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

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Cont.

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

74
Homework

• p. 315 's 149