The Mole Concept

What is a mole?

In chemistry, a mole is a counting unit. Abbreviated mol.

1 mol = 6.022x1023 representative particles. Avogadro’s Number (N)

What is a representative particle? A unit that represents what the mole

is counting.

About Scientific Notation in Calculators

How do you represent 6.022x1023 in a scientific calculator?

Do you punch in 6.022x10^23? You can, but there’s a better way.

For graphing calculators: Type in 6.022(2nd)(,)23 The calc. displays 6.022E23 This means “6.022x1023” to the calculator.

For other calculators there is usually a key marked “E” or “EXP” that allows you to enter numbers in scientific notation. Learn how to use your calculator!

Representative Particles

Substance / Representative Particle Elements / Atoms Ions / Ions Covalent Compounds / Molecules Ionic Compounds / Formula Units

Representative Particles

If you have 3 representative particles of water, what do you have? You have 3 water molecules. H2O is a molecular compound.

If you have 4 representative particles of table salt, what do you have? You have 4 NaCl formula units. NaCl is an ionic compound.

If you have 5 representative particles of gold, what do you have? You have 5 gold atoms. Gold is an element.

Representative Particles

3 water molecules

4 NaCl formula units

5 gold atoms

How Big is Avogadro’s Number?

6.022x1023 dollars would be enough to give every person on Earth $87 trillion. That’s $1 million a second for the next 2.8 years.

A modern computer that can make up to 100 million counts per second would take almost 200 million years to count to 6.022x1023.

If 6.022x1023 snowflakes fell evenly across the United States, the blanket of snow would cover every building in the country, including the tallest skyscrapers.

If you could travel at the speed of light (186,000 mi/s in a vacuum), it would take you more than 100 billion years (longer than the Universe has been around) to travel 6.022x1023 miles.

Mole-Particle Conversions

How many molecules of methane are in 4.50 mol methane? 1 mol CH4 = 6.022x1023 CH4 molecules

4.50 mol CH4 x

6.022x1023 molecules CH4

1 mol CH4

= 2.71x1024 molecules CH4

Mole-Particle Conversions

How many moles of iron are in 2.39x1024 iron atoms? 1 mol Fe = 6.022x1023 Fe atoms

2.39x1024 atoms Fe x6.022x1023 atoms Fe

1 mol Fe= 3.97 mol Fe

Molar Mass

Representative particles are too tiny to count directly. If we know how much these particles

weigh, we can count them by weighing them.

Molar mass = mass of 1 mole of a substance. Unique for every substance.

Calculating Molar Mass

Molar mass of an element = atomic mass. Atomic mass of Fe = 55.847 amu Molar mass of Fe = 55.847 g/mol

Every 55.847 grams of iron contains 6.022x1023 iron atoms! We can weigh a sample of pure iron

and then calculate how many atoms it contains!

Moles and Mass

What is the mass of 0.250 moles of magnesium? 1 mol Mg = 24.305 g Mg

0.250 mol Mg x24.305 g Mg

1 mol Mg= 6.08 g Mg

Moles and Mass

How many atoms of mercury are in 3.15x10-5 g Hg? Always go to moles first! 1 mol Hg = 200.59 g Hg 1 mol Hg = 6.022x1023 atoms Hg

3.15x10-5 g Hg x1 mol Hg

200.59 g Hg

= 9.46x1016 Hg atoms

6.022x1023 Hg atoms

1 mol Hgx

Calculating Molar Mass

Molar mass of a compound = Combined molar mass of its elements, multiplied by subscripts. Molar mass of H2O =

2 x H = 2 x 1.01 g/mol = 2.02 g/mol 1 x O = 1 x 16.00 g/mol = 16.00 g/mol Total = 18.02 g/mol

Every 18.02 grams of water contains 6.02x1023 water molecules!

Calculating Molar Mass

Molar mass of Fe(NO3)3

1 x Fe = 1 x 55.85 g/mol = 55.85 g/mol 3 x N = 3 x 14.01 g/mol = 42.03 g/mol 9 x O = 9 x 16.00 g/mol = 144.00 g/mol Total = 241.88 g/mol

Every 241.88 grams of iron(III) nitrate contains 6.02x1023 formula units of Fe(NO3)3!

Mole-Mass Conversions

What is the mass of 0.245 mol CH4? First, find molar mass of CH4:

1 x C = 1 x 12.01 g/mol = 12.01 g/mol 4 x H = 4 x 1.01 g/mol = 4.04 g/mol Total = 16.05 g/mol

0.245 mol CH4 x16.05 g CH4

1 mol CH4

= 3.93 g CH4

Mole-Mass Conversions

How many moles are in 26.64 grams of calcium acetate? First we need a formula:

Calcium acetate = Ca(C2H3O2)2

Next we need the molar mass: 1 x Ca = 1 x 40.08 g/mol = 40.08 g/mol 4 x C = 4 x 12.01 g/mol = 48.04 g/mol 6 x H = 6 x 1.01 g/mol = 6.06 g/mol 4 x O = 4 x 16.00 g/mol = 64.00 g/mol Total = 158.18 g/mol

26.64 g Ca(C2H3O2)2 x158.18 g Ca(C2H3O2)2

1 mol Ca(C2H3O2)2= 0.1684 mol Ca(C2H3O2)2

Mole-Mass-Particle Conversions

molesgrams particles

Use molar mass

Use 6.022x1023

The Commandments of Moles

Thou shalt not use 6.022x1023 when converting from grams to moles or from moles to grams. Thou shalt instead use the molar mass.

Thou shalt not use the molar mass when converting from particles to moles or from moles to particles. Thou shalt instead use 6.022x1023.

Thou shalt not attempt to convert from grams to particles in one step. Thou shalt instead go to moles first.