Stoichiometry Notes (Chapter 12)

Review of Molar Mass Recall that the molar mass of a compound is the mass, in grams, of one mole of that compound. To determine the molar mass of a compound, add together the molar mass of all of the elements in the compound.

Practice Problems: 1.What is the molar mass of nitrogen dioxide?

2. What is the mass in grams of 1.25 moles of sulfur trioxide?.

Practice problems: 3. When bees sting they release the compound isopentyl acetate, C 7 H 14 O 2. How many moles of isopentyl acetate are there in 2.00 grams of C 7 H 14 O 2 ?

Stoichiometry - shows the relationship between reactants and products in a chemical reaction. Now we will use our knowledge of chemical reactions and equations to solve problems.

One new conversion we need to know is the mole ratio – it relates the amounts of moles of any two substances involved in a chemical reaction. 2Al 2 O 3  4Al + 3O 2

Three types of problems involving stoichiometric calculations (mole ratios) are: 1. Mole to mole problems (1 step problems) 2. Mole to mass problems (2 step problems) 3. Mass to mass problems (3 step problems)

Practice Problem 4. According to the following equation how many moles of lithium hydroxide are required to react with 20.0 moles of CO 2 ? CO 2 + 2LiOH  Li 2 CO 3 + H 2 O

5. How many moles of ammonia, NH 3 are produced when 6.00 moles of hydrogen gas react with excess nitrogen gas? 3H 2 + N 2  2NH 3

6. What mass, in grams, of glucose is produced when 3.00 moles of water react with carbon dioxide? 6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2

7. What mass in grams of magnesium oxide is produced when 2.00 moles of magnesium react with oxygen?

8. The reaction below is run using 824 grams of NH 3 and excess oxygen, how many moles of NO are formed? How many moles of H 2 O are formed? 4NH 3 + 5O 2  4NO + 6H 2 O

9. If mercury (II) oxide decomposes, how many grams of mercury (II) oxide are needed to produce 125 grams of oxygen?

10. How many grams of SnF 2 are produced from the reaction of 30.0 grams of HF with Sn? Sn + 2HF  SnF 2 + H 2

11. What mass of aluminum is produced by the decomposition of grams of Al 2 O 3 ?

III. Limiting Reactants- the reactant that limits the amounts of the other reactants that can combine and the amount of the product that can form in a chemical reaction. The substance that is not used up completely in a reaction is called the excess reactant.

12. Silicon dioxide (quartz) is usually quite uncreative but reacts readily with hydrogen fluoride according to the following equation. SiO 2 (s) + 4HF (g)  SiF 4 (g) + 2H 2 O (l) If 2 grams of HF are exposed to 4.5 grams of SiO 2, which is the limiting reactant?

13. If 20.5 grams of chlorine is reacted with 20.5 grams of sodium, which reactant is in excess?

14. Using what you learned from problem #20, how much sodium chloride would be produced if 20.5 grams of chlorine is reacted with 20.5 grams of sodium?

15. Magnesium sulfate reacts with potassium. How much magnesium would be produced if 35 grams of magnesium sulfate is exposed to 45 grams of potassium?

Percent Yield Theoretical yield – maximum amount of product that can be produced from a given amount of reactant. This value can be calculated from working mass to mass problems.

Example: In Practice Problem 15 you determined that 2646 grams of Al could be produced from grams of Al 2 O 3. This will occur if the reaction occurs without ANY type of error!!

Actual yield – actual amount of a product obtained from a reaction. It must be experimentally obtained. Percent yield = Actual yield X 100 Theoretical yield

16. When 36.8 grams of C 6 H 6 react with an excess of Cl 2, the actual yield of C 6 H 5 Cl is 38.8 grams. What is the percent yield of C 6 H 5 Cl? C 6 H 6 + Cl 2  C 6 H 5 Cl + HCl

17. If 75 grams of CO react to produce 68.4 grams of CH 3 OH, what is the percent yield of CH 3 OH? CO + 3H 2  CH 3 OH

MASS A (if given) MOLE A ( given ) MOLE B ( wanted ) MASS B (If needed) Substance “A” Substance “B” Stoichiometry Island Diagram Use coefficients from the balanced chemical equation to go from Substance “A” to Substance “B” 1 mol A molar mass (g) A molar mass (g) B 1 mol B ____ mol B __________________ ____ mol A Use molar mass of Substance “A”” Use molar mass of Substance “B”