Stoichiometry

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Stoichiometry

• Stoichiometry is a method used in chemistry to
  determine the amount of materials used and
  produced in a chemical reaction. It is a vital
  part of all quantitative chemistry. Learn well.

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Step by Step Example Problem

• What mass of aluminum will be produced if excess
  aluminum chloride is reacted with 11.23g of
  magnesium?
• The word excess indicates that there will be more
  aluminum chloride than you will need.

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Step 1 Predict the Products

• What are the two products when aluminum chloride
  reacts with magnesium?
• Magnesium chloride and aluminum.

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Step 2 Write Proper Formulas

• It is important to write the proper formulas
  using subscripts. On your worksheet, write the
  formulas for aluminum chloride reacting with
  magnesium to form magnesium chloride and
  aluminum.
• AlCl3 Mg ? MgCl2 Al

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Step 3 Write Balanced Equation

• AlCl3 Mg ? MgCl2 Al
• Balance the equation on your worksheet.
• 2AlCl3 3Mg ? 3MgCl2 2Al

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Step 4 Write down your given

• You are given are 11.23g of magnesium, it is
  important that you write it all down the number,
  the units and the substance.
• 11.23g Mg

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Step 5 Convert grams to moles

• You are given are 11.23g of magnesium, convert
  this to moles of magnesium. Use factor label.
• 11.23g Mg moles Mg

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Step 5 Convert grams to moles

• 11.23g Mg 1 mole Mg 0.462 moles Mg
• 24.3g Mg

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Step 6 Moles to Moles

• This is Stoichiometry!!! Typically you should
  just continue on with the problem from step 5,
  but since we are breaking them up, start this
  step by writing down what you calculated in step
  5.
• 0.462 moles Mg
• Now use factor label to convert to moles of Al.
• (remember to use your balanced equation)

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Step 6 Stoichiometry

• 0.462 moles Mg 2 moles Al 0.308 moles Al
• 3 moles Mg

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Step 7 Convert moles to grams

• 0.308 moles Al 27.0g Al 8.32g Al
• 1 mole Al
• So we have determined that if 11.23g of
  magnesium reacts with excess aluminum chloride,
  it will produce 8.32g of aluminum.

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Practice Problem 1

• Do this practice problem and show all of your
  work on your worksheet. When you finish, go to
  the next screen to check your answer.
• How many grams of product will be produced from
  the composition of 34.21g of aluminum with excess
  chlorine?

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2Al 3Cl2 ? 2AlCl3
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Practice Problem 2

• Do this practice problem and show all of your
  work on your worksheet. When you finish, go to
  the next screen to check your answer.
• How many grams of sodium will react with 0.0045g
  of magnesium chloride?
• Notice that this problem wants to know the amount
  of the other reactant. No worries, it is done
  the same way as the other stoichiometry problems.

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2Na MgCl2 ? 2NaCl Mg
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Practice Problem 3

• If you decompose 1.25 moles of aluminum oxide,
  how many grams of aluminum can you produce?
• Notice this problem does NOT start with grams,
  therefore you can skip step 5. Show all your
  work on your worksheet and then go to the next
  frame to check your answer.

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2Al2O3 ? 4Al 3O2