Chapter Six Calculations: Formula Masses, Moles, and Chemical Equations.

Chapter SixChapter SixCalculations:

Formula Masses, Moles, and

Chemical Equations

Chapter 6 | Slide 2

Molecular and Formula Mass

Atomic mass unit (amu) = the unit of mass for atoms

Molecular mass is the ___ of the ______ of all atoms in a ________

Formula mass is the ___ of the ______ of all atoms in a _______ _____ of an _____ compound

Chapter 6 | Slide 3

Examples

Molecular mass of H2O is18 amu

Formula mass of Ca(NO3)2 is 1 Ca atom = 1 x 40.1 amu

2 N atoms = 2 x 14.0 amu

6 O atoms = 6 x 16.0 amu

Sum is 161.4 amu

Chapter 6 | Slide 4

Avogadro’s Number

6.02 x 1023

Avogadro’s number is equal

to __ ______ Makes working with large numbers easier

Instead of saying you have 12,000,000,000,000,000,000,000,000 molecules of NH3, you can say you have 19.9 ______ of NH3

Chapter 6 | Slide 5

How big is a mole?

1 mole = 6.02 x 1023 of anything1 mole of atoms is _______ atoms1 mole of baseballs is ______ baseballs1 mole of dimes is ______ dimes

If there were 1 mole of people, it

would take 120 trillion Earths to

accommodate them!

6.02 x 1023 = 1 mole

Chapter 6 | Slide 6

← Fig. 6.3 Everyday counting units.

Calculations: Formula Masses, Moles, and Chemical Equations cont’d

Chapter 6 | Slide 7

How big is a mole?

The mole is a term of convenience

1 mole = 6.02 X 1023

1 dozen = 12

1 gross = 144 or 12 dozen

1 ream paper = 500 sheets of paper

1 ton (mass) = 2000 pounds

Chapter 6 | Slide 8

Fig. 6.7In solving chemical-formula-based problems, the only “transitions” allowed are those between quantities (boxes) connected by arrows.


Chapter 6 | Slide 9

Calculations with Avogadro’s Number

How many CO2 molecules are there in 2.3 moles of CO2?

A Tums tablet has 7.224 x 1023 atoms of Ca. How many moles is this?

Chapter 6 | Slide 10

→Fig. 6.2

A basic process in chemical laboratory work is determining the mass of a substance. We don’t count or measure out moles directly.



The Mass of a Mole

The mass of a mole is not a set number of grams; it depends on the substance.


The Mass of a Mole

Consider: How much does a dozen weigh?

All depends on what dozen1 dozen pennies1 dozen eggs1 dozen bricks

The mass of a mole depends on the specific substance.


The Mass of a Mole Molar mass is the mass of one mole of molecules, atoms,

ions, or formula units Numerically equal to the substance’s formula mass The only difference is the units:

►Molecular weight: weight of a molecule, in amu►Molar mass: mass of one mole of a compound, in grams

Calculated the same way

Example: What is the molar mass of C8H9O2N?

Molar mass is a conversion factor between mass and moles!


Calculations Involving Molar Mass

How many grams are there in exactly three moles of NaCl?


Calculations Involving Molar Mass

How many moles are in 215 g of silver chloride (AgCl)?


Molar mass in grams


Chemical Formulas as Conversion Factors

The subscripts in a chemical formula tell you how many of each kind of atom are in one molecule

The subscripts also tell you how many moles of atoms of the various elements are present in 1 mole of the substance

Example: N2O4



Calculate the number of moles of hydrogen atoms in 42.11 mol of H2O.



Calculate the number of moles of Citronellal molecules, C10H18O, that contain 2.35 x 1025 moles of carbon atoms.


Fig. 6.7In solving chemical-formula-based problems, the only “transitions” allowed are those between quantities (boxes) connected by arrows.



The Mole and Chemical Calculations

Calculate the number of molecules of carbon dioxide in 129.7 g of carbon dioxide.


The Mole and Chemical Calculations

Calculate the mass of 4.14 x 1025 molecules of diethyl ether, C4H10O.


Chemical Equations

•Lavoisier: mass is conserved in a chemical reaction.•Chemical equations: symbolic descriptions of chemical reactions.•Two parts to an equation:

•reactants and •products:

2H2 + O2 2H2O


Mass is Conserved During a Chemical Reaction

When 16.90 g of compound CaS (left) is decomposed into its constituent elements the Ca and S produced (right) has an identical mass of 16.90 grams.


Chemical Equations are like Recipes!

Banana Chocolate Chip SoftiesThese cookies are nice change of pace from the regular chocolate chip cookie. They have extra sweetness form the ripe banana and milk chocolate chips. Prep Time: approx. 8 Minutes. Cook Time: approx. 11 Minutes. Ready in: approx. 20 Minutes. Makes 2 dozen (24 servings). Printed from Allrecipes, Submitted by Kim 1 1/4 cups all-purpose flour1 teaspoon baking powder1/2 teaspoon salt1/3 cup butter, softened1/4 cup light brown sugar1 ripe banana, mashed1 egg1 teaspoon vanilla extract3/4 cup milk chocolate chipsDirections1 Preheat oven to 350 degrees F (175 degrees C). Grease cookie sheets. Sift together the flour, baking powder and salt, set aside.2 In a medium bowl, cream together the butter and brown sugar. Beat in the banana and egg, then ALL RIGHTS RESERVED Copyright © 2004 www.allrecipes.com

The Thermite Reaction glycerin

2Al(s) + Fe2O3(s) ----> Al2O3(l) + 2Fe(l) Reactants Products

Similarities:•Name.

