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Stoichiometry and the Mole

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Title: Stoichiometry and the Mole


1
Chapter 8
  • Stoichiometry and the Mole

2
Objectives
  • Explain what a mole is.
  • Perform conversions with Avogadros Number and
    Molar Mass

3
Chemistry and Cooking
  • Chemistry is not so different from cooking
  • Amount to be made must be considered
  • Ingredients (reactants) must be measured
  • Cooking conditions must be correct
  • Techniques must be learned
  • These are the basics of stoichiometry
  • I wonder why there arent more chemistry
    channels? (Hmmmmmm?)

4
Stoichiometry
  • Definition Relationship between quantities of
    reactant and product in a chemical reaction.
  • Literally means Measure the elements
  • Answers the question of how much
  • Always based on a chemical reaction
  • The chemists recipe
  • You already know how to do stoichiometry!

5
Consider the FollowingDorothys Sour Cream
Raisin Bars
1.75 C Oatmeal 1.00 C Brown Sugar 1.00 C Butter 1.75 C Flour
4.00 Egg Yolks 3.00 Tbs. Corn Starch 2.00 C Sour Cream 1.50 C White Sugar
2.00 C Raisins Makes 24 Yummy Bars
Combine the Oatmeal, Brown Sugar, Butter, and
Flour with pastry cutter. Hold out 2.00C. Place
the rest in the bottom of a 9x13 greased pan.
Bake 15 min at 350 F. Combine yolks,
starch, sour cream, sugar and raisins in a pan.
Bring to a boil and pour over crust. Crumble hold
out over top. Bake 20 min. at 350 F
6
What Does This Tell Us?
  • A number of things!
  • The amount and type of ingredients
  • The conditions required
  • The number of bars made
  • Also
  • How to change the recipe
  • How to scale it up or down

7
How Many Bars Could You Make If
1.75 C Oatmeal 1.00 C Brown Sugar 1.00 C Butter 1.75 C Flour
4.00 Egg Yolks 3.00 Tbs. Corn Starch 2.00 C Sour Cream 1.50 C White Sugar
2.00 C Raisins Makes 24.0 Yummy Bars
  • You only have 1.00 Egg

Ans. 6.00 Bars
8
How Many Bars Could You Make If
1.75 C Oatmeal 1.00 C Brown Sugar 1.00 C Butter 1.75 C Flour
4.00 Egg Yolks 3.00 Tbs. Corn Starch 2.00 C Sour Cream 1.50 C White Sugar
2.00 C Raisins Makes 24.0 Yummy Bars
  • You have 3.00 C Oatmeal

Ans. 41.1 Bars
9
How Many Bars Could You Make If
1.75 C Oatmeal 1.00 C Brown Sugar 1.00 C Butter 1.75 C Flour
4.00 Egg Yolks 3.00 Tbs. Corn Starch 2.00 C Sour Cream 1.50 C White Sugar
2.00 C Raisins Makes 24.0 Yummy Bars
  • You have 4.33 pints sour cream

Calc. 103.92 Bars
Ans. 104 Bars
10
Chemical Reactions
  • 4.04 g of hydrogen gas and 32.00 grams of oxygen
    gas react to form 36.04 grams of water.
  • How many grams of water can be made from
  • 8.08 g hydrogen gas

Ans. 72.1 g water
11
Its Just That Easy
  • When Doing Stoichiometry
  • Keep the recipe (balanced equation) in mind
  • Set up ratios that are true
  • Check to see your answer makes sense
  • Check those significant figures

12
The Mole
From www.wildcru.org/research/farming/moles/mole1
.jpg
13
The Mole
  • One mole of anything is equal to
    6.022x1023particles
  • Also called Avogadros Number
  • Inconvenient for large things like hamburgers and
    hammers
  • Works well for small things like atoms, ions, and
    molecules
  • Mole Song (Michael Offutt Chemistry Songbag 1)

14
Conversions With Moles
  • Easy to convert from particles to moles and back
  • Use conversion factors
  • Set up a ratio that cancels units

