1. Aim # 4: How do we determine the molar mass of a compound?
H.W. # 4
Study pp
up to example 6.7 “calculating moles from mass”
Complete the handout sheet

2. When we say the atomic mass of hydrogen is 1
When we say the atomic mass of hydrogen is amu, how much hydrogen are we talking about?
I atomic mass unit(amu)- the unit of measurement used to express the mass of an atom.
1 amu = 1/12 the mass of one 12C atom
II atomic mass- the average mass (in amu) of one atom of an element.
e.g H 1 amu Ar 40 amu
What is it an average of?

3. III molar mass – the average mass of one mole of a substance
III molar mass – the average mass of one mole of a substance For an element, it is equal to the atomic mass expressed in grams e.g H 1 g Ar 40 g
If the atomic mass of an element is the mass (in amu) of one atom, what do you think the molecular mass of a compound represents?
IV molecular mass – the sum of all the atomic masses in a molecule.
e.g. H amu + 1 amu = 2 amu
H2O 2(1 amu) amu = 18 amu

4. V The molar mass, in grams, of one mole of molecules of an element or compound is equal to the molecular mass expressed in grams e.g. H2 1g + 1g = 2g H2O 2(1g) + 16g = 18g
VI If the compound is ionic, there are no molecules. The molar mass is the mass, in grams, of one mole of formula units.
e.g. NaCl g + 35g = 58g

5. Problem: calculate the molar mass of Ca(NO3)2
Element # of moles molar mass total
Ca g g
N g g
O g g
molar mass = 164g

6. Now you try it Problem: Calculate the molar mass of Mg(OH)2 Answer:
Element # of moles molar mass total
Mg g g
O g g
H g g
total = 58g

7. VII Calculating the molar mass of a hydrate
BaCl2•2H2O
Element # of moles molar mass total
Ba g g
Cl g g
H g g
O g g
total = 244g

8. Practice makes perfect
Calculate the molar mass of CuSO4•5H2O
Answer
Element # of moles molar mass total
Cu g g
S g g
O g g
H g g
total = 250g