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 Brophy’s presentation at http://teacherweb.com/TX/McNeilHS/brophy/photo2.aspx

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?v=xiVweBpjXJo&playnext=1&videos=kHh8a0fILuY&feature=mfu_in_order

• Mole 2 chem guy: http://www.youtube.com/watch?v=xqw2BWdKl1Q&feature=PlayList&p=C09489B20AE215DF&playnext=1&playnext_from=PL&index=19

• Mole 3 (Junior) :

• http://www.youtube.com/watch?v=O7qjYRYxkso&feature=PlayList&p=C09489B20AE215DF&playnext=1&playnext_from=PL&index=20

• 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-sNfM5Xo&feature=related

• Empirical formula for AP chem with chem. Guy: http://www.youtube.com/watch?v=fFXtX_8NgsQ&feature=related

The Basics of Stoichiometry and

Mole Calculations

Topics (review and new)

• Reading and writing

chemical equations

• Balancing Equations

• Types of reactions

• Formula Weights

• Formula Weights and Molar

Masses

• Percent composition

• Avagadro’s Number

(continued)

• Mole calculations

• Solving problems using dimensional analysis using (1) Avagadro’snumber, (2) GFM, or (3) mole ratios from coefficients of balanced equations

• Determining Empirical formulas and Molecular formulas

• Limiting reagents(reactants)

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)

2 H2 (g) + O2 (g) 2 H2O (l)

Reading Balanced Chemical Equations

Molecules

Particle diagrams

Mass (amu)

Amount (mol)

Mass (g)

2 H2 (g) + O2 (g) 2 H2O (l)

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

_____NaN3 (s) _____ Na (s) + _____N2 (g)

Practice Balancing Equations

• (example of a decomposition reaction that is

used in air bags in cars)

_____NaN3 (s) _____ Na (s) + _____N2 (g)

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)

___C3H8 (g) + ___O2 (g) ___ CO2 (g) + ___H2O (g)

Formula Weights

• Atomic masses of

atoms, molecules

and formula unit

• Atoms:

• Ex: Au (gold)

• One atom of Au

has a mass of

__________amu

• 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

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?

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

Percent Composition

What is the % carbon in MSG?

Step 1: FW for C5H8NO4 is ________amu

C

H

N

O

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?

% Composition of Carbon in C5H8NO4

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

Percent Composition

Avagadro’s number and The Mole

6.02 x 1023

602,000,000,000,000,000,000,000

The Mole

• A counting unit

• Similar to a dozen, except instead of 12, it’s 602 billion trillion 602,000,000,000,000,000,000,000

• 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

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.

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!

= 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

A Mole of Particles

Contains 6.02 x 1023 particles

1 mole C

1 mole H2O

1 mole NaCl

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!

Avogadro’s Number as

Conversion Factor

1. Number of atoms in 0.500 mole of Ala) 500 Al atomsb) 6.02 x 1023 Al atomsc) 3.01 x 1023 Al atoms

2.Number of moles of S in 1.8 x 1024 S atomsa) 1.0 mole S atomsb) 3.0 mole S atomsc) 1.1 x 1048 mole S atoms

Learning Check

Practice Problems – with /without

using your calculator

• How many hydrogen atoms are in 2.5 moles of

water? (Analyze, plan, solve, check)

• 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

Molar Mass or Gram-Formula

Mass (GFM)

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 amu’s 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 Avogadro’s 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 NUMBER3 just different units

Find the molar mass or gram-formula mass

(usually we round to the tenths place)

Learning Check!

A.1 mole of Br atoms

B. 1 mole of Sn atoms

= 79.9 g/mole

= 118.7 g/mole

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)

Problems involving Empirical Formulas

and Molecular Formulas

• Resources:

• PPT

• POGIL

• Practice problems (in-class, chapter problem

set, online practice quiz questions)

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.

Types of Formulas• Empirical Formula

The formula of a compound thatexpresses the smallest whole numberratio of the atoms present.

Ionic formula are always empirical formula

• Molecular Formula

The formula that states the actualnumber of each kind of atom found in onemolecule of the compound.

To obtain an Empirical Formula1. 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.

4. 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

A sample of a brown gas, a major air pollutant, isfound to contain 2.34 g N and 5.34g O. Determine aformula 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:

0.334 0.167ON 0.167 0.334 2

0.167 0.167

N O NO====

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?

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

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?

Predicting Quantities of Products

Based on Quantities of Reactants Use

(use concept/road map)

Grams of reactant

(Given)

Moles of reactant

Moles of product

Grams of product

(To Find_

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

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?

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

Activities and Problem set X

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 ______

• 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