15
Convert the Following
  • 6.0x1023 atoms gold to moles
  • Ans. 1.0 moles Au

16
Convert the Following
  • 9.12x1024 molecules hydrogen to moles
  • Calc. 15.144470
  • Ans. 15.1 mol H2

17
Convert the Following
  • 3.12 moles strontium to atoms
  • Calc.1.8788x1024
  • Ans.1.88x1024 atoms Sr

18
Moles in Equations
  • Coefficients in balanced equations represent
    ratios
  • 2H2 O2 ? 2H2O
  • This means 2 molecules of hydrogen react with 1
    molecule of oxygen to produce 2 molecules of
    water
  • Or
  • 2 moles of hydrogen react with 1 mole of oxygen
    to produce 2 moles of water

19
How Much Is A Mole?
  • 1 mole of any atom has a mass equal to its atomic
    mass in grams
  • This is an atoms Molar Mass (MM)
  • Mass of a Mole
  • Units of grams/mole (g/mol)
  • What is the Molar Mass of sodium?
  • 22.99 g/mol
  • What is the Molar Mass of aluminum?
  • 26.98 g/mol

20
About 1 Mole of Aluminum
27.405g
From http//www.tufts.edu/as/wright_center/person
al_pages/george_l/Al.jpg
21
Converting Grams and Moles
  • We often need to convert between grams and moles.
  • We measure in grams but,
  • Reactions are in moles
  • Use molar mass for grams and moles
  • Use Avogadros number for moles and particles

22
Convert the Following
  • 8.00 grams of helium to moles
  • Ans. 2.00 mol He

23
Convert the Following
  • 6.25 moles of chlorine to grams
  • Calc.221.5625
  • Ans. 222 g Cl

24
Convert the Following
  • And this is a tough one!
  • 49.00 grams of calcium to atoms
  • Calc.7.36222x1023
  • Ans. 7.362x1023 atoms Ca

25
Moles and Compounds
  • Formulas tell us mole information too
  • The formula H2O means
  • 2 moles of hydrogen and 1 mole of oxygen makes up
    1 mole of water
  • The formula Ca(NO3)2 mean
  • 1 mole of calcium, 2 moles of nitrogen, and 6
    moles of oxygen make 1 mole of calcium nitrate

26
Molar Mass and Compounds
  • Compounds have molar masses too.
  • Determine the moles of each element
  • Add the molar masses of each
  • Ex. Find the molar mass of water (H2O)
  • 2 moles of H and 1 mole of O
  • 2(1.01g/mol) 1(16.00 g/mol) 18.02g/mol
  • Always put 2 digits after the decimal pt.

27
Find The Molar Mass Of
  • Sodium Carbonate
  • Na22.99, C12.01, O16.00
  • Na2CO3
  • MM2(22.99)(12.01)3(16.00)
  • Ans.105.99 g/mol
  • Calcium Phosphate
  • Ca40.08, P30.97, O16.00
  • Ca3(PO4)2
  • MM3(40.08)2(30.97)8(16.00)
  • Ans.310.18 g/mol

28
Converting Moles and Mass
  • Conversions with compounds are just like elements
  • Use molar mass of the compound
  • Convert 2.33 grams of water to moles

29
Convert the Following
  • 2.34 g Sodium Carbonate to moles
  • 105.99 g/mol
  • Calc. 0.022077
  • Ans.0.0221 mol Na2CO3

30
Convert the Following
  • 1.87 moles of Calcium Phosphate to grams
  • 310.18 g/mol
  • Calc.580.0366
  • Ans.580. g Ca3(PO4)2

31
Calculate the Following
  • Grams of oxygen in 16.11 grams of Calcium
    Phosphate (MM 310.18 g/mol)
  • Calc. 6.64801
  • Ans. 6.648 g O

32
The Conversion Works!
33
Balanced Equations and Mass
  • What masses are required to complete the reaction
    below
  • 2H2 O2 ? 2H2O
  • Hydrogen need 2 moles
  • So 4.04 g are needed
  • Oxygen need 1 mole
  • So 32.00 g are needed
  • Water 2 moles produced
  • So 36.04 grams are produced

