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

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Presentation on theme: "Stoichiometry A measure of the quantities consumed and produced in chemical reactions."— Presentation transcript:

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, 63 copper (62.93 amu and % of copper atoms) and 65 Cu (64.93 amu and % of copper atoms). What is copper’s average atomic mass? Practice Do 22 p. 123 (207.2 amu)

4 The Mole Avogadro’s number x 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 C 10 H 6 O 3 Practice – # 33 p. 123 Practice - 1  g (1.0 x ) of C 7 H 14 O 2 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 (C 12 H 22 O 11 )? Practice – Calculate the mass percent of each element in C 10 H 14 O

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 % 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 = g/mol NPCl 2, g / mol CoC 4 O 4, 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 grams of this compound produces g of CO2 and g of H2O, what is the empirical formula of this compound? Practice – Do 71, and 73 p. 125 Do Free Response # 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 + O 2 → Fe 2 O 3 If the last element can be balanced by multiplying by a half, do so, then multiply all reactants and products by 2. NH 3 + O 2 → NO + H 2 O C 2 H 6 + O2 → CO 2 + H 2 O 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? Fe 2 O 3 (s) + 2Al(s) → 2Fe(l) + Al 2 O 3 (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


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