Stoichiometry | Chemistry for JAMB PDF Download

In a chemical reaction, the ratio in which the reactants combine or react with each other is fixed and known. This is known as Stoichiometry. Stoichiometry deals with the calculation of quantities of reactants and products in a chemical reaction. Stoichiometry is used primarily when dealing with different moles of chemical substances. Based on this principle, scientists measure the exact amount of materials required for a reaction. One can calculate the quantities in terms of moles, mass, and volume depending on how much information is given.

Chemical Reactions and Equations

• Chemical Reaction: A process where one or more substances (reactants) are transformed into new substances (products).
• Chemical Equation: An expression that illustrates the identities and quantities of reactants (on the left) and products (on the right) involved in a chemical reaction.

Patterns of Chemical Reactivity

• Recognizing patterns in chemical reactivity helps predict the products formed when reactants combine.

Formula Masses

• Empirical Formula: Represents the simplest ratio of elements in a compound, derived from percent composition.
• Molecular Formula: Represents the actual number of atoms of each element in a compound and can be derived from the empirical formula and the compound's molar mass.
• Percent compositions can be determined through methods like combustion analysis.

Avogadro's Number and the Mole

• Molecular Mass and Formula Mass: Calculated by adding the atomic masses of the elements in a compound's formula, measured in atomic mass units (amu).
• Mole: Defined as the amount of substance containing $6.022\times 10236.022\; \backslash times\; 10^\{23\}$6.022×1023 atoms (Avogadro's number) of a specific element.

Empirical Formulas from Analysis

• Chemical formulas indicate the number of each type of atom in a compound, while empirical formulas denote the simplest ratio.
• While combustion analysis can determine empirical formulas, it cannot definitively determine molecular formulas.

Quantitative Information from Balanced Equations

• Balanced chemical equations provide quantitative information, allowing calculations of reactant and product amounts.
• Stoichiometric calculations often involve volumes of solutions with known concentrations instead of masses.
• The coefficients in balanced equations indicate the number of moles of reactants and products.

Limiting Reactants

• Stoichiometry: Describes the relative amounts of reactants and products in a balanced chemical equation.
• Stoichiometric Quantity: The exact amount of a reactant required to completely react with another reactant.
• Limiting Reactant: The reactant that is consumed first in a reaction, determining the maximum amount of product formed.
• Excess Reactant: The reactant that remains unreacted after the reaction has gone to completion.

Stoichiometric calculations :

When solving stoichiometric problems, the first step is to determine what kind of problem you are dealing with. It could be mass, volume, or number of moles. The second step is to identify the reactants and products. Reactants are the elements present at the beginning of a chemical reaction, while products are the elements present at the end of it. In order to solve stoichiometric problems, follow these steps given below as most of the stoichiometric problems can be solved by following them.

• Firstly, Balance the chemical equation
• The second step involves the conversion of given substance units such as mass (g)  into moles 
• The third step involves the use of a stoichiometric ratio to get moles of the required substance in a chemical reaction.
• The fourth step involves the conversion of moles of required substance to the required units.

Let us take an example of the calculation of the mass of HCl needed for a complete reaction with 43.5 gm MnO2.

Step 1 requires a balanced equation

4HCl + MnO₂→MnCl₂ + 2H₂O + Cl₂

Step 2  involves converting the Mass of MnO2 into moles

Moles of MnO₂ =43.5/87 = 0.5

Step 3 involves use of a stoichiometric ratio 

For 1 mole of MnO₂ = 4 mole of HCl is required

For 0.5 mole of MnO₂ =  moles of HCl is required = 2 moles

Step 4 involves the conversion of 2 moles of HCl into its mass

Mass of HCl required = 2 x 36.5 = 73 gm     

• To solve Stoichiometric calculations, consider information like molecules, moles, mass, etc., together, which can lead to finding out the answers accurately, and having a required time period to solve these problems is essential.
• If there is an excess number of moles of an element in the balanced equation, there will be more moles of that element in the unknown reaction compared to what there are of each other element. Also, if there’s an excess number of molecules of one element compared to another in the balanced equation, then there will be a greater mass of that element’s molecules compared to any other element in an unknown reaction.

Some Stoichiometric calculations examples that will help you understand the concepts are :

 Ans: The chemical reaction that takes place when iron pyrites are burned and sulfur trioxide gas is released are :

4FeS₂ + 15O₂ —> 2Fe₂O₃ + 8SO₃

2. Solid copper oxide is a base that reacts with ammonia in the presence of excess amounts of water. What will be the reaction of the following experiment?

 Ans: According to the way the experiment is carried out, the equation that will be represented is :

2NH₃ + 3CuO —> 3Cu + 3H₂O + N₂

We learned about stoichiometric calculations and how they are performed during the practicals. Besides this, we learned more about stoichiometry formulas and stoichiometric equations that help us solve fundamental problems. In the end, we looked at some stoichiometric calculation examples and the formulas attached to them to solve the questions given. Apart from that, learning stoichiometry formulas and applying them to the problems will help the students to understand the concept better.