Chemical Stoichiometry:The Mole Concept

Mr. ForteAtascadero High School

Average Atomic Mass Modern Periodic Table is based on one atom

of Carbon-12, which weighs 12 amu exactly! All other nuclides are assigned a relative

mass when compared to C-12, in atomic mass units (amu)

A weighted average is calculated based on percent abundance of naturally occurring isotopes.

There is no single atom of an element that has the exact mass equal to its average atomic mass.

Calculating Average Atomic Mass

Average atomic mass is a weighted average.

Multiply relative mass by percent abundance for each naturally occurring isotope.

Add them all together.

Neon (Z=10)

There are two types of naturally occurring neon atoms, Ne-20 and Ne-21.

Ne-20 has a relative mass of 19.99943 amu and makes up 90.5% of all Ne atoms.

Ne-21 has a relative mass of 20.99931 amu and makes up 9.5% of all Ne atoms.

(19.99943)(0.905) + (20.99931)(0.095) =

20.179 amu

Mole Concept

A mole is a word that represents a number, much like a dozen.

One mole = 6.02 x 1023 of anything. The average atomic mass of any element

expressed in grams provides exactly one mole of atoms of that element!

This concept allows the chemist to count by weighing.

>, <, or = to, One Mole?

45.5 grams of Sodium (Z=11)

158 grams of Radon (Z=86)

55.847 grams of Iron (Z=26) 8.7 grams of Lithium (Z=3)

17.0 grams of Neon (Z=10) <

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Which Contains More Atoms?

25 g of Nickel (Z=28)

25 g of Zinc (Z=30)

8 g of Helium (Z=2)

25 g of Copper (Z=29)

25 g of Carbon (Z=6)

400 g of Lead (Z=82)

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Molar Mass

The mass in grams of any element or compound that provides exactly one mole of units of that substance is called its molar mass.

The molar mass of iron (Fe) is 55.8 grams

The molar mass of carbon dioxide, CO2 is 44.0 grams

Conversions

The molar mass of a substance is used to convert between mass (grams) and moles (number of particles).

Avogadro’s Number is used to convert moles or grams into an actual number of particles. It can be set equal to one mole or the molar mass of any substance.

What mass of carbon (C) is found in a 3.45 mol sample? Molar mass of carbon = 12.0 g/mol

3.45 mol (12.0 g C) = 41.4 g (1 mol C)

How many moles of fluorine (F) atoms are found in a 100. g sample? Molar mass of fluorine = 19.0 g/mol

100. g F (1 mol F) = 5.26 mol F atoms (19.0 g F)

How many actual atoms of zinc (Zn) are found in a 7.50 gram sample? (Zn = 65.4 g/mol)65.4 g Zn = 6.02 x 1023 atoms of Zn

7.50 g Zn (6.02 x 1023 atoms of Zn) = (65.4 g Zn) 6.90 x 1022 atoms of Zn

How many actual atoms of tin (Sn) are found in a 3.25 moles sample of tin?(1 mole Sn = 6.02 x 1023 atoms of Sn)

3.25 mol Sn (6.02 x 1023 atoms of Sn) = (1 mol Sn) 1.96 x 1024 atoms of Sn