Student Exploration: Stoichiometry (ANSWER KEY)


Student Exploration: Stoichiometry
Vocabulary: Avogadro’s number, balanced equation, cancel, coefficient, dimensional analysis, molar mass, mole, molecular mass, stoichiometry
Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
A 250 mL glass of orange juice contains 22 grams of sugar. How much sugar is in a two-liter (2,000 mL) bottle of orange juice? ___________________________________________
It requires two sticks of butter to make a batch of 20 cookies. How much butter will it take to make 150 cookies? ______________________________________________________
Gizmo Warm-up
Just as a cook follows a recipe to decide how much of each ingredient to add, a chemist uses stoichiometry to determine the amounts of substances involved in chemical reactions. The Stoichiometry Gizmo™ allows you to try your hand at figuring out the amounts of reactants and products that take part in a chemical reaction.
To begin, check that this equation is shown:
Fe2O3 + 3CO à 2Fe + 3CO2
Look at the coefficients (such as the “3” in 3CO) in front of each substance in the equation. The coefficients tell you how many molecules or atoms take part in a chemical reaction. In the spaces below, list the number of each molecule or atom in the equation:
Fe2O3 ____                CO ____                     Fe ____                      CO2 ____
In a balanced equation, the same number of each kind of atom is shown on each side of the equation. Calculate the number of iron (Fe), oxygen (O), and carbon atoms (C).
Reactants                       Iron: ____                   Oxygen: ____ Carbon: ____
Products            Iron: ____                   Oxygen: ____ Carbon: ____
Based on these values, is the equation balanced? __ ____________________________
Activity A:

Moles
Get the Gizmo ready:
  • Check that the equation is still:
Fe2O3 + 3CO à 2Fe + 3CO2
  • If not, click New equation until it reappears.
 
Introduction: A mole is:
A)   A mammal known for digging up gardens.
B)    A small, dark spot on the skin.
C)    A spy embedded within an enemy government.
D)    6.02 × 1023 particles of a substance.
E)    All of the above.
The correct answer, of
of
Infer: A mole of a substance has a mass in grams that is equal to the molecular mass. For example, a carbon atom has a list the molar mass of each substance:
Practice: Hydrogen has an atomic mass of 1.01 u. What is the molar mass of these substances?
H2O ____        CH4 ____        H2CO3 ____    C6H12O6 ___
Activity B:

Canceling units
Get the Gizmo ready:
  • Check that the equation is still:
Fe2O3 + 3CO à 2Fe + 3CO2
 

Introduction: While solving problems in stoichiometry, it is useful to pay attention to the units of the answer. The process of comparing units is called dimensional analysis. A common technique involves using conversion factors to convert from one unit to another. Units that appear in the numerator and denominator of a fraction can be canceled out. For example, converting 2 moles of carbon monoxide to grams involves multiplying by a conversion factor:
2 mol CO • 28.01 g CO = 56.02 g CO
1 mol CO
Notice that the conversion factor is equal to one because the numerator (28.01 g CO) and denominator (1 mol CO) are equivalent. The moles of CO on the top and bottom are canceled, leaving a final unit of grams.
Question: How do we solve problems in stoichiometry?
Observe: The first question is: “How many moles of carbon monoxide (CO) are required to react completely with 1.75 moles of iron (III) oxide (Fe2O3)?” (If this is not the question you see, click New question until it appears.)
What unit is given in the question? __________ ________________________
What quantity is asked for? _________________ ___________________
Find: Look for the tile that contains the units “mol Fe2O3” on top and “mol CO” on the bottom. Drag this tile down to the green strip at the bottom.
According to the tile, how many moles of CO react with one mole of Fe2O3? ____________
Analyze: To get an answer in moles of CO, you need to cancel the moles of Fe2O3. Turn on Show units.
What units are given to the right of the equals sign? _________ ____________
If these aren’t the units you want, click Flip tile. What unit is given now?
Calculate: If the units are correct, multiply or divide the numbers to solve the problem.
How many moles of CO will react with 1.75 moles of Fe2O3? _____ ____
Turn on Show numerical result. Were you correct? _________ _____________
(Activity B continued on next page)
Activity B (continued from previous page)
Practice: Turn off Show units and Show numerical result. Click New question, and use what you’ve learned to solve another stoichiometry problem. For each problem, list the units given, the units asked for, and the solution.
The problems in the Gizmo are given in random order, so you may have to click Next question several times to see a new problem. (Note: Each term in the equation is either a solid (s), liquid (l), gas (g), or an aqueous solution (aq).)
If you are stuck, try one of the following hints:
If the given unit is grams (or liters or particles), convert from that unit to moles first. Then convert to moles of the answer substance.
If the answer unit is grams, liters, or particles, find the number of moles of the answer substance first. Then convert the moles of answer substance to the desired unit.
If you have a calculator, try to calculate the solution to each problem yourself before turning on the Show numerical solution checkbox.
Problems:
What volume of carbon dioxide (CO2) will be produced if 2.90 moles of iron (Fe) is produced?
Given unit: ________              Answer unit: _____ __             Solution:
What mass of iron (Fe) can be obtained from 3.80 g iron (III) oxide (Fe2O3) reacting with excess carbon monoxide (CO)?
Given unit: __ __Answer unit: ___ ____                                  Solution: __
How many moles of iron (Fe) will be produced from 6.20 moles of carbon monoxide (CO) reacting with excess iron (III) oxide (Fe2O3)?
Given unit: __ __                     Answer unit: ___ _______       Solution: _ _
How many molecules of carbon monoxide (CO) are needed to react with excess iron (III) oxide (Fe2O3) to produce 11.6 g of iron (Fe)?
Given unit: ______                  Answer unit: ____                    Solution: 1.88x1023 CO
On your own: Click New equation to try solving problems with a new chemical equation. There are five equations in all, and five problems per equation. The Gizmo will keep track of how many problems you solve. Good luck!

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