Lecture 10/13/99, Chapter 4, Section 4

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Stoiciometry, Limiting Reagent, and % Yield.

  1. Over the years, the thermite reaction has been used for welding railroad rails, in incendiary bombs, and to ignite solid-fuel rocket motors. What masses of iron(III) oxide and aluminum are required for a theoretical yield of 15.0 g iron? What is the maximum mass of aluminum oxide that could be produced? How much aluminum oxide is actually produced from these starting materials if the yield is 93%? The thermite reaction is:

    Fe2O3(s) + 2Al(s) -> 2Fe(l) + Al2O3(s)

      Fe2O3(s) 2Al(s) -> 2Fe(l) Al2O3(s)
    produce       15 g  
    MW 159.7 g/mole 26.98 g/mole   55.85 g/mole 102.0 g/mole
    Produce       0.269 mole 0.134 mole
    Need 0.134 mole 0.269 mol      
    Need 21.4 g 7.26 g      
    Yield (theoretical)       15 g 13.7 g
    93% Yield       14.0 g 12.7 g
  2. Elixirs such as Alka-Seltzer use the reaction of sodium bicarbonate with citric acid in aqueous solution to produce a fizz. Balance the following reaction.
    1. What mass of C6H8O7 should be used for every 1.0*102 mg of NaHCO3?
    2. If 1.0*102 mg of C6H8O7 is used, which is the limiting reagent?
    3. What mass of CO2(g) could be produced by this mixture?
    4. If only 45 mg of CO2 is produced, what is the percent yield?
    5. How many grams of Na3C6H5O7 is actually produced?
        3 NaHCO3(aq) 1 C6H8O7 -> 3 CO2 3 H2O 1 Na3C6H5O7(aq)
      given 1.0*102 mg          
      given 0.10 g          
      MW 84.01 g/mole 192.1 g/mole   44.01 g/mole 18.02 g/mole 258.07 g/mole
      given 1.19x10-3 mole          
      need   3.97x10-4 mole        
      need   0.076 g        
      produce       1.19x10-3 mole 1.19x10-3 mole 3.97x10-4 mole
      Yield (theoretical)       0.052 g 0.021 g 0.102 g
      Actual Yield       0.045 g    
      % yield       86.5 %    
      Actual Yield         0.018 g 0.088 g