Mole

• A mole is a standard scientific unit for counting particles—like a “chemist’s dozen”.

• It represents 6.022 × 10²³ elementary entities (atoms, molecules, ions, or electrons). This number is called Avogadro’s number.

• One mole of any substance contains the same number of particles, even though the mass differs.

Molar Mass

• Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol).

• Calculated by adding the atomic masses of all atoms in the chemical formula (found in the periodic table).

• Examples:

  • H₂O: (2 × 1.008) + 16.00 = 18.02 g/mol

  • NaCl: 22.99 + 35.45 = 58.44 g/mol

Conversion

• Grams ↔ Moles:

  moles= mass (g)​ / molar mass (g/mol)

• Moles ↔ Particles:

  number of particles=moles×6.022×1023

Key Insight:

The mole lets chemists translate between the invisible world of atoms and measurable lab quantities.

Molarity

Molarity measures how much solute is dissolved in a given volume of solution

Things to keep in mind:
Higher molarity = more concentrated solution
Lower molarity = more dilute solution
Volume must be in liters, not mL

Dilution of Solution

Dilution is the process of reducing the concentration of a solution by adding more solvent, while keeping the amount of solute constant.

Dilution Formula

M1​V1​=M2​V2​

• M₁ and V₁: molarity and volume of the concentrated (stock) solution.

• M₂ and V₂: molarity and volume after dilution.

Example

• Prepare 250 mL of 0.25 M HCl from 1.0 M HCl stock:

  V1​ = (M2xV2)/​M1 ​​= (0.25×250mL)/(1.0) ​=62.5mL

• Measure 62.5 mL of 1.0 M HCl and add water until total volume is 250 mL.