Chemical Composition

Learning Targets

I can…calculate formula weight, molecular weight, and

molar mass.calculate percent composition given a chemical

formula.convert between the mass, moles, and number of

particles of a substance.

Atomic Masses: Counting Atoms by Weighing

Atoms are too tiny to be weighed in grams and kilograms. Instead, we use the atomic mass unit, or amuThe atomic mass unit =1.66x10-24g

For example, the average atomic mass for carbon atoms is 12.01 amu

Calculating Mass Using Atomic Mass Units (amu)

Calculate the mass, in amu, of a sample of aluminum that contains 75 atoms.1 Al atom= 26.98 amu75 Al atoms x 26.98amu/Al atom=2024 amu

Calculating Mass Using Atomic Mass Units (amu)

Calculate the mass, in amu, of a sample of nitrogen that contains 23 atoms.1 N atom = 14.01 amu23 N atoms x 14.01 amu/N atom=322.2 amu

Calculating the Number of Atoms from the Mass

Calculate the number of sodium atoms present in a sample that has a mass of 1172.49 amu1 Na atom = 22.99 amu1172.49 amu x 1 Na atom/ 22.99 amu=51.00 Na atoms

Calculating the Number of Atoms from the Mass

Calculate the number of oxygen atoms in a sample that has a mass of 288 amu.1 O atom = 16.00 amu288 amu x 1 O atom/ 16.00 amu=18.0 O atoms

The Mole

We have used atomic mass units for mass, but these are extremely small units.

In the laboratory a much larger unit, the gram, is the convenient unit for mass.

We will learn to count atoms in samples with masses given in atoms.

The Mole

We have a sample of aluminum that has a mass of 26.98g.

What mass of copper contains exactly the same number of atoms as this sample of aluminum?

26.98g sample Al = ? Grams of Cu

To answer this question:

We need to know the average atomic masses for aluminum and copper:Al= 26.98 amuCu= 63.55 amu

Which atom has the greater atomic mass, aluminum or copper?Copper

To answer this question:

If we have 26.98 g of aluminum, do we need more or less than 26.98 g of copper to have the same number of copper atoms as aluminum atoms?MoreWe need more than 26.98 g of copper because each

copper atom has a greater mass than each aluminum atom.

To answer this question:

What does this mean?A given number of copper atoms will weigh more

than an equal number of aluminum atoms.

How much copper do we need?aluminum atoms are 26.98 amuCopper atoms are 63.55 amu

26.98g of aluminum and 63.55g of copper contain exactly the same number of atoms

To answer this question:

Therefore:

26.98g of aluminum contains the same number of aluminum atoms as 63.55g of copper contains copper atoms.

Another example:

Now compare carbon and heliumDoes a 12.01g sample of carbon contain more or less

or the same number of atoms as 4.003g of helium?Carbon = 12.01 amuHelium = 4.003 amu

Both samples contain the exact same number of atoms

The Mole

The number of atoms present in all of these samples assumes special importance in chemistry.

The mole (abbreviated mol) can be defined as:The number equal to the number of carbon atoms in

12.01 grams of carbon. That is to say, one atomic mass unit is defined to be

1/12 of the mass of a carbon-12 atom.

The Mole

Techniques for counting atoms very precisely have been used to determine this number to be 6.022x1023

One mole of something consists of 6.022x1023 units of that substance is called:Avogadro’s Number

For example, Just as a dozen is 12 eggs…One mole of eggs is 6.022x1023 eggs

How do We Use the Mole in Chemical Equations?

12.01g sample of Carbon contains 6.022x1023 atoms

The atomic mass of hydrogen is 1.008 amuA 1.008 g sample of hydrogen contains 6.022x1023

atoms

The atomic mass of aluminum is 26.98 amuA 26.98 g sample of aluminum contains 6.022x1023

atoms

The point is:

A sample of any element that weighs a number of grams equal to the average atomic mass of that element contains 6.022x1023 atomsOr one mol of that element

Comparison of 1-Mol Samples of Various Elements

Element # of Atoms Present

Mass of Sample (g)

Aluminum 6.022x1023 26.98

Gold 6.022x1023 196.97

Iron 6.022x1023 55.85

Sulfur 6.022x1023 32.07

Boron 6.022x1023 10.81

Xenon 6.022x1023 131.3

Test Your Knowledge

Aluminum (Al), a metal with a high strength-to-weight ratio and a high resistance to corrosion, is often used for structures such as high-quality bicycle frames. Compute both the number of moles of atoms and the number of atoms in a 10.0g sample of aluminum.

In this case we want to change from mass to moles of atoms.10.0g of Al= ? Moles of Al atoms

Test Your Knowledge

The mass of one mol (6.022x1023 atoms) of aluminum is:26.98g

The sample we are considering has a mass of:10.0g

Its mass is less than 26.98g, so this sample contains:Less than 1 mol of aluminum atoms.

Test Your Knowledge

1 mol Al= 26.98 g Al

10.0 g Al x 1 mol Al/ 26.98 g Al=

0.371 mol Al

Test Your Knowledge

Next we convert from moles of atoms to the number of atoms.6.022x1023 Al atoms= 1 mol Al atoms0.371 mol Al x 6.022x1023 Al atoms/ 1 mol Al=2.23x1023 Al atoms

Molar Mass

A chemical compound is:A collection of atoms

For example,Methane consists of molecules each containing one

carbon atom and four hydrogen atoms (CH4)

How can we calculate the mass of 1 mol of methane?What is the mass of 6.022x1023 CH4 molecules?

Molar Mass

Each CH4 molecule contains:1 carbon atom4 hydrogen atoms

1 mol of CH4 molecules consists of:1 mol of carbon atoms4 mol of hydrogen atoms

How can we find the mass of 1 mol CH4?

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

Mass of 4 mol of H= 4 x 1.008g = 4.032g

Mass of 1 mol of CH4 =

16.04 g

Calculating Molar Mass

Calculate the molar mass of sulfur dioxide, a gas produced when sulfur containing fuels are burned.

The formula for Sulfur dioxide is:SO2

1 mol SO2 molecules =1 mol S atoms2 mol O atoms

Calculating Molar Mass

Mass of 1 mol of S=1 x 32.07 =32.07 g

Mass of 2 mol of O=2 x 16.00 =32.00 g

Mass of 1 mol of SO2 =64.07 g = molar mass