Reaction Stoichiometry Problems and Questions

  1. Define stoichiometry.
  2. What do the coefficients in a balanced chemical equation represent?
  3. What is a mole ratio, and where do the numbers come from?
  4. What is the “pathway” (series of dimensional analysis steps) used to solve most stoichiometry problems?
  5. How many grams of iron are produced when 0.75 g of ferric oxide is reacted with an excess of aluminum in the following balanced equation for the “thermite” reaction? 2Al + Fe2O3 → 2Fe + Al2O3
  6. How many grams of iron (III) oxide are formed when 1.50 grams of metallic iron is burned in a crucible? Iron burns according to the unbalancedequation: Fe + O2 → Fe2O3
  7. Sodium metal is reacted with an excess of water. How many grams of sodium are needed to produce 6.5 g of hydrogen gas? The unbalancedequation is: Na + H2O → NaOH + H2.
  8. A camping stove burns butane gas according to the reaction: C4H10 + O2 → CO2 + H2O (unbalanced). How many grams of oxygen are needed to completely react with 75.0 g of butane?
  9. Ferric oxide is reduced to elemental iron through the following single replacement reaction:

Fe2O3 + H2 → Fe + H2O (unbalanced). How many grams of iron can be produced if 5.00 g of ferric oxide is reacted with excess hydrogen gas?

  1. Define theoretical yield.
  2. Define actual yield.
  3. What equation is used to determine the percentage yield of a chemical reaction?
  4. What is the maximum percentage yield for any reaction?
  5. Suppose that a chemist works for a company that produces a compound through a chemical reaction. Why would the percentage yield of the reaction be important to the chemist (and the owners of the company)?
  6. A chemist calculates that a maximum of 3.82 g of product can be produced in a reaction, but only 2.76 g of material is actually produced. What is the percentage yield of the reaction?
  7. Tungsten (W) can be produced from its oxide according the following balanced chemical equation:

WO3 + 3H2 → W + 3H2O. What is the percentage yield if 56.9 g of WO3 yields 41.4 g of tungsten?

  1. In the commercial production of elemental arsenic, the following balanced reaction equation is used: 2As2O3 + 3C → 4As + 3CO2. If 8.87 g of arsenic (III) oxide is used in the reaction and 5.33 g of arsenic is produced, what is the percentage yield?
  2. Diiodine pentoxide is used in respirators because it reacts with dangerous carbon monoxide according to the following balanced equation: I2O5 + 5CO → I2 + 5CO2. What is the percentage yield if 2.00 g of carbon monoxide reacts within a respirator and produces 3.17 g of iodine?
  1. Explain the concept of limiting reagent.
  2. Suppose that you are making cookies and the recipe requires 2 eggs, 1 cup of sugar, and 2 cups of flour to make 24 cookies. However, you actually have 7 eggs, 2 cups of sugar, and 3 cups of flour available. What is the maximum number of cookies that you can make and what ingredient is the “limiting reagent?”
  3. In the production of copper from ore containing copper (II) sulfide, the ore is first roasted to change it to the oxide according to the equation: 2CuS + 3O2→ 2CuO + 2SO2. If 118 g of CuS and 56 g of oxygen gas are available, which reactant is limiting and how much CuO can be formed?
  4. Use the chemical equation in the previous problem to determine the maximum amount of CuO that can be formed from the reaction of 18.7 g of CuS and 12.0 g of oxygen. Which reactant is the limiting reagent?
  5. How much iron can be formed if 2.50 of aluminum reacts with 6.75 g of ferric oxide? 2Al+ Fe2O3 → 2Fe + Al2O3
  6. Copper (II) oxide and hydrogen gas react to form pure copper and water vapor according to the balanced equation: Cu2O + H2 → 2 Cu + H2O. Suppose that a chemist has 15.38 g of Cu2O and 0.25 g of hydrogen gas available. After carrying out the reaction, the chemist finds that 10.83 g of pure copper actually has been produced. What is the percentage yield? HINT: first determine the maximum amount (theoretical yield) of copper that can be produced, then determine the percentage yield using the actual yield.

Answers to Selected Problems: (5) 0.52 g Fe, (6) 2.15 g Fe2O3, (7) 150 g Na, (8) 269 g O2, (9) 3.50 g Fe, (15) 72.3%,

(16) 91.8%, (17) 79.3%, (18) 87.3%, (21) 93 g CuO, (22) 15.6 g CuO, (23) 4.72 g Fe,(24) 79.1% yield.