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Stoichiometry

Stoichiometry. Or… the cheeseburger problem. Cheeseburger Recipe. 1 hamburger bun 2 hamburger patties 2 slices of cheese 4 strips of bacon. Chemical Recipe. 2Na + Cl 2  2NaCl H 2 SO 4 + 2NaOH Na 2 SO 4 + 2H 2 O. How this all relates….

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Stoichiometry

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  1. Stoichiometry Or… the cheeseburger problem

  2. Cheeseburger Recipe • 1 hamburger bun • 2 hamburger patties • 2 slices of cheese • 4 strips of bacon

  3. Chemical Recipe • 2Na + Cl2 2NaCl • H2SO4 + 2NaOH Na2SO4 + 2H2O

  4. How this all relates… • Coefficients in chemical equations represent number of molecules, not mass of molecules

  5. How this all relates… • But, we don’t want to COUNT molecules for our chemical reactions • Counting is done by weighing!!

  6. How this all relates… • But, with different atomic masses, how can we compare the compounds? • Must use MOLES!!

  7. A Question… • If 12.5 g of C4H10 are burned in air, how many grams of carbon dioxide is formed?

  8. Step 1 • Write a BALANCED equation for the reaction! • It is no good if the equation is not balanced!

  9. Step 2 • Convert the known mass of the reactant of product (can work either direction!) to moles of that substance

  10. Step 3 • Use the balanced equation to set up appropriate mole ratios

  11. Step 4 • Use appropriate mole ratios to calculate the number of moles of reactant or product

  12. Step 5 • Convert from moles back to grams if required by the problem

  13. Another Problem • AgNO3 + KBr  AgBr + KNO3 • 64.18 g of silver bromide formed from how much silver nitrate?

  14. Remember the steps… • Balanced chem eqn? • Mass to moles? • Mole ratios? • Moles to grams?

  15. When there just isn’t Enough!!

  16. We had assumed… • That there was always enough of the other chemical for the reaction to complete itself.

  17. However… • This is not always the case! • In the following reaction: • 2H2(g) + O2(g) 2H2O(g)

  18. We need… • 2 moles H2 to react with 1 mole O2 to form 2 moles of H2O

  19. We have… • Only 4.0 moles H2 and 1.0 moles O2 • We can only form 2.0 moles of H2O • 2 moles H2 INXS!!

  20. A Limiting Reactant • 17.82 g sodium hydroxide reacts with 15.40 g phosphoric acid to give sodium phosphate and water. • Which reaction is limiting? • How much sodium phosphate is formed?

  21. Mg(s) + I2(s) MgI2(s) • Identify the limiting reagent • 100 atoms Mg and 100 molecules of I2 • 150 atoms Mg and 100 molecules of I2 • 200 atoms Mg and 300 molecules of I2 • 0.16 mol Mg and 0.25 mol I2 • 0.14 mol Mg and 0.14 mol I2 • 0.12 mol Mg and 0.08 mol I2 • 1.00 g Mg and 2.00 g I2

  22. AgNO3 + KBr  AgBr + KNO3 • 64.81 g of AgNO3 and 92.67g KBr • 1. Which reactant is limiting? Which INXS? • 2. How much silver bromide is formed? • 3. How much of the excess reactant is left over? • 4. If the actual yield of silver bromide is 54.77g, what is the percent yield?

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