What is Stoichiometry?

The branch of chemistry which tells us the quantitative relationship between reactants and products in a balanced chemical equation is called stoichiometry.

On the other hand, if the amount of one reactant is known and the amount of the products can be computed using empirical data, the amount of the other reactants can likewise be calculated.

This is illustrated in the image here, where the balanced equation is:

       CH₄ + 2 O₂ → CO₂ + 2 H₂O

Here, one molecule of methane reacts with two molecules of oxygen gas to yield one molecule of carbon dioxide and two molecules of water.

This particular chemical equation is an example of complete combustion. Stoichiometry measures these quantitative relationships and is used to determine the amount of products and reactants that are produced or needed in a given reaction.

 Describing the quantitative relationships among substances as they participate in chemical reactions is known as reaction stoichiometry. In the example above, reaction stoichiometry measures the relationship between the quantities of methane and oxygen that react to form carbon dioxide and water.

With the knowledge of atomic mass, molecular mass, the mole, Avogadro’s number, and the molar volume, we can make use of the chemical equations in a much better way and can get much useful information from them.

A chemical equation has certain limitations as well. They do not tell about the conditions and the rate of reaction. Chemical equations can even be written to describe a chemical change example that does not occur.

So, when the stoichiometric calculation is performed, we have to assume the following conditions.

• All reactants are completely converted into products.
• No side reaction occurs.
• While doing calculations, the law of conservation of mass and the law of definite proportion are obeyed.

The following type of relationships can be studied with the help of a balanced chemical equation.

Mass-mass Relationship

If we are given the mass of one substance, we can calculate the mass of the other substance and vice-versa.

Mass-mole Relationship

If we are given the mass of one substance, we can calculate the moles of the other substance and vice-versa.

Mass-volume Relationship

If we are given the mass of one substance, we can calculate the volume of the other substance and vice-versa.

Similarly, mole-mole calculations can also be performed.

---

Let us solve a problem to understand these relationships.

Solved problem

Magnesium metal reacts with HCl to give hydrogen gas. What is the minimum volume of HCl solution (27% by weight) required to produce 12.1 g of H_2. The density of HCl solution is .14 g/cm³.

Equation

Mg _(s) + 2HCl _(aq)     MgCl_{2 (aq)} + H_{2 (g)}

Solution

Mass of H₂ produced = 12.1 g

Molar mass of H₂         = 2.016 g/mol

Moles of H₂ = Mass of H₂ / Molar mass of H₂

                                    = 12.1/ 2.016 = 6.0 moles

To calculate the number of moles of HCl, compare the moles of H₂ with those of HCl.

                                    H₂           :       HCl

                                     1         :        2

                                      6        :        12

So, 12 moles of HCl are being consumed to produce 6 moles of H_2.

Mass of HCl     = Moles of HCl x Molar mass of HCl

                           = 12 x 36.5 = 438 g

We know that HCl solution is 27% by weight, it means that

27 g of HCL are present in HCL solution = 100g

1 g of HCL are present in HCL solution     = 100/27

438 g of HCL are present in HCL solution = 100/₂₇ ^x 438                                                                                                                                            

                                                                          = 1622.2 g   

Density of HCl solution = 1.14 g/cm³

Volume of HCl = Mass of HCl solution/ Density of HCl

                           = 1622.2/ 1.14 = 1423 cm³

To produce 12.1 g of H2 we required 1423 cm³ volume of HCL.

What is the importance of stoichiometry?

Stoichiometry is essential when quantitative information about a chemical reaction is required. However it is important to predict yields of chemical products.

What are the 4 steps of stoichiometry?

Almost all stoichiometric problems can be solved in just four simple steps:
Balance the equation.
Convert units of a given substance to moles.
Using the mole ratio, calculate the moles of substance yielded by the reaction.
Convert moles of wanted substance to desired units.

Why is it called stoichiometric?

Stoichiometry is a section of chemistry that involves using relationships between reactants and products in a chemical reaction to determine desired quantitative data. In Greek, stoikhein means element and metron means measure, so stoichiometry generally translated means the measure of elements.

What is a real life example of stoichiometry?

Soap, tires, fertilizer, gasoline, deodorant, and chocolate bars are just a few commodities you use that are chemically engineered, or produced through chemical reactions. Chemically engineered commodities all rely on stoichiometry for their production.

Who is the founder of stoichiometry?

Stoichiometry was first discovered by Jeremias Richter, a German chemist. It was Richter who coined the term stoichiometry, a tongue-twisting word that baffles students to this day. Stoichiometry was derived from stoikheion, Greek for “element”, and “metron”, meaning measure.