stoichiometry

• Stoichiometry


  The branch of Chemistry that deals with calculations that relate quantities of reactants and products
• Balancing chemical equations


  1. Make sure the chemical formula of each substance is correctly written

  2. Balance each element in the equation by putting a coefficient in front of each substance

  3. Start with the most complex chemical formula

  4. Balance polyatomic ions or radicals as a unit
• Balancing the equation 2KClO3 → 2KCl + 3O2


  1. The most complex chemical formula is KClO3, so start by counting

  2. K balance: 1 in the reactant, 1 in the product

  3. Cl balance: 1 in the reactant, 1 in the product

  4. O balance: 3 in the reactant, 2 in the product

  5. Put a coefficient of 2 before KClO3 and a coefficient of 3 before O2 to balance the oxygen atoms

  6. Balance the K and Cl by putting a coefficient of 2 before KCl
• Mole


  The unit used in the International System of units (SI) as a measure of the amount of substance. One mole of any substance contains as many individual entities as there are atoms in exactly 12 grams of C-12.
• Mole calculations


  • 0.125 moles Na = 7.53x10^22 atoms Na
  • 4.25x10^-3 moles Cl2 = 2.56x10^21 molecules Cl2
  • 32 grams CH4 = 2 moles CH4
• Molar mass


  The mass of substance present per mole. The molar mass of an element corresponds to its atomic mass, while the molar mass of a compound is the sum of the atomic masses of its component elements.
• Molar mass calculations


  • Molar mass of O2 = 2(16) = 32 g/mol
  • Molar mass of Fe2O3 = 2(55.85) + 3(16) = 159.7 g/mol
• Calculating percent composition


  1. Calculate the molar mass of the compound

  2. Calculate the mass% of each element
• Percent composition calculations


  • H2O: %H = 11.2%, %O = 88.8%
  • Fe2O3: %Fe = 69.94%, %O = 30.06%
• Determining empirical formula from percent composition

  1. Get the number of moles for each element

  2. Compare all number of moles with the smallest value

  3. Use the numbers obtained in the comparison ratio as subscripts of the element in the empirical formula
• Empirical formula calculations


  • CaCl2 from %Ca = 36.04%, %Cl = 63.96%
  • Unknown compound from %Na = 43.4%, %C = 11.3%, %O = 45.3%
• Limiting reagent


  The reactant that is completely consumed in a chemical reaction, causing the reaction to stop
• Limiting reagent examples


  • 2.5 moles Fe reacted with 3 moles S, Fe is the limiting reagent
  • 50 g FeO reacted with 25 g Al, determine the limiting reagent