34
Homework
  • p309 's 37,41,47,103,107,109

35
Objectives
  • Calcualte Mass Percent
  • Describe Empirical and Molecular Formulas
  • Calculate Empirical Formulas

36
Mass Percent
  • Definition The percentage, by mass, that each
    element contributes to the compound
  • How to
  • Find the Molar Mass of the compound
  • Divide the mass of each element by the MM
  • Multiply by 100.
  • Gives you percent

37
Example
  • Find the mass percent of Glucose
  • C6H12O6 C-12.01, H-1.01, O-16.00
  • C 39.99 H 6.73 O 53.28

38
Example
  • Find the mass percent of Sodium Carbonate
  • Na2CO3 Na-22.99, C-12.01, O-16.00
  • Na 43.38 C 11.33 O 45.29

39
Empirical Formulas
  • The formula with the simplest ratio of elements
    present
  • May or may not be the actual formula
  • Often determined from percent composition
  • Percent found by lab analysis
  • Combustion analysis or other means

40
Molecular Formulas
  • Actual formula of the compound
  • Formula is a whole number multiple of the
    empirical formula
  • Molar mass is a whole number multiple of
    empirical formula molar mass
  • CH2 Empirical Formula, 14.04 g/mol
  • C2H4- Molecular Formula, 28.08 g/mol

41
How To Find Empirical Formulas
  • If percent composition is given
  • Assume 100.00 grams
  • All percents become grams
  • Find the moles of each element
  • Divide each by the smallest number
  • If you have whole numbers write the formula OR
  • Multiply all number to get a whole number

42
Cont.
  • If masses are given
  • Start with the masses
  • Everything else is the same
  • When you write the formula
  • Ionic Compounds
  • Metals are usually written first
  • Covalent Compounds
  • Usually written Carbon, Hydrogen, Oxygen

43
Example
  • Find the empirical formula of a compound with the
    following percent composition
  • K 26.56, Cr 35.41, O 38.03

44
Example
  • A compound is composed of Chlorine and Oxygen.
    The percent of Chlorine is 38.77. What is the
    empirical formula?

45
Example
  • CH2 is the empirical formula of a compound that
    has a molar mass of 70.15 g/mol. What is the
    molecular formula?

46
Example
  • A Compound is made of only phosphorus and oxygen
    and has a molar mass of approximately 284 g/mol.
    The compound is made of 56.36 O. What is the
    molecular formula

47
Smores (Yum!)
  • Given the following
  • 1 bag 30 mallows, 1 bar16 rectangles
  • 2 grahams 4 rec. 1 mallow ? 1 smore
  • If you have 24 bars and plenty of grahams and
    mallows how many smores can you make?

48
Smores (Yum!)
  • Given the following
  • 1 bag 30 mallows, 1 bar16 rectangles
  • 2 grahams 4 rec. 1 mallow ? 1 smore
  • If you have 3 bags and plenty of bars and grahams
    how many smores can you make?

49
Objectives
  • Describe reaction stoichiometry
  • Determine mole ratios
  • Calculate theoretical, actual, and percent yields

50
Reaction Stoichiometry
  • Objectives
  • Describe reaction stoichiometry
  • Calculate theoretical yields for reactions
  • Compare actual yields to theoretical yields
  • Calculate the percent yield of a reaction
  • Determine a limiting reactant

51
Reaction Stoichiometry
  • A method for determining how much in chemical
    reactions
  • How much product can be made from X grams of
    reactant
  • How much reactant is necessary to make X grams of
    product

52
Mole Ratios
  • Calculations ALWAYS use mole ratios
  • Come from balanced equation
    2H2 O2 ? 2H2O
  • Mole Ratios could be

53
Process for Calculations
  • Write balanced equation
  • Find necessary molar masses
  • Set up calculation
  • Basic Setup
  • Grams A ? Moles A ? Moles B ? Grams B

