### Stoichiometry: Mole concept and limiting reactants

Stoichiometry is a branch of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. It is based on the concept of the mole, which is the unit of measurement for the amount of a substance.

Mole Concept

The mole is defined as the amount of a substance that contains the same number of particles as there are atoms in 12 grams of carbon-12. This number, known as Avogadro's number, is approximately 6.022 x 10^23 particles per mole.

Using the mole concept, we can convert between the mass, number of particles, and volume of a substance. For example, to convert from mass to moles, we divide the mass by the molar mass of the substance, which is the mass of one mole of the substance.

Limiting Reactants

In a chemical reaction, the reactants are consumed to form products. The reactant that is completely consumed in the reaction is known as the limiting reactant, because it limits the amount of product that can be formed.

To determine the limiting reactant, we need to compare the stoichiometric ratios of the reactants. The stoichiometric ratio is the ratio of the coefficients of the reactants and products in a balanced chemical equation. The reactant that has the smallest stoichiometric ratio is the limiting reactant.

Once we have identified the limiting reactant, we can use stoichiometry to calculate the theoretical yield of the product, which is the maximum amount of product that can be formed from the given amounts of reactants.

Example:

Consider the reaction between hydrogen gas and oxygen gas to form water:

2H2(g) + O2(g) → 2H2O(g)

If we have 4 moles of hydrogen gas and 2 moles of oxygen gas, which is the limiting reactant and what is the theoretical yield of water?

To determine the limiting reactant, we need to compare the stoichiometric ratios:

H2 : O2 = 2 : 1

4 moles of H2 = 4/2 = 2 moles of O2

Since we have only 2 moles of O2, which is the smaller amount, O2 is the limiting reactant.

Theoretical yield of water = 2 moles of O2 x (2 moles of H2O / 1 mole of O2) = 4 moles of H2O

In conclusion, stoichiometry is an important tool in chemistry that allows us to calculate the quantitative relationships between reactants and products in a chemical reaction. The mole concept and the concept of limiting reactants are essential in stoichiometry, as they allow us to convert between mass, number of particles, and volume of a substance, and to identify the reactant that limits the amount of product that can be formed.