Lesson 4: Wrap-Up
In this lesson, we continued our study of the stoichiometry of chemical reactions, that is, the relationships between the amounts of the various chemical reactants and products that are involved in chemical reactions.
Since we don't necessarily always combine just the right amount of each reactant needed in a reaction, sometimes there are reactants - called excess reactants (or excess reagents) - left over. It is the reactant that is used up - the limiting reactant (limiting reagent) - that determines when the reaction ends and the amount of each product that is formed. We learned how to determined which reactant will be limiting, how much product will be formed, and how much of each excess reactant will be left over.
The percent, by mass, of each element in a compound is called its percent composition. We reviewed how the percent composition of a compound can be determined from its formula and, in turn, how the percent composition (which we measured in the lab for magnesium oxide) can be used to determine the empirical formula of a compound.
Be sure to review the objectives and make sure you can do each one. Then take the self quiz and check your answers here.
Self Quiz
1. It is known that 4.0 g of NaCl reacts with 11.6 g of AgNO3. Which will be the limiting reagent when 1.0 gram of each are mixed together?
a. AgNO3
b. they will both react completely
c. NaCl
2. If 0.50 g of X reacts with 1.50 g of Y to give 2.00 g of Z, how much Z could be formed from mixing 0.75 g each of X and Y?
a. 1.0 g
b. 2.25 g
c. 0.25 g
d. 1.5 g
e. 3.0 g
3. Calculate the percentage by weight of salt in a solution containing 24 g of salt and 96 g of water.
a. 24%
b. 15%
c. 41%
d. 20%
Use this equation to answer the following questions:
2 Na + Cl2 
2 NaCl
4. How many grams of sodium chloride can be made from 3.0 moles of chlorine?
5. If one mole each of sodium and chlorine were allowed to react with one another, which would be the limiting reagent?
6. If 30 g of sodium and 40 g of chlorine were allowed to react with each other, which would be the limiting reagent?
7. If 40 g of sodium and 30 g of chlorine were allowed to react with each other, how much sodium chloride would be formed?
8. If 65.5 g of sodium and 100.0 g of chlorine were allowed to react with each other, what will be the excess reagent and how much of it will be left over?
9. If 0.500 mole of sodium and 0.750 mole of chlorine are allowed to react, how many grams of sodium chloride will be produced?
10. What is the percent composition (by weight) of sodium chloride?
Self Quiz Answers
- a
- a
- d (don't forget to use the total mass of the solution, not just the water, in the denominator)
- 350 g NaCl
- 1 mole Na (sodium) would be limiting
- 40 g Cl2 (chlorine) would be limiting
- 50 g of NaCl (1 sig dig); Cl2 is limiting
- Na will be excess; 64.7 g will be used and 0.8 g will be left over
- 29.2 g NaCl; Na is limiting
- 39.34% Na and 60.66% Cl