An Introduction…..

• Atomic Mass: Where can you find it?

• An atomic mass unit is defined as 1/12 the weight of the carbon-12 isotope. The old symbol was amu, while the most correct symbol is u (a lower case letter u).

• The atomic mass of any element is the “average” atomic mass and found on the periodic table.

Background: Atomic masses

Two problems with atomic mass1. Atomic masses do not convert easily to

grams2. They can’t be weighed (they are too

small)

Therefore, atomic mass is not that practical! And we need something that is……..

Background: Atomic masses

The Mole

No, not a little critter but a number………

The Mole• A counting unit!

• Similar to a dozen, except instead of 12, it’s 602,000,000,000,000,000,000,000

• 6.02 X 1023 (in scientific notation) often called Avogadro’s #

• This number is named in honor of Amedeo Avogadro (1776 – 1856)

Here is how the International Union of Pure and Applied Chemistry

(IUPAC) defines "mole:"

• The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

What is Avogadro's Number again?

one mole of ANYTHING contains

6.02 x 1023 entities.

In other words, there are 6.02 x 1023 anythings in one mole of anything!

• The mass of one mole is called “molar mass”

• The unit is g/mol

• Equal to the numerical value from periodic table, or

add the mass of the atoms together for a molecule

1 mole of C atoms = 12.0 g/mol

1 mole of Mg atoms = 24.3 g/mol

1 mole of O2 molecules = 32.0 g/mol

Molar Mass

Molar Mass of Compounds

• The molar mass (MM) of a compound is determined the same way, except you add up all the atomic masses for the molecule

Molar Mass of Compounds

Don’t forget the subscripts!!!!

Ex. Molar mass of CaCl21. Avg. Atomic mass of Calcium = 40.08g

2. Avg. Atomic mass of Chlorine = 35.45g

3. Molar Mass of calcium chloride = 40.08 g/mol Ca + (2 X 35.45) g/mol Cl 110.98 g/mol CaCl2

20

Ca  40.08

17Cl

35.45

FlowchartParticles (Atoms or Molecules)

Moles

Mass (grams)

Divide by 6.02 X 1023

Multiply by 6.02 X 1023

Multiply by atomic/molar mass from periodic tableDivide by

atomic/molar mass from periodic table

molar mass Avogadro’s number Grams Moles particles

Everything must go through Moles!!!

Calculations

Try it together:

• What is the molar mass (MM) of: lead (II) nitrate

Pb(NO3)2

Pb 207.2 g/molN 14.0 g/mol x 2O 15.99 g/mol x 6

207.2 + (14.0 x 2) + (15.99 x 6) = 331.14 g/mol

Finding MM Practise:

• Find the molar mass of the following:a) PbSO4 

b) Ca(OH)2

c) Na3PO4

d) (NH4)2CO3

e) C6H12O6

f) Fe3(PO4)2

g) Zn(C2H3O2)2

• Complete page 57 #16, 17, 18, 19

Day 2

Mass to Moles

How many moles of KBr are in a 34.91 g sample?

Find MM: 39.09 +79.9 = 118.9 g/mol KBr

x mol = 34.91 g x ___1 mol = 0.29 mol KBr 118.9 g

Moles to Mass

How many grams of CaCl2 are in a 0.450 mol sample?

Find MM: 40.07 +(35.4 x2) = 110.87g/mol CaCl2

x g = 0.450 mol x 110.87 g = 49.89 g CaCl2 1 mol

Mass to particles

Magnesium and oxygen react to form MgO ,how many molecules of MgO are contained in a 12.56g sample of MgO ?

Find MM: 24.3 +15.99 = 40.29 g/mol MgO

x molecules = 12.56 g x x 6.02 x 1023 molecules 1 mol

1 mol40.29 g

= 1.88 x 1023

molecules of MgO

Need to get atoms?

• Set up a conversion with atoms per molecule

Ex: (NH4)2S

How many individual atoms are there in this molecule?

2 + 8 + 1 = 11 atoms for 1 molecule

Mass to atoms

Magnesium and oxygen react to form MgO ,how many atoms of MgO are contained in a 12.56g sample of MgO ?

Find MM: 24.3 +15.99 = 40.29 g/mol MgO

x molecules = 12.56 g x x 6.02 x 1023 molecules 1 mol

1 mol

40.29 g

= 3.75 x 1023 atoms of MgO

x 2 atoms 1 molecule

Try it:

• Pg 59 # 20

• Pg 60 # 24

• Pg 63 # 28 (formula units = molecules)

• Pg 64 # 34