THE MOLE CONCEPT

• Objects are measured by counting or by weight (mass)

• It’s easier to measure large objects by counting– Elephants, cars, eggs

• It’s easier to measure small objects by weight (mass)– Nails, rice, pellets

THE MOLE CONCEPT

• Chemist handle chemicals in two ways– Chemicals that we can see and measure in

the lab

16 grams of CH4 or 32 grams of O2

– Molecules that we in our heads that move and combine to form new products

CH4 + 2 O2 → CO2 + 2 H2O

THE MOLE CONCEPT

• Through experiments we can find the relative mass of molecules– One molecule of CH4 has a mass of 16 a.m.u.

(atomic mass units)

– One molecule of O2 has a mass of 32 a.m.u.

• Knowing the mass of just one molecule is not very helpful in the lab because one molecule is just too small.

THE MOLE CONCEPT

• We can measure 16 grams of CH4 and 32 grams of O2

• These amounts are called the molecular (molar) mass in grams.

• When you measure out one gram molecular mass of a substance, you have weighed out the same number of molecules!!

THE MOLE CONCEPT

THE MOLE CONCEPT

SubstanceGram Molecular Mass (grams)

Number of Particles

(Molecules)

CH4 16 g 6.02 x 1023

O2 32 g 6.02 x 1023

CO2 44 g 6.02 x 1023

H2O 18 g 6.02 x 1023

THE DOZEN ANALOGY

CoinMass of one coin

Dozen Mass

Number of Coins

Penny 2 g 24 g 12 pennies

Nickel 4 g 48 g 12 nickels

Dime 1.5 g 18 g 12 dimes

Quarter 7 g 84 g 12 quarters

CH4 + 2 O2 → CO2 + 2 H2O

• Since each gram molecular mass of a substance contains the same number of particles (molecules). From the equation above we know that one molecule of CH4 will combine with 2 molecules of O2 to produce one molecule of CO2 and 2 molecules of H2O.

THE MOLE CONCEPT

• The gram molecular mass is a bridge between the measurements we made in lab (what we can see) and the way that individual molecules (that we cannot see) react.

• We shorten the name of gram molecular mass to the word MOLE– Scientists have found that a mole contains 6.02 x 1023

molecules – This is 602,000,000,000,000,000,000,000 molecules!!

THE MOLE CONCEPT

Defining the Mole

• Definition of the mole (mol)– One mole is the amount of any substance that

contains the same number of units as the number of atoms in exactly 12 grams of Carbon-12

– 1 mole of any substance = 6.02 x 1023 units of that substance. (unit = molecules, formula units, or atoms)

– 6.02 x 1023 is known as Avogadro’s Number

• The central relationship between the mass of one atom and the mass of one mole of those atoms is the atomic mass of an element obtained from the periodic table and is expressed in atomic mass units (a.m.u.)

• The a.m.u. is numerically the same as the mass of one mole of atoms of the element expressed in grams

Defining the Mole

1 Fe atom has a mass of 55.85 a.m.u.

1 mole of Fe atoms has a mass of 55.85 grams

55.85 grams of Fe contains 6.02 x 1023 atoms

Defining the Mole

•A similar relationship holds for compounds.

•The molecular mass (or formula units) of a compound expressed in atomic mass units is numerically the same as one mole of the compound expressed in grams

Defining the Mole

1 molecule of H2O has a molecular mass of 18.02 a.m.u.

1 mole of H2O has a mass of 18.02 grams

18.02 grams of H2O has 6.02 x 1023 molecules

Defining the Mole

• A mole of any ordinary object is a staggering amount, however, atoms, molecules are not ordinary objects. They are much smaller than ordinary objects.

• Therefore 1 mole is not such a staggering amount!

THE MOLE CONCEPT

1 mole of any substance = 6.02 x 1023 units of that substance.

• If you spread 1 mole of un-popped popcorn kernels across the USA the entire country would be covered in popcorn to a depth of over 9 miles.

• If 1 mole of marbles were spread over the surface of the Earth, our planet would be covered by a 50-mile thick layer of marbles.

• 1 mole of soft drink cans would cover the surface of the earth to a depth of over 200 miles

IMAGINE:

WE DIG CHEMISTRY