.

The equations may include free electrons, electrically charged molecules (or ions), and hydrated compounds. If the original equation was unbalanced, the field with this equation is highlighted in light pink in the stoichiometry calculator.

You can manually enter the chemical equation or directly paste it from another internet source. You can enter an unbalanced or a balanced chemical equation. Then, click on the calculate option. The results will show you a balanced chemical equation with the number of moles of each compound in the equation and their molar mass in grams.


Stoichiometry is one of the most practical applications in chemistry because it allows researchers to bridge the gap between the macroscopic quantities their instruments can compute and the raw data of atoms and molecules whirling around inside the test tubes.

Frequently Asked Questions

Q1. Why is stoichiometry important?

Answer: Stoichiometry is a branch of chemistry that looks into quantities of reactants and products involved in a chemical reaction. Stoichiometry is important in studying chemistry and industrial chemistry because you cannot mix reactants without knowing how much of each is needed!

2. How do you determine the stoichiometry in chemical reactions of reactants and products?

Answer: The coefficients in the balanced equation give you the ratios of moles. For example

CH4+ 2O2 → CO2 + 2H2O

This reaction doesn’t only explain to you that one molecule of CH4 collaborates with two molecules of O2 and forms one molecule of CO2 with two molecules of H2O. It means that one mole of CH4 and two moles of O2 react and form one mole of CO2 with two moles of H2O.

3. What is a limiting reagent?

Answer: Many times, during a chemical reaction between two or more reactants, a reactant that presents in the minimum amount gets absorbed after some time. Hence, the reactant which gets absorbed first limits the product’s formation. Therefore, it is known as a limiting reagent.

4. In what ways can I use stoichiometry in Chemistry?

Answer: Stoichiometry can be used in many different ways:

  • To calculate how much product a chemical reaction is expected to produce.
  • To calculate the percent yield of a chemical reaction that failed to produce its expected output.
  • To calculate how much of a reactant is necessary to react with another reactant.
  • To determine which two reactants will be consumed first in a chemical reaction.
  • To determine the formula of an unknown compound.
  • To determine the formula mass of an unknown compound.
  • To determine the concentration of a solution.




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