STOICHIOMETRY

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STOICHIOMETRY
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What is stoichiometry?

• Stoichiometry is the quantitative study of
  reactants and products in a chemical reaction.

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What You Should Expect

• Given Amount of reactants
• Question how much of products can be formed.
• Example
• 2 A 2B 3C
• Given 20.0 grams of A and sufficient B, how many
  grams of C can be produced?

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What do you need?

• You will need to use
• molar ratios,
• molar masses,
• balancing and interpreting equations, and
• conversions between grams and moles.
• Note This type of problem is often called
  "mass-mass."

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Steps Involved in Solving Mass-Mass
Stoichiometry Problems

• Balance the chemical equation correctly
• Using the molar mass of the given substance,
  convert the mass given to moles.
• Construct a molar proportion (two molar ratios
  set equal to each other)
• Using the molar mass of the unknown substance,
  convert the moles just calculated to mass.

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Mole Ratios

• A mole ratio converts moles of one compound in
  a balanced chemical equation into moles of
  another compound.

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Example

• Reaction between magnesium and oxygen to form
  magnesium oxide. ( fireworks)
• 2 Mg(s) O2(g) 2 MgO(s)
• Mole Ratios
• 2 1 2

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Practice Problems

• 1) N2 3 H2 ---gt 2 NH3
• Write the mole ratios for N2 to H2 and NH3 to
  H2.
• 2) A can of butane lighter fluid contains 1.20
  moles of butane (C4H10). Calculate the number of
  moles of carbon dioxide given off when this
  butane is burned.

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Mole-Mole Problems

• Using the practice question 2) above
• Equation of reaction
• 2C4H10 13O2 8CO2
  10H2O
• Mole ratio
• C4H10 CO2
• 1 4 bases
• 1.2 X
  problem
• By cross-multiplication, X 4.8 mols of CO2
  given off

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Mole-Mass Problems

• Problem 1 1.50 mol of KClO3 decomposes. How many
  grams of O2 will be produced? k 39, Cl 35.5,
  O 16
• 2 KClO3 2 KCl 3 O2

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Three stepsGet Your Correct Answer

• Use mole ratio
• Get the answer in moles and then
• Convert to Mass. Simple Arithmetic
• Hello!
• If you are given a mass in the problem, you will
  need to convert this to moles first. Ok?

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Lets go!

• 2 KClO3 2 KCl 3 O2
• 2 3
• 1.50 X
• X 2.25mol
• Convert to mass
• 2.25 mol x 32.0 g/mol 72.0 grams
• Cool!

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Try This

• We want to produce 2.75 mol of KCl. How many
  grams of KClO3 would be required?
• Soln
• KClO3 KCl
• 2 2
• X 2.75
• X 2.75mol
• In mass 2.75mol X 122.55 g/mol
• 337 grams zooo zimple!

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Mass-Mass Problems

• There are four steps involved in solving these
  problems
• Make sure you are working with a properly
  balanced equation.
• Convert grams of the substance given in the
  problem to moles.
• Construct two ratios - one from the problem and
  one from the equation and set them equal. Solve
  for "x," which is usually found in the ratio from
  the problem.
• Convert moles of the substance just solved for
  into grams.

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Mass-Volume Problems

• Just follow mass-mass problem to the penultimate
  level

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Like this

• There are four steps involved in solving these
  problems
• Make sure you are working with a properly
  balanced equation.
• Convert grams of the substance given in the
  problem to moles.
• Construct two ratios - one from the problem and
  one from the equation and set them equal. Solve
  for "x," which is usually found in the ratio from
  the problem.
• Convert moles of the substance just solved for
  into Volume.

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Conversion of mole to volume

• No of moles Volume
• Molar volume
• Can you remember a similar equation?

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Molar volume

• The molar volume is the volume occupied by one
  mole of ideal gas at STP. Its value is 22.4dm3

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Practice Problems

• Calculate the volume of carbon dioxide formed at
  STP in dm3' by the complete thermal
  decomposition of 3.125 g of pure calcium
  carbonate (Relative atomic mass of Ca40, C12,
  O16)
• Solution
• Convert the mass to mole
• Molar mass of CaCO3 40 12 (16 x 3)
  100gmol-1
• Mole mass/molar mass
• 3.125/100 0.03125mol

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Practice Problems

• As per the equation,
• Mole ratio 1 1
• problem 0.03125mol X
• X 0.03125mol of CO2
• Convert mole to volume slide 17
• Volume (0.03125 x 22.4)dm3
• 0.7dm3

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