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Stoichiometry

Stoichiometry. Where are we??. Stoichiometry. The numerical relationships of the molar quantities of products and reactants in a chemical reaction Must always balance the equation in order to correctly complete any stoichiometric problem!. Two types of data….

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Stoichiometry

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  1. Stoichiometry

  2. Where are we??

  3. Stoichiometry • The numerical relationships of the molar quantities of products and reactants in a chemical reaction • Must always balance the equation in order to correctly complete any stoichiometric problem!

  4. Two types of data… • Qualitative: tells us what is involved (types of elements, physical changes, etc.) • Quantitative: tells us how much of each substance is involved • Can be involved in both reactants and products in many different units

  5. How to be successful Must be able to… • Balance equations • Find molecular and formula mass • Find how many elementary particles are in a mol • Know the molar volume of a gas • Use dimensional analysis (dot line) with correct units

  6. Finding mols • Step 1: Balance the equation • Step 2: Start with given • Step 3: Find molar ratio • Step 4: End with mol unknown

  7. A balanced equation tells us the relative molar ratio of the species reacting and the products formed for example, in the following balanced equation. . . 1 C3H8 + 5O2 3 CO2 + 4H2O 1 5 for every one mole of propane, there are five moles of oxygen this makes the mole ratio of propane to oxygen. . . 1 mol C3H8 5 mol O2

  8. the mole ratio is used to predict the amount of reactants necessary for a reaction and the amount of products formed for example . . . how many moles of oxygen are necessary to react completely with 3.0 moles of propane? C3H8 + 5O2 3 CO2 + 4H2O X 5 mol O2 3.0 mol C3H8 = 15 mol O2 1 mol C3H8 also notice that we put what we were looking for on top and what we knew on the bottom notice we multiplied by the mole ratio

  9. try this how many moles of carbon dioxide are formed from 6.0 moles of oxygen gas? C3H8 + 5O2 3 CO2 + 4H2O 6.0 mol O2 X 3 mol CO2 3.6 mol CO2 = 5 mol O2

  10. you are not always given the amount of reactants or products in moles these value may be given in grams, kilograms, or numbers of molecules it is up to you to review how to convert between mass, moles, and number of particles

  11. Let’s look at an example How many grams of propane are necessary to produce 28.6 grams of water? C3H8 + 5O2 3 CO2 + 4H2O #mol H2O= 28.6g/18.02g/mol = 1.587 mol H2O 1 mol C3H8 1.587 mol H2O X = 0.397 mol C3H8 4 mol H2O mass(g) C3H8=0.397 mol x 44.11 g/mol = 17.5 g C3H8 it is extremely important that you notice that everything must be converted to moles so that a mole ratio can be used in this case, we converted from mass to moles to moles to mass

  12. now try this What mass of propane is required to make 1.314x 1023 molecules of carbon dioxide? (HINT: remember to convert to moles) C3H8 + 5O2 3 CO2 + 4H2O 1.314x1023 molecules x 1 mol x 6.02 x1023 molecules 1 mol C3H8 x 3 mol CO2 3.209 g C3H8 =

  13. now try this What mass of propane is required to make 1.314x 1023 molecules of carbon dioxide? (HINT: remember to convert to moles) C3H8 + 5O2 3 CO2 + 4H2O 1.314x1023 molecules x 1 mol = 6.022 x1023 molecules 0.218 mol CO2 0.218 mol CO2 x 1 mol C3H8 0.0727 mol C3H8 = 3 mol CO2 mass(g) C3H8=0.0727 mol x 44.11 g/mol= 3.208 g C3H8

  14. Mass-Mass Problems • How many grams of silver chloride can be produced from the reaction of 17.0 g of silver nitrate with excess sodium chloride? • All steps the same, except express the moles of unknown substances in terms of grams • Answer: 14.4 g AgCl

  15. Mass-Volume Problems • How many liters of carbon dioxide at STP is produced when 400. g of calcium carbonate reacts with excess HCl? • Same set up, except express mols in terms of liters • Answer: 89.6 L of CO2

  16. Volume-Volume Problems • What volume of hydrogen gas will combine with 480. L of nitrogen gas to produce NH3 at STP? • Express mols in terms of Liters • Answer: 1440 L H2

  17. Let’s take a look at the LR help-sheet… • Which is the excess reactant when 560 g of MgI2 and 360 g of Br2 react, and what mass remains? • What mass of I2 is formed in the same process?

  18. Try this problem… • Mercury and Bromine will react with each other to produce mercury (II) bromide. • a. What mass of the product can be produced from the reaction of 10.0g of Hg and 9.00 g of Br2? What mass of which reagent is left unreacted? b. What mass of HgBr2 can be produced from the reaction of 5.00mL mercury (density = 13.6 g/mL) and 5.00mL bromine (density = 3.10g /mL)?

  19. Theoretically, the amount of product obtained IF all the limiting reagent is used to form product • Theoretical Yield • Assuming that no side rxns take place, no equilibrium, no product loss, theoretical yield is possible • These assumptions are not valid in the real world

  20. x 100 Theoretical yield • Actual Yield: the amount of product actually obtained in carrying out any reaction • # is obtained through your experiments and measurements in the lab • Percent Yield: the percentage of the theoretical yield actually obtained in carrying out the rxn. % yield = actual yield

  21. Percent Error

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