Chemistry Essentials

Unit 7

Chemical Quantities and

Stoichiometry

Chapter Warm Up

1. If you had 144 eggs, how many dozen eggs

would you have?

2. If you baked 6.5 dozen cookies, how many

cookies did you bake?

3. If a recipe called for 2.5 cups of flour to

make 3 dozen cookies, how much flour

would you need to make 13.25 dozen

cookies?

Chemical Quantities

Equations --- A Second Look

What do the coefficients tell us?

How many atoms or molecules are involved in the

reaction

BUT….

Atoms and molecules are microscopic

Can’t see them, so can’t count them

So…..

How to count?

Come up with a new quantity– amount of substance

Whose unit is

THE MOLE!!!!(not an adorable furry creature)

(not a skin growth)

A UNIT

The Mole

Quantity:Amount of substance (n)

Symbol: n

Unit: Mole(mol)

1 mole of anything = 6.02 x 1023 of that thing

6.02 x 1023 known as Avogadro’s Number (NA)

Like a dozen is 12 of anything, a mole is

6.02 X 1023 of anything

Examples

1) How many moles of apples are in 3.25 x 1056

apples?

3.25 x 1056 apples 1 mole apples = 5.40 x 1032 moles apples

6.02 x 1023 apples

2) How many apples are in 7.50 moles of

apples?

7.50 moles apples 6.02 x 1023 apples = 4.52 x 1024 apples

1 mole apples

Practice

1) The population of the Earth is about 7.10 x

109 people. How many moles of people is

this?

1.18 x 10-14 moles people

2) The United States has a population of about

5.25 x 10-16 moles of people. How many

people is this?

3.16 x 108 people

Being Practical

How do we actually count/measure this stuff?

We can’t count it, so we must find another way

to measure it

We can measure mass, so how do we compare #

of atoms or molecules to mass or the numbers

of moles of atoms or molecules to mass?

Need to know the mass of 1 mole of the element

or compound

For elements(atoms)

Called the average atomic mass

From PT

We know it in amu’s

It’s the same value in grams/mole

Examples

1)What is the average atomic mass of gold

(grams/mole)?

196.97 grams Au/ 1 mole Au

Practice

Give the average atomic mass (grams/mole) for

a) sodium

b) iron

c) lawrencium

For compounds known as formula weight(FW)

or molar mass(MM)

Sum of masses of all atoms in compound

Expressed in grams/mole (g/mol)

Examples

1) What is the molar mass of NaCl?

22.990 g/mol + 35.453 g/mol = 58.443g NaCl/ 1

mol NaCl

2) What is the molar mass of Cu3(PO4)2?

3(63.546g/mol) + 2(30.974g/mol) + 8(15.999g/mol)

= 190.638 g/mol +61.948 g/mol + 127.992 g/mol

= 380.578 g Cu3(PO4)2 / 1 mol Cu3(PO4)2

Practice

Calculate the molar mass of the following.

1) CdS

144.48g CdS/1 mole CdS

2) (NH4)3PO4

149.12 g (NH4)3PO4/ 1 mole (NH4)3PO4

Chemical Calculations --

Stoichiometry

Calculating how much of a product can be made

from a given amount of reactant or how much

of a reactant is needed to form a certain

amount of a product

Based on balanced chemical equations

Coefficients represent atom (molecules) of a

substance or moles of a substance

2Al(s) + 3I2(s) → 2AlI3(s)

Means 2 atoms of aluminum react with 3 molecules of

iodine to form 2 molecules of aluminum iodide

Means 2 moles of aluminum react with 3 moles of

iodine to form 2 moles of aluminum iodide

DOES NOT MEAN 2 grams of aluminum react with 3

grams of iodine to form 2 grams of aluminum iodide

CalculationsFollow patterns or “road maps” from have to want

moles A to moles B (have moles A want moles B)

One step

Road Map

moles A → moles B

Use mole ratio (mole bridge)

Allows us to go from one substance to another

Fraction made with coefficients from balanced chemical

equation

Want/have

Example

The reaction of aluminum and iodine to produce

aluminum iodide is 2Al(s) + 3I2(s) → 2AlI3(s).

1) How many moles of AlI3 can be produced

from 5.75 moles of Al?

5.75 moles AlI3

2) How many moles of I2 are needed to produce

9.75 moles of AlI3?

14.63 moles I2

3) How many moles of Al are needed to fully

react with 3.50 x 106 moles of I2?

2.33 x 106 moles Al

mass of A to mass of B (have mass A want mass B)

Multiple steps

Remember—chemical equations are not written

in terms of mass

Must go from mass to moles to use the chemical

equation

Road Map

mass A → moles A → moles B→ mass B

mass → moles uses average atomic mass(element) or

molar mass(compound or diatomic)

moles A → moles B uses mole ratio

Examples

In the reaction 2Al(s) + 3I2(s) → 2AlI3(s),

How many grams of AlI3 can be produced from

165g Al?

2.49 x 103 g AlI3

How many grams of I2 are needed to

produce 3.50 x 103 grams of AlI3?

3.27 x 103 g I2

How many grams of Al are needed to

fully react with 1.50 x 10-3 grams of

I2?

1.06 x 10-4g Al

THE

END !