The Mole

Review of Terms• Atomic mass: the weighted average

of the masses of the existing isotopes of an element

• Formula mass: the sum of all of the atomic masses of all atoms in a compound as represented in a chemical formula

• Atomic mass units (AMUs): 1/12 the mass of a carbon-12 atom; unit of measurement for the mass of an atom

CountingThere are many things that are too

numerous to count. Can you think of any?

Scientists’ Problems• Problem 1: There are too many

atoms in a sample of a substance to count them all.

• Problem 2: Atomic and formula masses are measured in AMUs, but scientists measure mass in grams in the laboratory.

What They Needed• A: A way to count atoms without

having to count them individually.• B: A value to relate AMUs to grams.

Question:How many atoms are necessary to

make up a mass in grams that is equal to an element’s atomic mass in

AMUs?

Solution:

What’s a Mole?• A mole (mol) is a number equal to

the number of carbon atoms in exactly 12 grams of pure carbon-12 = Avogadro’s number.

• It is a unit, like a dozen or a ream, used to represent a particular number.

What’s a Mole?So just how many atoms are in 12

grams of carbon-12?

602 000 000 000 000 000 000 000

6.02 x 1023

How big is that?Well, a mole of marbles would cover

the earth in a stack 50 miles deep!

Representative Particles1 mole always equals 6.02 x 1023 of

something…but what?

A mole of a(n) … = 6.02 x 1023

element atomsionic compound formula units

molecular substance molecules

Molar Mass• So, the atomic mass of an element is

also the mass of 1 mole (6.02 x 1023 atoms) of that element

1 mole of H = 1.008 grams of H = 6.02 x 1023 H atoms

Molar Mass• The mole establishes a relationship

between AMUs and grams.• Molar mass: The mass in grams of

one mole of a compound.

NaClFormula mass = 58.44 AMUMolar mass = 58.44 g/mol

Calculating Molar Mass

CH4

Mass of 1 mol of C = 1 x 12.01 g = 12.01 g

Mass of 4 mol of H = 4 x 1.008 g = 4.03 g

Mass of 1 mol of CH4 = 16.04 g

Molar Mass of CH4 = 16.04 g/mol

What We Know About the Mole

• It establishes a relationship between atomic mass units and grams.

• 1 mole = 6.02 x 1023 representative particles

AnalogiesEqualities can act as conversion

factors between units.• How many eggs are there in 5½ dozen

eggs?5.5 dozen x = 66 eggs

• How many nickels are in 11 rolls that each contain 40 nickels?

11 rolls x = 440 nickels

12 eggs1 dozen

40 nickels 1 roll

Conversion Factors• How can the mole be used for

conversions?•Moles and mass•Moles and # of rep. particles

Moles MassConversion factor: Molar Mass

• The mass of one mole of a substance can be used to determine the mass of a sample if the number of moles is given.

• The mass of one mole of a substance can be used to determine the number of moles making up a sample if the mass in grams is given.

Moles MassCalcium carbonate, CaCO3 (also called

calcite), is the principal mineral found in limestone, marble, chalk, pearls, and

the shells of animals such as clams.

a. Calculate the molar mass of calcium carbonate.

b. A certain sample of calcium carbonate contains 4.86 mol. What is the mass in

grams of this sample?

Moles MassJuglone, a dye known for centuries, is

produced from the husks of black walnuts. It is also a natural herbicide (weed killer) that kills off competitive plants around the black walnut tree but does not affect grass and other noncompetitive plants. The formula for juglone is C10H6O3. A sample of

1.56 g of pure juglone was extracted from black walnut husks. How many moles of juglone does this represent?

Moles Representative Particles

Conversion Factor: Avogadro’s Number

• Avogadro’s number can be used to calculate the number of particles in a given number of moles of a substance.

• Avogadro’s number can be used to calculate the number of moles made up by a given number of particles of a substance.

Moles Representative Particles

Sucrose, or table sugar, has the formula C12H22O11. If 0.85 mol of sucrose are used in a recipe, how

many molecules of sucrose are being added?

Moles Representative Particles

Formaldehyde, H2CO, was used in the past to preserve biological samples. How many moles of formaldehyde

does 1.51 x 1023 molecules represent?