Full text

(1)

Molar Mass

Learning Target

Students will be able to explain molar mass and perform calculations.

(2)

Atomic Mass -Revision

• Mass of atoms expressed in grams are very small:

• It is possible to determine these tiny masses

by using a mass spectrometer.

• With this instrument, the mass of a fluorine atom was found to be 3.155 x 10

-23

g

• These values are inconveniently small and impractical to work with

It is more useful to compare the relative masses of atoms using a reference isotope as a standard

• The isotope chosen is

12

C. It has been arbitrarily assigned a mass of exactly 12 atomic mass units (amu)

Learning Target

Students will be able to explain molar mass and perform calculations.

(3)

Atomic Mass Unit-Revision

An atomic mass unit (amu) is defined as

1

/

12

the mass of

12

C atom.

• The atomic mass of any other atom is determined by comparing it with the mass of the

12

C atom.

• With these units, a He with a mass of 4.0026 amu, has about one-third the mass of a

12

C atom.

Most elements occur naturally as mixtures of isotopes.

• A Nickel-60 atom has about five times the mass of a

12

C atom

• *Question: What is the atomic mass of an atom if its mass is approximately equal to 4.5 times as much as

12

C atom?

Learning Target

Students will be able to explain molar mass and perform calculations.

(4)

Atomic Mass Unit-Revision

• Question: What is the atomic mass of an

atom if its mass is approximately equal to 4.5 times as much as 12 C atom?

4.5 X 12 amu = 54 amu

Learning Target

Students will be able to explain molar mass and perform calculations.

(5)

Masses of Substances

• If it were possible to mass individual atoms, 7 helium atoms would have the same mass as 2 nitrogen atoms.

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(6)

Numbers for Counting Quantities

• 1 pair = 2

• 1 dozen = 12

• 1 baker’s dozen = 13

• 1 score = 20

• 1 mole = 602 billion trillion

= 602,000,000,000,000,000,000,000 = 6.02 X 10 23

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(7)

Avogadro’s Number

6.02 X 10

23

is the number of particles in 1 mole of a substance

• 1 mole of paper clips is 602 billion trillion paper clips

• 1 mole of baseballs is 602 billion trillion baseballs

• 1 mole of Mg atoms is 602 billion trillion Mg atoms

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

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*The Mole

One mole samples of a solid, a liquid, and a gas.

Which would contain more representative particles: a mole of water molecules or a mole of chalk

molecules (CaCO 3 )?

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

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Do Now

• If the relative masses are based upon 12 C, and that mass is said to be the mass of one mole when 12amu as the standard:

• *Question: Can you figure out the

approximate mass of one mole of 6 Li atoms?

Explain your reasoning.

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*Mass of a Mole

• If the relative masses are based upon 12 C, and that mass is said to be the mass of one mole when 12amu as the standard:

• *Question: Can you figure out the

approximate mass of one mole of 24 Mg atoms? Explain your reasoning.

Learning Target

Students will be able to explain molar mass and perform calculations.

(11)

Mass of a Mole

• Question: Can you figure out the approximate mass of one mole of

24

Mg atoms? Explain your reasoning.

• You know that a mole of

12

C atoms has a mass of exactly 12 g and that a

12

C atom has an atomic mass of 12 amu.

• Compare one mole of

12

C with one mole of

24

Mg.

• A single

24

Mg atom is twice as massive as a

12

C atom

• Similarly, one mole of

24

Mg atoms should have twice as much mass as one mole of

12

C atoms

• By definition we know the mass of 1 mole of

12

C atoms is 12.0 g. Therefore, the mass of 1 mole of

24

Mg should be

2 X 12 g = 24.0 g

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(12)

The Mole

• Avogadro’s Number helps us since the

masses of atoms are too small to work with.

• The mass of 1 mole (602 billion trillion=6.02 X 10

23

) of an atom is its molar mass

• This is a number we can measure.

• Remember, the mass of a fluorine atom was found to be 3.155 x 10

-23

g

• The mass of a mole of fluorine atoms is _________ g

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(13)

A mole is always the same number (6.02 x 10

23

), but 1 mol samples of different substances will have

different masses.

