Moles

Concept of Moles

  • A mole is a unit of measurement in chemistry used to express amounts of a chemical substance.
  • It’s defined as the amount of any substance that contains as many particles as there are in 12 grams of carbon-12.
  • This number, known as Avogadro’s number, is approximately 6.022 x 10^23.

Molar Mass

  • The molar mass of a substance is the mass of one mole of that substance. It is measured in grams per mole (g/mol).
  • The molar mass of an element is numerically equal to the element’s relative atomic mass. For example, the molar mass of oxygen (O) is approximately 16 g/mol.
  • For a compound, it’s calculated by adding up the relative atomic masses of its constituent elements. For example, water (H2O) has a molar mass of approximately 18 g/mol (2*1 for Hydrogen + 16 for Oxygen).

Moles and Chemical Equations

  • Moles provide a bridge between the atom world (microscopic) and the real world (macroscopic).
  • In a balanced chemical equation, the coefficients in front of the formulas represent the number of moles of each substance involved in the reaction.

Calculations Involving Moles

  • To calculate the number of moles in a given mass of a substance, use the equation: moles = mass (g) / molar mass (g/mol).
  • To determine the mass of a certain number of moles of a substance, use the equation: mass (g) = moles x molar mass (g/mol).

Concentration and Moles

  • The concentration of a solution, measured in mol/L or M (molar), is calculated by dividing the number of moles of solute by the volume of solution in liters. The equation is: concentration (M) = moles of solute / volume of solution (L).

Analysis Using Moles

  • Understanding moles is crucial to many analytical chemistry techniques such as titration.
  • Titration uses a known concentration of a reactant (the titrant) to determine the concentration of an unknown reactant by finding the volume at which the reaction between the two is complete (the equivalence point).
  • The stoichiometric ratio of reactants from the balanced chemical equation and the concept of moles are used to calculate the unknown concentration.

Moles in Gas Laws

  • The ideal gas law, given as PV = nRT, uses the concept of moles (n) to relate the pressure (P), volume (V) and temperature (T) of a gas. Here R is the ideal gas constant.
  • In particular, Avogadro’s Law states that equal volumes of gases, at the same temperature and pressure, contain equal numbers of particles, or moles. Therefore, the volume of gas is proportional to number of moles if temperature and pressure are kept constant.