Reacting Quantities

Reacting Quantities


  • Reacting quantities refer to the amounts of substances involved in a chemical reaction.
  • This concept uses the ratio of moles of each substance, which can be determined from a chemical equation.
  • It’s based on the principle that matter can neither be created nor destroyed (Law of Conservation of Mass).

Key Points

  • A balanced chemical equation provides the number of moles of reactants and products in a chemical reaction.
  • The stoichiometric coefficients in the balanced equation give the molar ratio of the reactants to the products.
  • For instance, in the equation 2H2 + O2 → 2H2O, 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water.
  • The molar mass of a substance (measured in grams per mole) can be used to calculate the mass of a reactant or product if the number of moles is known, and vice versa.

Calculation of Reacting Quantities

  • The formula n = m/M can be used, where:
    • n = number of moles
    • m = mass of substance (grams)
    • M = molar mass of substance (grams per mole).
  • To find the mass of a substance required or produced, rearrange the equation to m = n*M.

Understanding Limiting Reactants

  • The limiting reactant is the substance that is completely used up in a chemical reaction.
  • Determination of the limiting reactant is crucial as it determines the maximum amount of product that can be formed.
  • Excess reactant(s) remain after the reaction has gone to completion.


  • Understanding reacting quantities is central to carrying out chemical calculations, planning industrial chemical processes, and conducting laboratory reactions.
  • It is used in areas of scientific research and industry to predict the yield of a reaction to make it as effective and cost-efficient as possible.
  • This understanding can also help avoid the production of unwanted or hazardous by-products.