score _____ /10 pts. Name _________________________________ Class ______ Date _______________________

7.1 Stoichiometry and Percent Yield

Mole Ratios An example:

The combustion of propane is used to heat many rural homes in winter. Balance the equation below for

the combustion of propane:

____C3H8 + ____O2 ____CO2 + ____H2O

1. When one molecule of propane reacts, how many molecules of oxygen are required?

2. If there were a dozen molecules of propane burned, how many molecules of oxygen would be

required?

3. One mole of propane burns, how many moles of oxygen are required?

4. As one mole of propane burns, how many moles of carbon dioxide are released?

a. How many moles of water are released?

A mole ratio is the ratio of coefficients between compounds in an equation. Mole ratios are used to

predict the amounts involved in a reaction.

Practice 5. Refer to the balanced equation for the combustion of propane:

a. How many moles of water would result when 1.45 moles of propane are burned?

i. (

)

b. If 2.35 moles of CO2 are produced in this reaction, how many moles of water would be

produced? (Use the mole ratio of CO2 and H2O and dimensional analysis to calculate the answer.)

c. Why is this statement false? “If 10 grams of propane burn, then fifty grams of oxygen burn.”

d. If 27.3 moles of carbon dioxide are produced during the combustion of a certain amount of propane:

i. How many moles of propane were required? (Use the mole ratio of CO2 and C3H8 from the balanced reaction to calculate the answer.)

ii. How many grams of propane is this?

e. If you have 410 grams of propane and want to know how many grams of oxygen are required to burn it, you can follow these steps:

i. Find the number of moles of propane present. (Use dimensional analysis and molar mass)

ii. Moles of propane are related to moles of oxygen by the ratio from the balanced coefficients in the equation. Find the number of moles of oxygen needed to burn the number of moles calculated from part i.

iii. Convert the number of moles calculated from part ii into grams using the molar mass of oxygen (don’t forget BrINClHOF.)

6. Verify that this statement is correct. “If 315 grams of propane combust, approximately 515 grams of water will be produced.”

7. Consider the decomposition of ammonia gas into nitrogen gas and hydrogen gas. f. If all of 385 grams of ammonia decomposes, how many grams of hydrogen gas will be produced?

g. How many grams of nitrogen gas would be produced?

Percent Yield: When a reaction occurs in the lab, there are always some factors that limit the efficiency of the reaction.

For example, consider when sodium metal reacts with chlorine gas to produce salt.

Write the balanced chemical reaction

Calculate the amount of salt produced if 23.0 grams of sodium reacted completely.

The calculation above provides the exact amount that is possible to produce, however, inefficiencies

always lessen this amount. Reactions in the lab will never produce the amount possible. As

procedures are improved you may reduce inefficiencies and get close, but you will never make the exact

amount possible.

Practice: 8. What are some practical things (sources of “error”) that may happen in the lab that would lead

to the production of less than 100% of what is possible?

9. According to the law of conservation of mass, it is always impossible to produce more product

than what is calculated, however, after a reaction you may often measure the resulting mass

and find that there is more than what you calculated was possible. What errors could lead to

the appearance of more product than is theoretically possible?

10. If you started your sodium and chlorine reaction with 40.5 grams of sodium metal and excess

chlorine gas, how many grams of sodium chloride should be produced?

Calculating Percent Yield Percent Yield is the measure of how much product you actually obtain in a reaction compared to the

theoretical amount possible. The following formula is used to calculate percent yield.

Actual yield is the amount of product measured in the lab. Theoretical yield is the calculated amount

possible to produce.

11. In number 10, you calculated that 40.5 grams of sodium could possibly produce 103 grams of

NaCl. If you actually ran the experiment and only collected 84.0 g NaCl, what would be your

actual yield?

a. What is the theoretical yield?

b. Use the formula above and calculate the percent yield.

12. Consider the combustion of 215.0 grams of butane, C4H10, with plenty of oxygen. If you are able to collect 295.3 grams of water from the reaction, what was your percent yield?

a. First, balance the equation.

i. ___C4H10 + ___O2 ___CO2 + ___H2O

b. Second, calculate the theoretical yield using molar masses and mole ratios from the balanced equation.

i. (

) (

) (

)

c. Third, use the theoretical yield calculated and the actual yield given to calculate % yield.

i.

13. Consider the double replacement reaction of excess sodium chloride reacting with 203.9 grams of silver

nitrate. If you are able to produce 150.4 grams of silver chloride in the lab, what is your percent yield?

14. If you make 46.8 grams of calcium carbonate by reacting 87.5 grams of calcium nitrate with excess sodium carbonate, what is your percent yield?

Stoichiometry Flow Chart Draw a flow chart that shows all the necessary steps to convert from grams, particles, or liters of a gas at

STP of the reactants in a reaction, to grams, particles, or liters of a gas (at STP) of products in a reaction.