•What ingredients are added together.

•What is made.

•The physical state of ingredients.

•How much of each ingredient is needed.

•How many products are made.

•Conditions needed for the reaction to occur.


Chemical Equations: The Numbers

Coefficient: numbers in front of the chemical formulas; give ratio of reactants and products. CONVERSION FACTORS!!!

►Changing these changes the number of molecules

Formula subscript: number in the middle of chemical formulas; give the number of each kind of atoms in individual molecules.

►Changing these changes the identity of the compound

2Na + Cl2 2NaCl


The Numbers in Chemical Equations


Balanced Chemical Equations

Chemical Equations must be balancedThere must be equal numbers of atoms of each element

on both sides of the equation (both sides of the arrow)►1. Write the correct symbols and formulas for all of the _______

and ____________.►2. Count the number of each type of _____ on BOTH sides of

the __________. ►3. Insert ____________ (numbers to the left of the compound

formulas) until there are the equal numbers of each kind of _______ on both sides of the equation.


Practice: Balancing Chemical Equations

Solid sodium nitride decomposes to form solid sodium metal and nitrogen gas


Hints on Balancing Equations

1. When there is no coefficient written, the coefficient is assumed to be 1

2. To balance the equation insert COEFFICIENTS. 3. NEVER alter the subscripts because that would

change the chemical formula which would change the identity and properties of the substance.

4. Start with atoms that only show up in only one compound on each side of the equation arrow.

5. The only way to learn to balance equations is through practice.


More Practice: Balancing Chemical Equations

NH3 + O2 N2 + H2O

Na2SO4 + C Na2S + CO2


More Practice:Balancing Combustion Reactions

C2H6 + O2 CO2 + H2O

C3H6 + O2 CO2 + H2O

NH3 + O2 NO + H2O


Chemical Equations as Conversion Factors: Mole Ratios

The coefficients in an equation may be used to generate conversion factors used in problem solving. These conversion factors are called “mole ratios.” 4Fe (s) + 3O2 (g) 2Fe2O3 (s)

MOLE RATIOS (from equation coefficients) ARE CONVERSION FACTORS BETWEEN: number of moles of one compound and number of moles of another compound.


S’mores Stoichiometry

1 S’more

+

+

2 GC

3 Cp

1 Mm

What are the ratios?


Stoichiometry

Chemical Stoichiometry: using mass and quantity relationships among reactants and products in a chemical reaction to make predictions about how much of a product will be made.

Example: Let’s say that you have lots of marshmallows and chocolate but only 6 graham crackers. How many s’mores can you make? Use the mole ratios from the equations!


Conversion Factors: A Reminder

Molar Mass: 1 mole X = # g X

Avogadro’s Number: 1 mole = 6.02 X 1023

Mole Ratios: from coefficients found in chemical equations 2 H2 + O2 2 H2O

2 moles H2 = 1 mole O2 = 2 moles H2O


Fig. 6.9 In solving chemical-equation-based problems, the only “transitions” allowed are those between quantities (boxes) connected by arrows.



Practice with Mole Ratios

Example: How many moles of CO2 can be produced from 1.20 moles of C4H10 in the following reaction? 2 C4H10 + 13O2 8CO2 + 10H2O



How many moles of O2 are required to completely react with 1.20 moles of C4H10?



A can of butane lighter fluid contains 1.20 moles of butane (C4H10). Calculate the number of moles of carbon dioxide given off when this butane is burned.


More Practice with Mole Ratios

How many moles of O2 react with 2.03 moles of CS2?

3O2 + CS2 CO2 + 2SO2


More Practice with Mole Ratios

How many moles of SO2 are produced when 2.03 moles of CS2

react? 3O2 + CS2 CO2 + 2SO2


Stoichiometry Calculations, Part I

1. Some sulfur is present in coal in the form of pyrite (FeS2, also known as “fool’s gold”). When it burns, it pollutes the air with SO2, as follows:

FeS2 (s) + O2 (g) Fe2O3 (s) + SO2 (g)

What mass of SO2 is produced by the combustion of 38.8 g of FeS2?


Stoichiometry Calculations, Part II

FeS2 (s) + O2 (g) Fe2O3 (s) + SO2 (g)

How many grams of O2 are needed to react with 38.8 g of FeS2?


Stoichiometry Calculations, Part III

Over the years, the thermite reaction has been used for welding railroad rails, in incendiary bombs, and to ignite solid-fuel rocket motors. The reaction is:

Fe2O3(s) + 2Al(s) -> 2Fe(l) + Al2O3(s)

What masses of iron(III) oxide and aluminum must be used to produce 15.0 g iron?


Stoichiometry Calculations, Part IV

Automotive airbags inflate when sodium azide, NaN3, rapidly decomposes to its constituent elements. The equation for the chemical reaction is2NaN3 (s) 2Na (s) + 3N2 (g)

The gaseous N2 so generate inflates the airbag. How many moles of NaN3 would have to decompose in order to generate 2.53 x 108 molecules of N2?

Chapter Six Calculations: Formula Masses, Moles, and Chemical Equations.

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