Mole Concept & Stoichiometry

Chemistry is often called the "science of counting atoms," but atoms are far too small to count individually. The mole concept gives us a practical way to relate the microscopic world of atoms to the macroscopic world of grams and litres that we measure in the lab.

What is a Mole?

A mole is a counting unit, just like a "dozen" means 12. One mole contains exactly 6.022 x 10²³ particles (atoms, molecules, ions, etc.). This number is called Avogadro's number (Na).

1 mole of any substance = 6.022 x 10²³ particles = Molar mass in grams

Molar Mass

The molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). It is numerically equal to the atomic/molecular mass in amu.

Examples:

• Carbon (C): 12 g/mol
• Water (H₂O): 2(1) + 16 = 18 g/mol
• Calcium carbonate (CaCO₃): 40 + 12 + 3(16) = 100 g/mol

Stoichiometry

Stoichiometry uses balanced chemical equations to calculate quantities of reactants and products.

Steps for stoichiometric calculations:

1. Write and balance the chemical equation
2. Convert given quantity to moles
3. Use mole ratios from the balanced equation
4. Convert to desired unit

Worked Example

How many grams of CO₂ are produced when 10 g of CaCO₃ decomposes completely?

CaCO₃ --> CaO + CO₂

Step 1: Moles of CaCO₃ = 10/100 = 0.1 mol

Step 2: From the equation, 1 mol CaCO₃ produces 1 mol CO₂

Step 3: Moles of CO₂ = 0.1 mol

Step 4: Mass of CO₂ = 0.1 x 44 = 4.4 g

Limiting Reagent

When two reactants are mixed, the one that runs out first is the limiting reagent. It determines the maximum amount of product formed.

Example: 2H₂ + O₂ --> 2H₂O. If you have 3 mol H₂ and 2 mol O₂:

• H₂ needs: 3 mol H₂ requires 1.5 mol O₂ (available: 2 mol) -- enough O₂
• O₂ needs: 2 mol O₂ requires 4 mol H₂ (available: 3 mol) -- not enough H₂
• H₂ is the limiting reagent. Maximum H₂O = 3 mol.

Percentage Yield

Percentage yield = (Actual yield / Theoretical yield) x 100%

Nepal Connection

Limestone (CaCO₃), found abundantly in Nepal, is heated to produce quicklime (CaO) used in construction. Stoichiometry helps calculate how much limestone is needed for a given amount of quicklime.

Key Takeaways

• 1 mole = 6.022 x 10²³ particles = molar mass in grams
• Balanced equations provide mole ratios for calculations
• The limiting reagent determines maximum product yield
• Percentage yield compares actual to theoretical output