Calculations based on stoichiometry

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 out 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.

Stoichiometric equation :

All amount related calculations for a chemical reaction  are collectively known as stoichiometry . It always requires balanced equation of the reaction. Stoichiometry is based on law of conservation of mass .By convention, atoms are denoted with a subscript zero for uncombined elements, an integer for uncombined compounds, and a superscript for negative ions.

• In balanced equation, the number associated with all the substances is called stoichiometric coefficient of those substances.The value of stoichiometric coefficients of substance may be changed but the ratio among them is constant. Example

• Since the review of fundamental principles of stoichiometry, it is now time to do some example problems.Suppose a reaction involves breaking of  bonds (for example, between hydrogen and oxygen to form water), and this bond energy isn’t accounted for in other ways (for example, as heat or sound). In that case, it must be counted on the side with the atoms being created.

Stoichiometric Formulas :

A balanced chemical equation gives the chemical formulas of the reactants and the amount of each reactant that reacts to produce the amount of product . 

2H2 + O2 → 2H2O

Mass of  reactants ( H2 and O2  is  equal to mass of product (H2O) i.e 18 g. That means mass is conserved.The number of water molecules can be calculated if the masses of hydrogen and oxygen gas are known. Recognising patterns of chemical reactivity will enable you to predict whether certain substances will combine to form products. As a result, stoichiometry itself can feel like it’s narrowly focused on numbers and maths. Stoichiometry formulas will help you solve and balance the equation with complete steps.

Some Important Terms :

• Avogadro Number: Avogadro’s number is a non-negative integer that has no chemical significance. This number is known as the number of particles present in 12 grams of carbon-12.

• Molar Mass: Molar mass is mass of one mole substance and is represented by the formula Molar mass = Mass of the substance/ moles where 1 mole of substance = 6.02 × 1023 unit particle of that element. 

• Formula Mass: The formula mass of a compound is calculated by summing the masses of each atom present in the given formula. For example, the formula mass of ammonium sulphide (NH4S) is calculated as 1(14) + 4(32) = 102

For Example, if a molecule of Methane is reacting with two molecules of Oxygen, then what are the products received by the person. Write the equation of it?

Here, we will determine the products by writing the equation, and the Stoichiometric Formulas will work according to it. So, the equation is :

CH4 + 2 O2 → CO2 + 2 H2O

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
• Second step involves conversion of given substance units such as mass (g)  into moles 
• Third step involves use of stoichiometric ratio to get moles of required substance in chemical reaction.
• Fourth step  involves conversion of moles of required substance to the required units .

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

Step 1 requires balanced equation

4HCl + MnO2→MnCl2 + 2H2O + Cl2

Step 2  involves converting Mass of MnO2 into moles

Moles of MnO2 =43.5/87 = 0.5

Step 3 involves use of 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 conversion of 2 moles 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 :

1. When iron pyrites are burned in the presence of oxygen gas, sulphur 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 sulphur trioxide gas is released are :

4FeS2 + 15O2 —> 2Fe2O3 + 8SO3

1. 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

Conclusion :

In this material, 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 calculations examples and the formulas attached with them to solve the questions given. Apart from it, learning stoichiometry formulas and applying them to the problems will help the students to understand the concept better.