The Basics of Stoichiometry and Mole Calculations

1
The Basics of Stoichiometry and Mole Calculations

• References and Resources
• Our TB Ch. 3 of Chemistry The central Science
  AP version (10th edition)
• Powerpoint and in-class work
• POGIL activities
• Online resources for our TB (in particular
  Practice Quiz and e-book)
• Concept map for Stoichiometry and problem solving
  (handout)
• Video lectures from chem guy (see next page)
• Chem tours from ch. 3 of the W.W. Norton online
  book by Gilbert
• http//www.wwnorton.com/college/chemistry/gilbert2
  /contents/ch03/studyplan.asp
• Some figures and examples in this PPT file have
  been taken from Melissa Brophys presentation at
  http//teacherweb.com/TX/McNeilHS/brophy/photo2.a
  spx

2
Resources Video lectures for Chemistry

• Chemguy videos (google AP chemistry and chemguy
  videos use in sequence)
• (good, short, video lectures that are
  appropriate for regular, honors, or AP chemistry)
• chem Guy video for mole 1 http//www.youtube.com
  /watch?vxiVweBpjXJoplaynext1videoskHh8a0fILuY
  featuremfu_in_order
• Mole 2 chem guy http//www.youtube.com/watch?vxq
  w2BWdKl1QfeaturePlayListpC09489B20AE215DFplay
  next1playnext_fromPLindex19
• Mole 3 (Junior)
• http//www.youtube.com/watch?vO7qjYRYxksofeature
  PlayListpC09489B20AE215DFplaynext1playnext_f
  romPLindex20
•  
• Stoichiometry video with worked example for
  predicting amount of product from 2.6 mol of one
  of reactant see http//www.youtube.com/watch?v-
  W9-sNfM5Xofeaturerelated
• Empirical formula for AP chem with chem. Guy
  http//www.youtube.com/watch?vfFXtX_8NgsQfeature
  related

3
The Basics of Stoichiometry and Mole
Calculations

• Topics (review and new)

• (continued)

• Reading and writing chemical equations
• Balancing Equations
• Types of reactions
• Formula Weights
• Formula Weights and Molar Masses
• Percent composition
• Avagadros Number

• Mole calculations
• Solving problems using dimensional analysis using
  (1) Avagadros number, (2) GFM, or (3) mole
  ratios from coefficients of balanced equations
• Determining Empirical formulas and Molecular
  formulas
• Limiting reagents(reactants)

4
Chemical Equations

• Symbolic representations or descriptions of
  chemical reactions
• Meaning of the symbols, subscripts, parenthesis,
  signs, arrows, co-efficients, states of matter,
  etc. Vocab. terms reactants, products,
  catalyst, reaction conditions , etc.
• Use of particle diagrams (to represent)
• Conservation of Mass

• Resources POGIL activity Chemical Reaction
  Equations (To Do)

5
Reading Balanced Chemical Equations

• Molecules
• Particle diagrams
• Mass (amu)
• Amount (mol)
• Mass (g)

6
Balancing Equations

• Objective find the smallest whole number ratio
  of coefficients for reactants and products that
  respects the law of conservation of mass.
• How?
• Trial and error to some extent
• Can Not change subscripts in chemical formulas
• but you Can change co-efficients in front of
  the formulas
• Start by balancing those elements that occur in
  the fewest chemical formulas
• Move back and forth checking the atom count each
  time a co-efficient is changed

7
Practice Balancing Equations

• (example of a decomposition reaction that is used
  in air bags in cars)

8
Practice Balancing Equations (Index card
activity to hand in)

• (example of a combustion reaction)
• (Hint Start with the element that occurs in the
  fewest chemical formulas)

9
Formula Weights

• Calculating formula weights in amu using chemical
  formula and the atomic masses from the periodic
  table
• Ex NaCl
• One formula unit of NaCl has a mass of ______amu
• Ex C6H12O6 (glucose)
• One molecule of glucose has a mass of ______amu

• Atomic masses of atoms, molecules and formula
  unit
• Atoms
• Ex Au (gold)
• One atom of Au has a mass of __________amu

10
Percent Composition

• What is the percent carbon in C5H8NO4 (the
  glutamic acid used to make MSG monosodium
  glutamate), a compound used to flavor foods and
  tenderize meats?

