The mole concept involves the concept

of the weights of substances.

We must first understand the

meaning of the terms "atomic weight," and "molecular weight,"

Atomic weights represent the relative weights of different

atoms.

Atomic, and molecular weights are expressed in atomic

mass units.

Chemists invented the concept of the mole

because they needed a way to ''count" atoms

and molecules.

Since atoms and molecules are far too

small and far too numerous to actually

count,

we must be able to achieve this "counting"

by some indirect method.

Take Ba + I2 >> Ba I2 We can see that if we add one

atom of Ba and one molecule of I2 it will

form the desired product

Well we could just count out the atoms that we need to the

exact amount then do the reaction so there is

no waste

It would take the entire population of the world a gazillion

years to count the atoms in the smallest

piece of matter.

The problem is we can’t see them and the

numbers are unbelievably large.

Ba = 137.3 amu and I2 = 2(126.9) =

253.8amu

well we can see that I2 is heavier than Ba.

How much heavier?

253.8/137.3 = 1.85 it is 1.85 times heavier.

If you had a football player and a cheer leader. Say the football player weighed 253.8lb and the

cheerleader weighed 137.3lb

the football player would be 1.85 times

heavier than the cheerleader

What if the cheerleader and

football player had Identical twins

2football player/ 2cheerleader = ?

1.85 What about 10football player/

10cheerleader?

100000football player/ 100000cheerleader?

Equal 1.85

So (X) football player/ (X)

cheerleader = 1.85

The ratio of weights of a large but equal number of objects is the same as the ratio of weights of one of each of these

objects.

1( gazillion) / 2 ( gazillion) = 1/ 2

=.5

Your in the lab and you want to run an

experiment and don’t want to waste

anything

With great skill you do the following

you reach in and pull out exactly 253.8

grams of I2 and 137.3 grams of Ba

So if I’m holding a sample of I2 that is 1.85 times heavier

than my sample of Ba. I can say something

very important

THE POINT! At this point we know that the number of I2 molecules and the number of Ba

molecules are the same.

Just as 10 football players weighed 1.85 times more than 10

cheerleaders

If I didn’t know how many football players and cheerleaders there were but, I knew the ratio was 1.85 then I would at least know they were equal.

The mole is defined as a quantity of a substance

whose mass is equal to its atomic or molecular weight expressed in

grams instead of amu's.

A mole of any substance contains the

same number of "particles" as a mole

of any other substance.

Ba =137.3 amu A mole of Ba = 137.3

grams

I2 = 253.8 amu A mole of I2 =253.8

grams

A mole of anything has the same number of “things” as a mole of anything else. A mole of people or a

mole of cars

A dozen eggs = 12 eggs, but how many

people in a dozen people?

What a dozen or a mole of something

weighs, depends upon what it is.

Well if there are 12 in a dozen than how

many are there in a mole? A LOT!!

The number of particles in a mole is equal to 6.02 E23, an unimaginably large

number.

6.02E23 = 6.02 X1023

602,000,000,000,000,000,000,000 of

something

When the molecular weight of a substance is expressed in grams, it is referred to as the

molar mass of the substance.

• Find the molar mass of H2O

• 2H = 2(1) = 2 grams

• 1O = 1(16) =16 grams

• H2O = 18 grams

•Calculate the molar mass of HNO3

•H = 1(1) = 1 grams

•N = 1(14) = 14 grams

•O = 3(16) = 48 grams

•HNO3 = 63 grams

• Calculate the molar mass of HC2H3O2

•H = 1(4) = 4 grams

•C = 2(12) = 24 grams

•O = 2(16) = 32 grams

• HC2H3O2 = 60 grams

Practice

• 1) Na2S

• 2) Al(OH)3

• 3) (NH4)2SO4

• 4) C6H12O6

Answer

Converting

• Set up to find the number of eggs in 4 dozen

dozen dozen

eggseggs

4 dozen1 dozen

48 eggs12 eggs

=

=

Converting

• Set up to find the number of Dozens in 43 eggs

eggs eggs

dozen1dozen

43 eggs12 eggs

3.6 dozen 1 dozen

=

=

Mols KCl Mols KCl

Grams KClGrams KCl

1.  What is the mass of 2.0 moles of KCl ?

2 Mols KCl 1 Mols KCl

141 G KCl75.5G KCl

=

=

What is the mass of 0.75 moles of CuSO4 ?

Mols CuSO4 Mols CuSO4

Grams CuSO4Grams CuSO4

.75 Mols CuSO4 1 Mols CuSO4

119.7 G CuSO4159.55G CuSO4

=

=

Calculate the number of mols in 30.2 grams of LiNO3

• One molar mass of LiNO3 = 68.94g

g LiNO3g LiNO3

Mol LiNO3

Mol LiNO3

30.2 g LiNO368.94 g LiNO3

.43 Mol LiNO3

1 Mol LiNO3

=

=

How many atoms are in a. 2.56 mols selenium

Mols SeMols Se

Atoms SeAtoms Se

2.56Mols Se1 Mols Se

1.54E24 Atoms Se6.02E23Atoms Se

=

=