Stoichiometry

# Stoichiometry

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## Stoichiometry

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1. Stoichiometry A measure of the quantities consumed and produced in chemical reactions

2. Atomic Mass • Is measured in either amu or in grams / mol • 1 amu is exactly 1/12 the mass of a Carbon - 12 atom so it serves as the standard for all other atoms • There are no atoms in nature that have the mass stated on the periodic table • These are averages of the most stable isotopes of that atom

3. Calculating the Average Mass of an Element • There are two isotopes of copper, 63copper (62.93 amu and 69.09 % of copper atoms) and 65Cu (64.93 amu and 30.91 % of copper atoms). What is copper’s average atomic mass? • Practice Do 22 p. 123 (207.2 amu)

4. The Mole • Avogadro’s number • 6.022 x 1023 of anything • Like a dozen • Relationship between the mole and amu to get grams / mole

5. The Mole • Determining mass of a sample of atoms • Determining the number of moles of atoms from mass • Calculating number of atoms from mass

6. Molar Mass • The mass of one mole of a substance • Just add together the elements mass • What is the mass of C10H6O3 • Practice – # 33 p. 123 • Practice - 1 g (1.0 x 10-6 ) of C7H14O2 contains how many molecules and how many atoms of carbon?

7. Percent Composition • (Part / Whole) x 100 • What is the percent of Carbon in table sugar (C12H22O11)? • Practice – Calculate the mass percent of each element in C10H14O

8. Determining Formulas • Empirical Formula • The simplest whole-number ratio of atoms in a compound • 3 steps • Determine the number of moles of each element • Divide all elements by the lowest mole number • Multiply all moles by a number that will make all whole if necessary

9. Determining Formulas cont… • Empirical Formulas cont… • What is the empirical formula of a compound made from 43.64% phosphorus and 56.36 % oxygen by mass? • Practice - # 64 and 66 p. 125

10. Determining Formulas cont… • Molecular Formula • The actual formula of a compound • Two things are needed to calculate it • The empirical formula of a compounds • The molar mass of the compound

11. Determining Formulas cont… • Molecular formulas cont…. • SNH molar mass = 188.35 g/mol • NPCl2, 347.64 g / mol • CoC4O4, 341.94 g / mol • A compound is 41.39% C, 3.47% H, and the rest is oxygen, the molar mass is 116 g / mol • Practice – Do 68 p. 125

12. Determining Formulas cont… • A compound consists of only C, H, and O. Combustion of 0.1156 grams of this compound produces 0.1638 g of CO2 and 0.1676 g of H2O, what is the empirical formula of this compound? • Practice – Do 71, and 73 p. 125 • Do Free Response # 3 2006 part (a)

13. The meaning of a chemical equation • Gives you the number of each reactant required and the number of each product produced • Physical states are represented by (s), (l), (g), (aq) • The number of each type of atom must be the same on both sides of the equation.

14. Balancing Chemical Equations • Whenever you see an equation the first thing you should ask yourself is whether or not it is balanced. • Most equations can be balanced by inspection • Balance all element other than H and O first, then do H, and finally O

15. Balancing Chemical Equations cont… • Hints: • Make the odd one even • FeO + O2→ Fe2O3 • If the last element can be balanced by multiplying by a half, do so, then multiply all reactants and products by 2. • NH3 + O2→ NO + H2O • C2H6 + O2 → CO2 + H2O • Practice – do 79 p. 126

16. Stoichiometric Calculations • Calculating masses of reactants or products from known amount • Balance equation • Convert known mass to moles of that same substance • Use mole ratio to convert to moles of other substance • Covert moles of other substance to grams if necessary

17. Stoichiometric Calculations cont… • Sample: What masses of iron III oxide and aluminum must be used to produce 15.0 g of Fe from the following equation for the thermite reaction? • Fe2O3(s) + 2Al(s) → • 2Fe(l) + Al2O3(s) • Practice – do # 88 p. 127

18. Calculations involving a limiting reactant • Unless the starting mole amounts of reactant are the exact right ratio (which is rare), then one of the reactants will run out before the other reactant(s) does. • This is the limiting reactant • It is the reactant we base all answers on • The reactant left over is known as the reactant in excess

19. Calculations involving a limiting reactant • If you are given a starting amount of more than one reactant than you have a limiting reactant problem. • If the problem says anything like reacts with excess of another reactant it is not a limiting reactant problem

20. Calculations involving a limiting reactant • Example of determining which reactant will run out first • Example: # 93 p. 127 • Practice – do 94 p. 127 • Example - # 95 p. 127 • Practice - # 96 p. 127

21. Percent Yield • (Actual Yield / (Theoretical Yield) x 100 • Actual is the amount actually produced (given to you in the problem) • Theoretical is the mathematical calculation we do to determine how much we should get

22. Percent Yield cont… • Sample # 99 p. 128 • Practice # 100 p. 128