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Stoichiometry – Multi Step Conversions

Stoichiometry – Multi Step Conversions

  • Stoichiometry is the calculation of quantities in chemical reactions.
  • When you know the quantity of one substance in a reaction, you can calculate the quantity of another substance consumed or created in the reaction.
  • A quantity can be grams, moles, liters, molecules, atoms, ions, formula units or particles.
  • Theoretical yield is the maximum amount of product that will form during a reaction. (how much you expect to make)
    • Example: 1 box of muffin mix says it will make 12 muffins
  • Any time you are calculating the amount of product produced you are calculating theoretical yield.
  • Actual yield is the amount of product that actually forms when the reaction is carried out in a laboratory.
    • Example: when used the 1 box of muffin mix only makes 11 muffins (instead of the 12 it theoretically should have made).

 

  • Stoichiometry Calculations General Steps
    • The first step is to convert the give substance measurement to moles. (if not starting with moles)
    • Next use the mole ratio to switch between substances
    • Finally convert to the desired substance to the correct unit for the final answer.
    • Remember:
      • 1 mol = (molar mass) grams
        • RECAL: molar mass is the SUM of ATOMIC MASSES
      • 1 mol = 22.4 Liters
      • 1 mol = 6.02 x 1023 particles (ptl)
      • _____ mol A = ____ mole B     [called the mole ratio]
      • These are the ONLY conversion you can do.
    • Things to remember
      • Don’t let the words in the problem confuse you. If it doesn’t have a number or a unit it probably is not important.  Especially when given the complete chemical equation. 
      • You MUST have a balanced chemical equation to do ANY mole to mole conversions.
      • The coefficients in the balanced chemical reaction are used in the mole ratio ONLY
      • Mole ratios are the ONLY place that you can switch substances.
      • Can NOT do this [], you can only go from gram to mole, volume to mole, particles to mole
      • The starting amount (# given in the problem) is only written ONE time, and never in a conversion fraction.
      • Any time you have a reactant in excess it does NOT affect calculations (you can ignore it)
      • Some students find using the mole map helpful

                            

mole map