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Solving Stoichiometry Problems

Objectives:

    1.    Name four major categories of stoichiometry problems.

    2.    Explain how to solve each type of stoichiometry problems.

Notes:

It is important to remember that solving stoichiometry problems is very similar to following a recipe.  Once you know the recipe you can modify it using the same ratios to make the product for more or less people.

There are 4 major categories of stiochiometry problems.  It is important to remember, though, that in every situation you need to start out with a balanced equation.

1.    Mole-Mole Problems

Problem:  How many moles of HCl are needed to react with 0.87 moles of Al?

        Step 1: Balance The Equation & Calculate the Ratios

            2Al:6HCl (1:3)        2Al:2AlCl3 (1:1)        2Al:3H2 (1:1.5)

        Step 2:  Find the Moles of the Given

            0.87 moles of aluminum are reacted with hydrochloric acid

          Step 3: Calculate the moles using the ratios

            moles HCl = 0.87molAl x  3molHCl/1molAl = 2.6 mol HCl

           

 

2.    Mass-Mass Problems (Strategy: Mass g Mole g Mole g Mass)

Problem:  How many grams of Al can be created decomposing 9.8g of  Al2O3?

        Step 1: Balance The Equation & Calculate the Ratios

 

            2Al2O3:4Al (1:2)        2Al2O3:3O2 (1:1.5)

Step 2:  Find the Mass of the Given

            9.8g Al2O3 are decomposed

Step 3: Calculate the moles of the given (mol/g)

            9.8g Al2O3 x (1mol Al2O3/102g Al2O3) = 0.096 mol Al2O3

       

 

Step 4: Calculate the moles using the ratios

        0.096 mol Al2O3 x (2 mol Al/1 mol Al2O3) = 0.19 mol Al 

      

        Step 5: Calculate the mass using the new moles

        0.19 mol Al x (27g Al/1 mol Al) = 5.1g Al

       

 

3.    Mass-Volume Problems (Strategy: Mass g Mole g Mole g Volume)

        Problem:  How many liters of H2 are created from the reaction of 20.0g K?

        Step 1: Balance The Equation & Calculate the Ratios

 

            2K:2H2O (1:1)    2K:2KOH (1:1)    2K:1H2 (2:1)

Step 2:  Find the Mass of the Given

            20.0g K are used in the reaction

Step 3: Calculate the moles of the given (mol/g)

            20.0g K x (1 mol K / 39g K) = 0.513 mol K

           

Step 4: Calculate the moles using the ratios

            0.51 mol K x (1mol H2 /2mol K) = 0.266mol H2

           

        Step 5: Calculate the volume using the new moles

        0.266 mol H2 x (22.4L H2 /1mol H2) = 5.75L H2

       

4.    Volume-Volume Problems

        Problem:  How many liters of SO2 will be produced from 26.9L O2?

Step 1: Balance The Equation & Calculate the Ratios

            2O2:1S2 (2:1)    2O2:2SO2 (1:1)    

Step 2: Find the volume of the given

            26.9L O2

Step 3: Calculate the moles of the given

            26.9L O2 x (1 mol O2 / 22.4L) = 1.20 mol O2

       

Step 4: Calculate the moles using the ratios

            1.20 mol O2 x (1mol SO2 /1mol  O2) = 1.20 mol SO2

       

        Step 5: Calculate the volume using the new moles

        1.20 mol O2 x (1mol SO2 /1mol  O2) x (22.4L /1mol) = 26.9L SO2

       

 

 

 

 

 

 

Last modified: January 30, 2004