Mole Ratio
MM A
MM B
54
Assumptions
  • Base your calculations off of the given reactant
  • Assume that there is plenty of the other reactant
  • We say that other reactant is in excess
  • Dont worry about the reactant that is not stated
  • You wont use it in your calculations

55
Theoretical Yield
  • Maximum mass of product you can obtain in a
    reaction
  • This is what you are calculating for
  • When the reaction is run you may not get exactly
    what you calculated
  • Could be more or less
  • This is called the actual yield

56
Actual Yield
  • Amount of product obtained when a reaction is
    carried out
  • Reasons for differences in theoretical and actual
    yield
  • Loss of product
  • Residue in reaction vessel
  • Equilibrium
  • Moisture in product

57
Percent Yield
  • Percent of theoretical yield obtained when a
    reaction is carried out.
  • Represents the efficiency of a reaction
  • Goal is to have a high percent yield
  • Equation

58
Example
  • 84.12g of O2 is reacted with excess NO2 according
    to the following equation.
    2NO O2 ? 2 NO2
  • What mass of NO2 is formed? O232.00
    g/mol, NO246.01g/mol

59
Example
  • What mass of NO is required to react with 84.12
    grams of O2 according to the equ. 2NO O2 ? 2
    NO2
  • O2 32.00 g/mol, NO 30.01 g/mol

60
Example
  • What mass of NO is required to produce 39.12
    grams NO2 if excess O2 is present 2NO O2 ? 2
    NO2
  • NO 30.01 g/mol, NO2 46.01 g/mol

61
Example
  • What mass of Silver Bromide is produced when 2.33
    grams of Silver Nitrate is mixed with excess
    Sodium Bromide?
  • AgNO3 NaBr ? AgBr NaNO3
  • AgBr187.77g/mol, AgNO3169.88g/mol

62
Example
  • The previous reaction was run and only 0.88 grams
    of AgBr were produced. Find the percent yield.

63
Homework
  • p.311 's 83,84,85

64
Who Bought the Smores Stuff?
  • Given the following
  • 1 bag 30 mallows, 1 bar16 rectangles
  • 2 grahams 4 rec. 1 mallow ? 1 smore
  • If you have 24 bars, 3 bags and plenty of grahams
    how many smores can you make?

65
Objectives
  • Describe limiting and excess reactants
  • Determine the limiting reactant for a reaction
  • Calculate theoretical yields from limiting
    reactants

66
Limiting Reactant
  • Reactant that is consumed first in a chemical
    reaction
  • Left over reactant is considered to be excess
    reactant
  • Limiting reactant is not necessarily the one with
    the smallest mass
  • Depends on mole ratio

67
Limiting Reactant
  • Limiting reactant problems will give you the mass
    of both reactants
  • Both masses need to be considered
  • You will need to figure out which reactant is
    limiting
  • The reactant that gives the smallest product mass
    is the limiting reactant
  • The other is in excess
  • The reactants could run out simultaneously

68
Process
  • Consider the reaction A B ? C
  • Convert grams of A to grams of C
  • Convert grams of B to grams of C
  • The reactant that gives the smallest mass of C is
    the limiting reactant AND
  • The smallest mass of C is the theoretical yield

69
Example
  • 152.50 g of CO reacts with 24.50 g of H2 What
    mass of CH3OH can be produced? CO 2H2 ? CH3OH
  • CO 28.01, H2 2.02, CH3OH 32.05

70
Cont.
  • What mass of the excess reactant is left? CO
    2H2 ? CH3OH
  • CO 28.01, H2 2.02, CH3OH 32.05

71
Example
  • 92.7 g of N2 react with 265.8 g of NH3. What
    mass of Ammonia is produced? N2 3H2 ?
    2NH3
  • N2 28.02, H2 2.02, NH3 17.04

72
Cont.
  • What mass of the excess reactant is left? N2
    3H2 ? 2NH3
  • N2 28.02, H2 2.02, NH3 17.04

73
Cont.
  • Determine the percent yield if only 82.0 grams of
    ammonia are produced

74
Homework
  • p. 315 's 149
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