MOLES!!!. Atomic mass Review the mass of an atom expressed in atomic mass units is an average of the...

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