 Stoichiometry and Stoichiometric Calculations

You may ask why do we need to measure the elements in chemical reactions? What happens when your mom adds extra salt to your favourite pasta? Do you eat it? No! Similarly, without a proper study of Stoichiometry, we can’t know what quantity of elements are perfect for any suitable reaction. In this chapter, we will understand the concept of Stoichiometry and look how we can make the stoichiometric calculations from those reactions.

Suggested Videos         What is Stoichiometry?

The branch of stoichiometry deals with the calculation of various quantities of reactants or products of a chemical reaction. The word “stoichiometry” itself is derived from two Greek words “stoichion” that means element and “metry” means to measure. We have the following two sub-sections in this concept of stoichiometry.

• Gravimetric analysis
• Volumetric analysis

We can solve the problems on gravimetric and volumetric analysis by using the two well-known concepts of mole concept and the concept of equivalence. To understand Stoichiometric calculations, we must be able to understand the relationship between the various reactants and products in a chemical reaction. For a reaction to be balanced, both sides of the equation should have an equal number of elements. We use various stoichiometry coefficients to adjust the number of each element on both sides of the reaction. These are the number of atoms that we write to balance the reaction.

Browse more Topics under Some Basic Concepts Of Chemistry

Let us now discuss the various conversion factors to solve stoichiometric problems. We have to follow a few steps for the same. They are:

1. Balancing the given equation.
2. Giving the substance the unit as a mole.
3. Calculating the number of moles.

Stoichiometric Calculations are Based on Chemical Formulas

Let’s learn some terms used in Stoichiometry first.

• Formula Mass: It is the sum of the atomic weights of the various atoms present in the molecule of the substance. For example, we can calculate the formula mass of Na2S as 2(23) + 1(32) = 78
• Avogadro number: Avogadro’s number is the total number of particles present in one mole of a substance. It is the number of atoms present in exactly 12g of C-12. Avogadro number is valued as 6.022 × 1023.
• Molar Mass: It is the sum of the total mass of all the atoms that constitute a molecule per mole.

We can explain the mole ratio of reactants and products with the help of the following reaction:

2Na + 2HCl  2NaCl + H2

From the above reaction, we can see that 2 moles of Na react with 2 moles of HCl. This reaction forms 2 moles of NaCl ( Sodium Chloride) and 1 mole of H2. Hence, for a given amount of sodium, let us say x mole, we get the x mole of NaCl in reaction with x mole of HCl. The hydrogen gas produced will be x/2 moles.

Solved Example for You

Q: What is a limiting reagent?

Ans: Limiting reagent is an important concept in a chemical calculation. It refers to a reactant which is present in a minimum stoichiometric quantity for a given chemical reaction. This particular reactant is fully consumed in the  chemical reaction. So, we make all the calculations related to various products or in a sequence of reactions on the basis of limiting reagent.

The limiting reagent is necessary to find out in a chemical reaction that involves more than two reactants. In that case, we need to understand which one is the limiting reagent for analysing the chemical reaction.

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