11
Percent Composition

• What is the percent carbon in C5H8NO4?
• Step 1 First calculate the Formula weight for
  the compound (FW for C5H8NO4 is ________amu)
• Step 2 composition (mass of part mass of
  whole) X 100
• or
• element ( atoms of that element)(atomic
  weight of the element) x 100
• Formula weight of the compound

12
What is the carbon in MSG?

• Step 1 FW for C5H8NO4 is ________amu
• C
• H
• N
• O

13
Composition of Carbon in C5H8NO4

• Step 2 C (mass of part mass of whole) X
  100
• or
• element ( atoms of element)(atomic weight of
  the element) x 100
• Formula weight of the compound
• Answer?

14
Percent Composition

• What is the percent carbon in C5H8NO4 (the
  glutamic acid used to make MSG monosodium
  glutamate), a compound used to flavor foods and
  tenderize meats?
• a) 8.22 C
• b) 24.3 C
• c) 41.1 C

15
Avagadros number and The Mole
6.02 x 1023
602,000,000,000,000,000,000,000
16
The Mole

• A counting unit
• Similar to a dozen, except instead of 12, its
  602 billion trillion 602,000,000,000,000,000,000,0
  00
• 6.02 X 1023 (in scientific notation)
• This number is named in honor of Amedeo _________
  (1776 1856), who studied quantities of gases
  and discovered that no matter what the gas was,
  there were the same number of molecules present

17
How big of a unit is a mole?

• If you had Avogadro's number of unpopped popcorn
  kernels, and spread them across the United States
  of America, the country would be covered in
  popcorn to a depth of over 9 miles.

18
The Mole (abbreviated as mol)

• 1 dozen cookies 12 cookies
• 1 mole of cookies 6.02 X 1023 cookies
• 1 dozen cars 12 cars
• 1 mole of cars 6.02 X 1023 cars
• 1 dozen Al atoms 12 Al atoms
• 1 mole of Al atoms 6.02 X 1023 atoms
• Note that the NUMBER is always the same, but the
  MASS is very different!

19
A Mole of Particles Contains 6.02 x 1023
particles

• 6.02 x 1023 C atoms
• 6.02 x 1023 H2O molecules
• 6.02 x 1023 NaCl formula units
• (technically, ionic compounds are not molecules
  so they are called formula units)
• 6.02 x 1023 Na ions and
• 6.02 x 1023 Cl ions

1 mole C 1 mole H2O 1 mole NaCl
20
Avogadros Number as Conversion Factor

• 6.02 x 1023 particles
• 1 mole
• or
• 1 mole
• 6.02 x 1023 particles
• Note that a particle could be an atom OR a
  molecule!

21
Learning Check

• 1. Number of atoms in 0.500 mole of Al
• a) 500 Al atoms
• b) 6.02 x 1023 Al atoms
• c) 3.01 x 1023 Al atoms
• 2.Number of moles of S in 1.8 x 1024 S atoms
• a) 1.0 mole S atoms
• b) 3.0 mole S atoms
• c) 1.1 x 1048 mole S atoms

22
Practice Problems with /without using your
calculator

• How many hydrogen atoms are in 2.5 moles of
  water? (Analyze, plan, solve, check)

23
Molar Mass or Gram-Formula Mass (GFM)

• The Mass of 1 mole (in grams)
• Equal to the numerical value of the average
  atomic mass (get from periodic table)
• 1 mole of C atoms 12.0 g
• 1 mole of Mg atoms 24.3 g
• 1 mole of Cu atoms 63.5 g

24
Other Names Related to Molar Mass or Gram-Formula
Mass (GFM)

• Molecular Mass/Molecular Weight If you have a
  single molecule, mass is measured in amus
  instead of grams. But, the molecular mass/weight
  is the same numerical value as 1 mole of
  molecules. Only the units are different. (This
  is the beauty of Avogadros Number!)
• Formula Mass/Formula Weight Same goes for
  compounds. But again, the numerical value is the
  same. Only the units are different.
• THE POINT You may hear all of these terms which
  mean the SAME NUMBER just different units

25
Learning Check!

• Find the molar mass or gram-formula mass
  (usually we round to the tenths place)

79.9 g/mole

1. 1 mole of Br atoms
2. 1 mole of Sn atoms

118.7 g/mole
26
Practice Problems use mole road map or concept
map to help you solve

