MOLES!!!

Atomic mass Review

the mass of an atom expressed in atomic mass units

is an average of the isotopes for that element

listed on the periodic table

What is a mole?

A mole is a counting number (like a dozen or a gross)

the number of atoms in exactly 12 g of carbon-12

Molar mass

the mass in grams of one mole of a substance (g/mol)

same number as the atomic mass but the units are different

Example

The average atomic mass of an Fe atom is 55.85 amu.

The molar mass of Fe is 55.85 g/mol

Conversions

To convert between moles and mass:

mass = # of moles x ___g/1 mol

use molar mass

Directions

Add the examples to your 3.4 notes

Copy the problems and solve the problems.

Example #1

Find the mass of 3.000 moles of Fe.

Answer #1

3.000molFe x55.85g Fe = 167.6g Fe1 mole Fe

Example #2

Find the mass of 2.25 moles of Helium.

Answer #2

2.25 mol He x 4.00g He = 9.00 g He

1 mol He

Conversion #2

Converting from mass to moles.Mass (g) x 1 mole = # moles

molar mass (g)

Example #3

How many moles of carbon in 49.5 g of carbon?

Answer #3

49.5 g C x 1 mol C = 4.12 mol C

12.01 g C

Avogadro’s number

the number of atoms/particles in one mole of a substance

= 6.022 x 1023 atoms/particles

1 mole of anything is 6.022 x 1023 of that object

Conversion #3

Determine the number of atoms from # moles

# moles x 6.022 x 1023atoms = #atoms1 mole

Example #4

Determine the number of atoms in 4.00 moles of carbon.

The substance doesn’t matter if it is in moles.

Answer #4

2.41 x 1024 C atoms

4.00 mole C x 6.022 x 1023 C atoms1 mole C

Conversion #4

To convert number of atoms to moles9.05 x 1025 Fe atoms x 1 mol Fe

6.022 x 1023 Fe atoms = 150. moles of Fe

Example #5

Calculate the number of moles in 3.20 x 1023 atoms of iron.

The identity of the substances is not important when converting between moles & # of atoms.

Answer #5

0.531 moles of Fe

3.20 x 1023Fe atoms x 1 mole Fe

6.022 x 1023Fe atoms

Practice

Add to your notes:

Practice 1-4 p. 102 & practice 1-3 p. 103

Copy problem; Show work; Include units in work & answer.

0.850g H x 1 mol H = 0.842 molH

1.01g H

Practice Problem Answers

P. 102:

1. 238 g U 2. 1.2g U

3. 0.842 mol H, 0.859g H

4. 486.0 g Pb,

0.01132 mol Pb

p. 103

1. 4.2 x 1023 atoms Fe

2. 0.4796 mol Ag

3. 0.58 mol Os

Homework

p. 104: 4,5,6-10,12,13