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