2.3 Amount of Substance

The mole and the Avogadro’s constant

• A mole of a substance is the amount of substance that contains the same amount of stated elementary units as there are atoms in 12 g of C-12.

• The number of atoms is 12 g of C-12 is 6.02 x 10²³ . This number is also known as the Avogadro’s constant, L.

• Examples:
  • 1 mol of He contains 6.02 x 10²³ He atoms.
  • 1 mol of CO2 contains 6.02 x 10²³ CO2 molecules but 3 x (6.02 x10²³) atoms.
  • 1 mol of NaCl contains 02 x 10²³ NaCl units, Na⁺ and Cl⁻ ions.

Moles and mass

Moles and volumes

• Volume occupied by a gas depends on the amount of gas, temperature and pressure. In other words the volume of a gas is not fixed
• Avogadro’s law states that for equal volumes of all gases, under the same conditions, contain the same number of moles.
• Hence, equal number of moles of any gas, under the same conditions, would occupy the same It does not depend on the nature of gas.
• At room temperature of 20 ℃ and a pressure of 1 atm, one mole of any gas occupies 24 dm³.
• At standard temperature and pressure (s.t.p), which is 0 ℃ and 1 atm, one mole of any gas occupies 22.4 dm³.
• Complete combustion of hydrocarbon produces water and carbon dioxide. The general equation is as follow:
• In incomplete combustion, the possible products are carbon dioxide, carbon monoxide, carbon soot and water.

Moles and concentration of solutions

• A solution is a homogeneous mixture of two or more substance
• The substance presents in small quantity is called the solute while the substance present is larger quantity is called the solvent
• Concentration is the amount of solute present in a fixed quantity of solution
• Concentration is expressed in terms of g dm⁻³. Concentration in mol dm⁻³ is called molar concentration or molarity.