Stoichiometry

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Stoichiometry

• (Stoo-eek-ee-aam-et-ry)

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9.1 Using Chemical Equations

• How many sandwiches can you make? 20 pieces of
  bread, 44 slices of tomatoes, 332 slices of meat
  and 4 slices of cheese.

?

• 1 piece of bread 3 tomato slices 3 slices
  meat 2 cheese slices ? 1 sandwich

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• F frame, W wheel
• Wheels come in packages of 3
• Frames come in packages of 2
• A bike factory is manufacturing bikes. A bike is
  made from one frame and two wheels. Write a
  balanced equation for producing bikes. (A bike,
  of course, is FW2).
• ___ F2 ___ W3 ? ____ FW2

?

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• ___ F2 ___ W3 ? ____ FW2

?

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• If we order 12 cases of frames, how many cases of
  wheels will we need?
• Use dimensional analysis! (Applause)
• The numbers in the parenthesis come from the
  balanced chemical equation

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Using Chemical Equations

• The coefficients can represent
• the number of molecules and atoms OR
• The moles of molecules and atoms
• N2(g) 3H2(g) ?
  2NH3(g)
• 1 molecule N2 3 molecules H2 ? 2 molecules NH3
• 1 mol N2 3 mol H2 ? 2 mol
  NH3

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• Mole Ratio The ratio between two chemicals in a
  balanced chemical equation. It is used as a
  conversion factor.
• ____ Fe ____ O2 ? ____ Fe2O3
• 4. The mole ratio for O2 to Fe2O3 is
• ___ mol O2 ___ mol Fe2O3
• 5. How many moles of O2 are needed to form 21.0
  moles of Fe2O3?
• (Convert 21.0 mol Fe2O3 to mol O2)
• 21.0 mol Fe2O3 ______________

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• Stoichiometry (stoik ee ahm ih tree)Using mole
  ratios to convert from an amount of one substance
  to the amount of another substance
• If you know the amount of chemical A and you want
  to know the amount of chemical B, then it is just
  converting moles of A to moles of B using the
  mole ratio.

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• 6. Questions may ask for the answer as a mass (in
  grams) rather than moles.
• Example How many grams of Fe2O3 are
• formed when 6.00 moles of O2 react?
• a. Convert ____________ to ____________
• b. Still use the mole ratio, but then convert
  mol Fe2O3 to g Fe2O3 a separate step.
• 6.00 mol O2 mol Fe2O3 __________
• mol O2

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• 7. Or, the question could start with the mass of
  the given chemical.
• What mass (in grams) of Fe is needed to react
  with 525 g of O2?

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Limiting Reactant and Percent Yield
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A new adventure in sales

• We are going to sell hot dogs on the street (to
  raise money for chemistry experiments).
• Each hot dog we sell will require one bun and one
  hotdog, so we order 200 packages of each.
• But then we find that hot dogs are in packages of
  10 and buns are in packages of 8.
• So, which will we use up first? Hot dogs or
  buns?

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So

• Since the hot dog business was to unprofitable
  (400 extra hot dogs got moldy while in storage
  waiting for the state fair), we go back to bikes.

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Bike stoichiometry

• F frame, W wheel
• Wheels come in packages of 3
• Frames come in packages of 2
• 3 packages of frames and 4 packages of wheels
  make six bikes
• 3 F2 4 W3 ? 6 FW2
• We have 21 cases of frames left in storage, but
  no wheels. How many packages of wheels must we
  buy?
• Oops. Actually, I just found 24 packages of
  wheels under my desk. So, do we use up the
  wheels, or do we have extra wheels?

