Mole Ratio and Stoichiometry

The word Stoichiometry comes from the Greek:  

- 'Stoichio' → element  

- 'Metry' → measure  

Literally, it means “measurement of elements.”

Stoichiometry is the branch of chemistry that involves calculations and expresses the quantitative relationships between reactants and products in a chemical reaction.  

The mole ratio in which reactants combine and products form defines the stoichiometry of the reaction.  

The Mole Concept  

- A mole represents a fixed number of particles (atoms, ions, molecules, or electrons).  

- This number is approximately 6.02 × 10²³, known as Avogadro’s constant.  

- Thus, 1 mole = Avogadro’s number of particles.  

Formulae for Calculating Moles  

• n = m/Mm
• n = N/L
• n = CV/1000
• n = v/Vm  
• n = CL 

Where:  

- n = mole  

- m = mass  

- Mm = molar mass  

- N = number of particles  

- C = molar concentration  

- V = volume  

- Vm = molar volume  

- L = Avogadro’s number 

Valency  

Valency is the combining power of an element, equivalent to the number of valence electrons in its outermost shell.  

- Hydrogen and metals generally have positive valencies.  

- Non‑metals usually have negative valencies.  

- Sometimes, a group of atoms combine to form a single unit called a radical.  

 - Radicals may have positive or negative valency, depending on their constituent atoms.  

Formation of Compounds:  

- When two or more elements combine, they exchange valencies.  

- These valencies are written as subscript numbers attached to the elements in the chemical formula.