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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 6.022×10236.022 \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.

Question for Stoichiometry
Try yourself:
Which term describes the exact amount of a reactant required to completely react with another reactant in a chemical reaction?
View Solution

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.

Question for Stoichiometry
Try yourself:What is the mass of H2O produced when 10 grams of H2 reacts with excess O2 according to the following balanced equation: 2H2 + O2 → 2H2O
View Solution

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

1. When iron pyrites are burned in the presence of oxygen gas, sulfur trioxide gas is released. How it can be written according to an equation?

 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.

Question for Stoichiometry
Try yourself:What is the balanced chemical equation for the reaction between magnesium and hydrochloric acid to form magnesium chloride and hydrogen gas?
View Solution

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FAQs on Stoichiometry - Chemistry for JAMB

1. What is stoichiometry and why is it important in chemistry?
Ans.Stoichiometry is the branch of chemistry that deals with the calculation of reactants and products in chemical reactions. It is important because it allows chemists to predict the quantities of substances consumed and produced in a reaction, ensuring that reactions are conducted efficiently and safely.
2. How do you perform stoichiometric calculations using a balanced chemical equation?
Ans.To perform stoichiometric calculations, first write and balance the chemical equation. Then, use the coefficients from the balanced equation to convert between moles of reactants and products. This involves using mole ratios to find the amount of substance needed or produced.
3. What is the mole ratio and how is it used in stoichiometry?
Ans.The mole ratio is the ratio of moles of one substance to the moles of another substance in a balanced chemical equation. It is used in stoichiometry to relate the quantities of reactants and products, allowing for the calculation of how much of one substance is needed to react with a given amount of another.
4. Can stoichiometry be applied to solutions and gases?
Ans.Yes, stoichiometry can be applied to solutions and gases. For solutions, molarity (moles per liter) is used to relate the concentration of reactants. For gases, the ideal gas law can be applied to convert between moles and volume, using the conditions of temperature and pressure.
5. What common mistakes should be avoided in stoichiometric calculations?
Ans.Common mistakes in stoichiometric calculations include failing to balance the chemical equation, misinterpreting mole ratios, and not converting units properly. It is crucial to double-check that all calculations are based on balanced equations and that the correct units are used throughout the process.
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