Molar Masses

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(14)

Dozen Analogy

A dozen is the same number (12) whether we have a dozen limes or a dozen eggs.

However, a dozen limes do not have the same mass as a dozen eggs.

Similarly, a mole is always the same number, 6.02 x 10

23

, but one mole samples of different substances will have different masses.

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(15)

Interpretations of The Mole

Mole

6.02 x 10

23

Molar mass of

Representative Particles the pure substance

One mole of

12

C is 6.02 x 10

23

atoms of

12

C. One mole of

12

C is 12 g of atoms of

12

C.

One mole of

4

He is 6.02 x 10

23

atoms of

4

He. One mole of

4

He is 4 g of atoms of

4

He.

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(16)

One Mole of Manganese

The atomic mass of manganese is 54.94 amu Therefore the molar mass of manganese is 54.94 g/mol.

When you measure 54.94 g of manganese on a balance, you indirectly count 6.02 x 10 23 atoms of manganese.

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

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Atoms in a paper clip

Paper clips are made of a basic steel, which is mostly iron and a small amount (less than

1.7%) of Carbon. Assuming that the amount of carbon in a paper clip is negligible, let’s

figure out how many atoms of iron are in a paper clip.

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(18)

Atoms in a paper clip

• Mass of paper clip ___1.01_____ g

____________atoms Fe

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(19)

*Do Now: Molar Mass

Determine the molar mass of NaCl using molar masses from the Periodic Table.

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

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* Measuring a Mole

• List the steps you take to measure out one mole of aluminum in the laboratory.

• How many aluminum atoms are in your 1 mole sample? Explain your reasoning.

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(21)

Measuring a Mole Activity

1. Determine the molar mass of given elements.

2. Physically measure out 1 mole of each substance (element) that is available.

3. Complete the following equation for each substance:

1 mole of [substance] = ____________ particles of [substance] = _______ g [substance]

4. Discuss this measurement process within the lab

group and with the class. Consider one dozen limes and one dozen eggs to explain the difference of

molar masses.

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(22)

*Do Now

• You need 1 mole of water for a reaction. How would you find this (step by step)?

Learning Target

Students will be able to explain molar mass and perform calculations.

(23)

Conservation of Mass

How would the mass

of this assembled barbell

compare with the mass of the unassembled barbell?

How do you think the combined mass of one

carbon atom and two oxygen atoms compares with the mass of a molecule of carbon dioxide (CO 2 )?

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(24)

Mass of a Backpack

• The mass of your backpack is the sum of the mass of the pack plus the

masses of the textbooks, notebooks, pencils, lunch, and miscellaneous

items you put into it.

• You could find its mass by

determining the mass of each item separately and adding them

together.

Similarly, the mass of a mole of a compound equals the sum of the masses of every particle that makes up the compound.

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(25)

Molar mass of SO 3

Sulfur trioxide is composed of one atom of sulfur and three atoms of oxygen.

1 mol SO3= 1 mol S atoms + 3 mol O atoms

– Using the periodic table, the mass of one mole of each element present in SO3can be determined.

Sulfur = 32.06 g/mol Oxygen = 15.99 g/mol =

1 x 32.06g/mol + 3 x 15.99g/mol = 80.06 g/mol

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(26)

Counting by Measuring Mass

Think how can you measure 1 mole of water without counting individual H 2 O molecules.

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(27)

Molar mass of H 2 O

To find the molar mass of a compound, find the number of grams of each element in one mole of the compound. Then add the masses of

elements in the compound.

*What is the molar mass of water (H2O)?

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(28)

*Molar Mass

• Determine the molar mass of C 12 H 22 O 11

(sucrose) using molar masses from periodic table.

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

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*Molar Mass

Determine the molar mass of C 8 H 18 (octane) using molar masses from periodic table.

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(30)

Moles and Gases

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

(31)

*Do Now

You need one mole of table salt (NaCl) for an experiment.

a. List the steps you take to measure out one mole of table salt (NaCl)

b. Is it possible to count Avogadro’s number of salt particles individually?

Learning Target

Students will be able to explain molar mass and Avogadro’s number and perform calculations.

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