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• Mole Ratio The ratio between two chemicals in a
  balanced chemical equation. It is used as a
  conversion factor.
• 4 Fe 6 O2 ? 2 Fe2O3
• The mole ratio for O2 to Fe2O3 is 6 mol O2 / 2
  mol Fe2O3

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9.2 Calculating mass

• You can think of a chemical equation as
• Number of molecules, moles AND grams
• N2(g) 3H2(g) ?
  2NH3(g)
• 1 molecule N2 3 molecules H2 ? 2 molecules NH3
• 1 mol N2 3 mol H2 ? 2 mol
  NH3
• 28 g N2 3 x 2.02 g H2 ? 2 x 17g
  NH3

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9.2 Calculating mass

• How many grams of NH3 can be made from 38.4 grams
  H2?
• N2(g) 3H2(g) ? 2NH3(g)
• The equation only gives the mole ratio, so we
  need to convert grams to moles.
• Plan
• Grams H2 ? moles H2 ? moles NH3 ? grams NH3

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38.4 g H2
grams NH3
17.04 g/ mol NH3
2.02 g/ mol H2
2 mol NH3/ 2 mol H2
moles H2
moles NH3
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9.3 Limiting Reactants and Percent Yield
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• Imagine that we want to make sandwiches (and I
  love sandwiches).

?

• We have 20 pieces of bread, 44 slices of
  tomatoes, 332 slices of meat and 4 slices of
  cheese.
• How many sandwiches can you make?
• Yes, two. We need to buy more cheese if we want
  to make more, and I want to watch the soccer
  game, so I cant go to the store right now.

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Determining the Limiting Reactant

• In the sandwich example, cheese was the limiting
  reactant (also called limiting reagent).
• The limiting reactant is completely used up in a
  reaction.

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• 3 F2 4 W3 ? 6 FW2
• We have 21 cases of frames left in storage, but
  no wheels. How many packages of wheels must we
  buy? (Show work!)
• Oops. Actually, I just found 24 packages of
  wheels under my desk. So, do we use up the
  wheels, or do we have extra wheels?

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• N2(g) 3H2(g) ? 2NH3(g)
• To use all 16.0 mol N2, you would need 48.0 mol
  H2.
• So what if you had 16.0 mol N2 and 19.0 mol H2.
  Which would you use up first?

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Now, with chemicals

• N2 3 H2 ? 2 NH3
• You have 16.0 moles of N2. How many moles of H2
  would be required?

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Using the Limiting Reactant

• You have 16.0 mol N2 and 19.0 mol H2. How many
  moles of NH3 will form?
• Since limiting reactant controls the amount of
  product formed, just convert from 19.0 mol H2 to
  mol NH3.
• How many moles of the excess reactant would
  remain?

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Example 25.0 kg N2 react with 5.00 kg H2. How
much ammonia (NH3) will be made?

• N2(g) 3H2(g) ? 2NH3(g)
• Calculate moles of H2 and N2.
• Calculate how many moles N2 you need to react
  with H2. Use the mole ratio.
• 3. Ask yourself Do you have enough H2?
• Yes ? H2 is excess, N2 is limiting.
• No ? H2 is limiting, N2 is excess
• 4. Use the limiting reactant to calculate grams
  NH3.

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Tin(II) fluoride is used in some toothpastes.

• It is made by reacting tin with hydrogen fluoride
  to produce tin(II) fluoride and hydrogen gas.
• Write a balanced chemical equation for this
  reaction.
• You have 3.15 mol of Sn and 4.96 mol HF.
• Which is the limiting reactant (show work!)?
• How many moles of tin(II) fluoride will form?
• You have 21.1 g Sn and 84.5 g HF.
• Which will be limiting?
• What mass of tin(II) fluoride will form?

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Percent Yield

• In a real experiment, usually you get less
  product than you would expect. Sometimes not all
  the chemicals react.
• Theoretical Yield calculated amount of product
• Actual Yield amount of product actually
  collected in lab
• Knowing both of these, you can calculate percent
  yield.
• yield actual yield ? 100
  theoretical yield

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Example

• In an experiment, Jenny collected 57.1 g of
  product. However, stiochiometrically she
  calculated that she should have gotten 66.8 g.
  Find the percent yield.
• 57.1 g ? 100 85.5 yield 66.8 g