Stoichiometry Tutorial
Using "THE MAP" to Solve Problems

Chemists are often responsible for designing a chemical reaction and analyzing the products obtained from it. One significant part of designing a chemical reaction is determining the amount of each reactant that is needed and the amount of each product that will be produced. In order to do so, chemists use stoichiometry, the quantitative relationships between reactants and products.

"THE MAP" shown below is your best friend when it comes to solving stoichiometry problems. Whether you need to convert from mass of one substance to mass of another substance, from moles of one substance to moles of a different substance, or simply from grams of one substance to moles of that same substance, "THE MAP" tells you exactly how many (and which) steps you will have to use and which conversion factors you will need. Make your life simpler......memorize "THE MAP!"

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Solving a stoichiometry problem using "THE MAP" is kind of like planning a trip. If you were planning a trip from Oklahoma City to Chicago, you would first find OKC on the map and then follow the major highways to Tulsa, Springfield, MO, St. Louis, and then Chicago. You have to do the same thing when using "THE MAP" to solve a stoichiometry problem. First, find your starting point on the map. Next, find your ultimate destination on the map. Finally, "simply" follow the steps outlined on the map to get where you want to be!

Let's work through a couple of examples to see how "THE MAP" is used to solve stoichiometry problems.

Converting from Moles A to Grams of B

C₃H₈ (g) + 5 O₂ (g) --> 3 CO₂ (g) + 4 H₂O (l)

The first step in solving this problem is to use "THE MAP" to plan your "trip" (i.e. decide where you're starting and where you will be when you've solved the problem). THE MAP for this problem looks like this:

"THE MAP" tells you that you need exactly two steps:

• First, use the molar ratio to convert mol C3H8 to mol H₂O

• Second, use the molar mass of water to convert mol H₂O to grams H₂O.

1. Write down the units you're looking for (including the formula for the compound) and put an equal sign.

   g H2O

   =

2. Write the information you were given in the problem (in this case, the number of moles of propane). Don't forget to include the formula for the compound you're starting with:

   g H2O

   =

   0.152 mol C3H8

3. Put a multiplication sign after the number of moles and draw a line.

   g H2O	=	0.152 mol C3H8	x	__________

4. Above and below the line, write the molar ratio. Since you want to get rid of the units you started with (in this case "mol C₃H₈), put the units "mol" and the formula of the compound you started with on the bottom. Put the units "mol" and the formula of the compound you're looking for on the top.

   g H2O	=	0.152 mol C3H8	x	mol H2O
   				mol C3H8

5. Use the coefficients from the balanced chemical equation to complete the molar ratio. Place the coefficient in front of the formula that you started with on the bottom and the coefficient in front of the formula that you are looking for on the top.

   g H2O	=	0.152 mol C3H8	x	4 mol H2O
   				1 mol C3H8

6. Cancel out your units and compare them to the ones you want to finish with. Since they are not the same, put a multiplication sign after the molar ratio and draw a line.

   g H2O	=	0.152 mol C3H8	x	4 mol H2O	x	_______
   				1 mol C3H8

7. Write the molar mass above and below the line so that the mol of water cancel out leaving you with grams of water.

   g H2O	=	0.152 mol C3H8	x	4 mol H2O	x	18.0 g H2O
   				1 mol C3H8		1 mol H2O

8. Cancel out your units and do the math. Report the answer with the correct units and the correct number of significant figures.

   g H2O	=	0.152 mol C3H8	x	4 mol H2O	x	18.0 g H2O
   				1 mol C3H8		1 mol H2O
   g H2O	=	10.9 g H2O

Converting From Grams of A to Grams of B

Example: How many grams of CO₂ will be produced by the combustion of 15.0 g of propane?

C₃H₈ (g) + 5 O₂ (g) --> 3 CO₂ (g) + 4 H₂O (l)

The first step in solving this problem is to use "THE MAP" to plan your "trip" (i.e. decide where you're starting and where you will be when you've solved the problem). THE MAP for this problem looks like this:

To solve this problem, you will need exactly three steps:

• First, use the molar mass to convert grams C₃H₈ to moles C₃H₈

• Second, use a molar ratio to convert moles C₃H₈ to moles CO₂

• Third, use molar mass to convert moles of CO₂ to grams of CO₂

1. Write down the units you're looking for (including the formula for the compound) and put an equal sign.

   g CO2

   =

2. Write the information you were given in the problem (in this case, the number of grams of propane). Don't forget to include the formula for the compound you're starting with:

   g CO2

   =

   15.0 g C3H8

3. Put a multiplication sign after the number of moles and draw a line. Above and below the line, write the molar mass of propane. Write it so that "g C₃H₈" cancel out leaving you with mol C₃H_8.

   g CO2	=	15.0 g C3H8	x	1 mol C3H8
   				44.0 g C3H8

4. Cancel out your units and compare them to the ones you want to end up with. Since they are not the same, multiply by the molar ratio. Remember to write the molar ratio so that "mol C₃H₈" cancels out. Use the coefficients from your balanced chemical equation to complete your molar ratio.

   g CO2	=	15.0 g C3H8	x	1 mol C3H8	x	3 mol CO2
   				44.0 g C3H8		1 mol C3H8

5. Cancel out your units and compare the ones that are left to the ones you want. Since they are not the same, multiply by the molar mass of carbon dioxide. Remember to write the molar mass as a conversion factor so that "mol CO₂" cancels out leaving you with "g CO₂"

   g CO2	=	15.0 g C3H8	x	1 mol C3H8	x	3 mol CO2	x	44.0 g CO2
   				44.0 g C3H8		1 mol C3H8		1 mol CO2

6. Cancel out your units and compare the ones that are left to the ones you want. Since they are the same, do the math. Report your answer using the correct units and the correct number of signficant figures.

   g CO2	=	15.0 g C3H8	x	1 mol C3H8	x	3 mol CO2	x	44.0 g CO2
   				44.0 g C3H8		1 mol C3H8		1 mol CO2
   g CO2	=	45.0 g CO2

   NOTE: By now you've noticed that the molar masses of propane and carbon dioxide are identical. This is NOT very common. In most cases, your reactants and products all have different molar masses.

Practice Problems

Use the following balanced equation to solve the problems below:

Fe₂O₃ (s) + 6 HCl (g) --> 2 FeCl₃ (s) + 3 H₂O (g)

1. How many grams of FeCl₃ can be produced using 0.295 mol Fe₂O_3?
2. How many grams of HCl are needed to react with 27.62 g of Fe₂O_3?
3. How many moles of HCl are needed to produce 15.5 g of FeCl_3?

Check Answers