• How many atoms of copper are there in a 3 gram
  sample of copper?
• (Interconverting mass and numbers of particles)

27
Problems involving Empirical Formulas and
Molecular Formulas

• Resources
• PPT
• POGIL
• Practice problems (in-class, chapter problem set,
  online practice quiz questions)

28
Chemical Formulas of Compounds

• Formulas give the relative numbers of atoms or
  moles of each element in a formula unit - always
  a whole number ratio (the law of definite
  proportions).
• NO2 2 atoms of O for every 1 atom of N
• 1 mole of NO2 2 mol of O atoms to every 1 mol
  of N atoms
• If we know, or can determine, the relative number
  of moles of each element in a compound, we can
  determine a formula for the compound.

29
Types of Formulas

• Empirical Formula
• The formula of a compound that expresses the
  smallest whole number ratio of the atoms present.
• Ionic formula are always empirical formula
• Molecular Formula
• The formula that states the actual number of
  each kind of atom found in one molecule of the
  compound.

30
To obtain an Empirical Formula

• 1. Determine the mass in grams of each element
  present, if necessary.
• 2. Calculate the number of moles of each
  element.
• 3. Divide each by the smallest number of moles to
  obtain the simplest whole number ratio.
• If whole numbers are not obtained in step 3),
  multiply through by the smallest number that will
  give all whole numbers
• Be careful! Do not round off numbers
  prematurely

31

• A sample of a brown gas, a major air pollutant,
  is found to contain 2.34 g N and 5.34g O.
  Determine a formula for this substance.
• require mole ratios so convert grams to moles
• moles of N 2.34g of N 0.167 moles
  of N
• 14.01 g/mole
• moles of O 5.34 g 0.334
  moles of O
• 16.00 g/mole
• Empirical Formula

32
Calculation of the Molecular Formula

• A compound has an empirical formula of NO2. The
  colourless liquid, used in rocket engines has a
  molar mass of 92.0 g/mole. What is the molecular
  formula of this substance?

33
Empirical Formula from Composition

• A substance has the following composition by
  mass 60.80 Na 28.60 B 10.60 H
• What is the empirical formula of the substance?
• Consider a sample size of 100 grams
• This will contain 28.60 grams of B and 10.60
  grams H
• Determine the number of moles of each element
• Determine the simplest whole number ratio of each
  element

34
Determination of Empirical Formulas using
combustion analysis data for organic compounds

• Experimental method that uses the mass of
  starting materials and masses of water and carbon
  dioxide produced to solve for the emp. Formula of
  organic compounds.
• Steps?

35
Predicting Quantities of Products Based on
Quantities of Reactants Use(use concept/road
map)
36
How do we do this?

• Predicting Quantities of Products Based on
  Quantities of Reactants
• Start from a balanced equation
• Use concept/road map
• Use dimensional analysis with (a) molar mass/ GFM
  and (b) mole ratios as the basis of conversion
  factors

37
Practice Problem (Practice ex. P. 103).

• The decomposition of potassium chlorate is
  commonly used to prepare small amounts of oxygen
  gas in the laboratory. How many grams of oxygen
  gas can be prepared from 4.50 g of potassium
  chlorate?

38
Limiting reagents(reactants) problems(Start with
POGIL Limiting reactants)

• Start with the balanced chemical equation
• Determine moles of reactants
• Setup tables (initial, change and end (final)
  or ICE charts
• Identify the limiting reagent/reactant
• Finish the problem using dimensional analysis

39
(No Transcript)
40
Activities and Problem set X

• Ch 3 Problems TO DO write out questions and
  answers show work
• all GIST, sample practice exercises,
  Visualizing concepts,
• end of chapter 3 exercises 10, 11 (b,e,g), 12
  (d,g), 14, 15, 17, 19, 23 (b,d), 26d, 27, 29, 33,
  35, 37, 41, 43, 47, 53, 59, 63, 67, 68, 71, 73,
  75, 80, 105

• TB ch. 3 all sections required for regents, SAT
  II and AP exams
• POGIL activities on chemical reaction equations,
  limiting reactants, etc
• Lab activities
• Determination CuxOy formula
• Percent composition of water in a hydrate
• Others - TBD
• Road/Concept map (provided)
• Online practice quiz due by ______