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Stoichiometry (Stoo-eek-ee-aam-et-ry)

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9.1 Using Chemical Equations 1 piece of bread + 3 tomato slices + 3 slices meat + 2 cheese slices 1 sandwich + + + How many sandwiches can you make? 20 pieces of bread, 44 slices of tomatoes, 332 slices of meat and 4 slices of cheese.

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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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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 N 2(g) + 3H 2(g) 2NH 3(g) 1 molecule N molecules H 2 2 molecules NH 3 1 mol N mol H 2 2 mol NH 3

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Mole Ratio: The ratio between two chemicals in a balanced chemical equation. It is used as a conversion factor. ____ Fe + ____ O 2 ____ Fe 2 O 3 4. The mole ratio for O 2 to Fe 2 O 3 is ___ mol O 2 : ___ mol Fe2O 3 5. How many moles of O 2 are needed to form 21.0 moles of Fe 2 O 3 ? (Convert 21.0 mol Fe 2 O3 to mol O 2 ) 21.0 mol Fe 2 O 3 ______________ =

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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 Fe 2 O 3 are formed when 6.00 moles of O 2 react? a.Convert ____________ to ____________ b.Still use the mole ratio, but then convert mol Fe 2 O 3 to g Fe 2 O 3 a separate step mol O 2 mol Fe 2 O 3 __________ mol O 2

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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 O 2 ?

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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 F W 3 6 FW 2 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 O 2 2 Fe 2 O 3 The mole ratio for O 2 to Fe 2 O 3 is 6 mol O 2 / 2 mol Fe 2 O 3

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9.2 Calculating mass You can think of a chemical equation as –Number of molecules, moles AND grams N 2(g) + 3H 2(g) 2NH 3(g) 1 molecule N molecules H 2 2 molecules NH 3 1 mol N mol H 2 2 mol NH 3 28 g N x 2.02 g H 2 2 x 17g NH 3

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9.2 Calculating mass How many grams of NH 3 can be made from 38.4 grams H 2 ? N 2(g) + 3H 2(g) 2NH 3(g) The equation only gives the “mole” ratio, so we need to convert grams to moles. Plan: Grams H 2 moles H 2 moles NH 3 grams NH 3

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38.4 g H g/ mol H 2 moles H 2 2 mol NH 3 / 2 mol H 2 moles NH g/ mol NH 3 grams NH 3

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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 can’t 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 F W 3 6 FW 2 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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N 2(g) + 3H 2(g) 2NH 3(g) To use all 16.0 mol N 2, you would need 48.0 mol H 2. So what if you had 16.0 mol N 2 and 19.0 mol H 2. Which would you use up first?

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Now, with chemicals N H 2 2 NH 3 You have 16.0 moles of N 2. How many moles of H 2 would be required?

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Using the Limiting Reactant You have 16.0 mol N 2 and 19.0 mol H 2. How many moles of NH 3 will form? –Since limiting reactant controls the amount of product formed, just convert from 19.0 mol H 2 to mol NH 3. How many moles of the excess reactant would remain?

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Example: 25.0 kg N 2 react with 5.00 kg H 2. How much ammonia (NH 3 ) will be made? N 2(g) + 3H 2(g) 2NH 3(g) 1.Calculate moles of H 2 and N 2. 2.Calculate how many moles N 2 you need to react with H 2. Use the mole ratio. 3. Ask yourself: Do you have enough H 2 ? Yes H2 is excess, N2 is limiting. No H2 is limiting, N2 is excess 4. Use the limiting reactant to calculate grams NH 3.

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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. a.Write a balanced chemical equation for this reaction. b.You have 3.15 mol of Sn and 4.96 mol HF. i.Which is the limiting reactant (show work!)? ii.How many moles of tin(II) fluoride will form? c.You have 21.1 g Sn and 84.5 g HF. i.Which will be limiting? ii.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 g 100% = 85.5% yield 66.8 g

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