Chapter 9: Stoichiometry
I. Introduction to Stoichiometry
A. Reaction Stoichiometry involves the mass relationships between reactants and products in a chemical reaction.
1. Information from a balanced formula equation is used to do the calculations.
2. These calculations allow us to predict amounts of substances used or produced.
B. Reaction Stoichiometry Problems.
1. Given and unknown quantities are amounts in moles.
2. Given is an amount in moles and the unknown is a mass that is often expressed in grams.
3. Given is a mass in grams and the unknown is an amount in moles.
4. Given is a mass in grams and the unknown is a mass in grams.
5. To do all of these types of problems, we need to know how to get the information used to convert.
a. Mole Ratio: a conversion factor that relates the amounts in moles of any two substances involved in a chemical reaction. It is obtained from the balanced formula equation.
b. Molar mass: we have always done this calculation.
c. We use these two pieces of information as conversion factors.
II. Ideal Stoichiometric Calculations
A. A chemical equation allows us to make predictions about how much of the reactants and products will be used or produced during the reaction under IDEAL conditions.
B. Conversions of Quantities in Moles
1. The general plan for this type of conversion is as follows:
amt. of given substance (in mol) ® amt. of unk substance (in mol).
2. Therefore, the mathematics are as follows:
given quantity * conversion factor = unknown quantity
3. The conversion factor is the mole ratio. The ratio comes from the balanced formula equation.
4. Practice, practice, practice.
C. Conversions of Amounts in Moles to Mass
1. The general plan for this type of conversion is as follows:
amt. of given substance (in mol) ® amt. of unk substance (in mol) ® mass of unknown substance (in grams).
2. Therefore, the mathematics are as follows:
given quantity * conversion factor(MR) * conversion factor(MM) = unknown quantity
3. The first conversion factor is the mole ratio. The second conversion factor is the molar mass.
4. Practice, practice, practice.
D. Conversions of Mass to Amounts in Moles
1. The general plan for this type of conversion is as follows:
mass of given substance (in g) ® amt. of given substance (in mol) ® amt. of unk substance (in mol) ® mass of unknown substance (in g).
2. Therefore, the mathematics are as follows:
given quantity * conversion factor(MM) *conversion factor(MR) = unknown quantity
3. The first conversion factor is the molar mass of the given substance. The second is the molar ratio from the balanced equation.
4. Practice, practice, practice.
E. Conversions of Mass to Mass
1. The general plan for this type of conversion is as follows:
mass of given (in g) ® amt. of given substance (in mol) ® amt. of unk substance (in mol) ® mass of unknown (in g).
2. Therefore, the mathematics are as follows:
given mass * conversion factor(MM) * conversion factor(MR) * conversion factor(MM) = unknown mass.
3. The first conversion factor is the molar mass of the given. The second conversion factor is the molar ratio from the balanced equation. The third conversion factor is the molar mass of the unknown.
4. Practice, practice, practice.
III. Limiting Reagents and Percent Yields
A. A limiting reagent (reactant) is the reactant that limits the amount of the other reactants that can combine and the amount of product that can form in a chemical reaction.
B. An excess reagent (reactant) is the reactant that is not completely used up in a reaction.
C. In these types of problems, we are given the amounts of both reactants, and told to find the amount of an unknown produced. We must determine the limiting reagent, then use the information of it to do the calculations.
1. We answer the question of which is the limiting reagent by the following steps:
a. Pick one of the reactants.
b. Convert that amount to the amount of the other reactant.
c. Compare the values. If the calculated amount is more than the actual amount, the substance not picked in step a is the LR. If the calculated amount is less than the actual amount, the substance picked in step a is the LR.
2. Once the LR is determined, use that value for ALL calculations in the problem.
3. Practice, practice, practice
D. Percent Yields
1. All of the amounts calculated to this point, are theoretical yields. Theoretical yield is the maximum amount of product that can be produced from a given amount of reactant.
2. When the reaction is completed in a lab environment, the product is measured. The amount of a product obtained from a reaction is called the actual yield of the product.
3. We use these values to calculate the percent yield, which is a measure of the efficiency of the experiment. Percent Yield is the ratio of the actual yield to the theoretical yield, multiplied by 100.
- % yield = (Actual/Theoretical) * 100.
4. Practice, practice, practice.