Download Aim # 4: How do we determine the molar mass of a compound?

Aim # 4: How do we determine the molar
mass of a compound?
H.W. # 4
Study pp. 165-168
up to example 6.7 “calculating moles
from mass”
Complete the handout sheet
When we say the atomic mass of hydrogen is
1.00794 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?
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 1g
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. H2 1 amu + 1 amu = 2 amu
H2O 2(1 amu) + 16 amu = 18 amu
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 23g + 35g = 58g
Problem: calculate the molar mass
of Ca(NO3)2
Element
Ca
N
O
# of moles molar mass total
1
40g
40g
2
14g
28g
6
16g
96g
molar mass = 164g
Now you try it
Problem: Calculate the molar mass of
Mg(OH)2
Answer:
Element
Mg
O
H
# of moles molar mass total
1
24g
24g
2
16g
32g
2
1g
2g
total = 58g
VII Calculating the molar mass of a hydrate
BaCl2•2H2O
Element # of moles
Ba
1
Cl
2
H
4
O
2
molar mass total
137g
137g
35.5g
71g
1g
4g
16g
32g
total = 244g
Practice makes perfect
Calculate the molar mass of CuSO4•5H2O
Answer
Element
Cu
S
O
H
# of moles molar mass total
1
64g
64g
1
32g
32g
9
16g
144g
10
10g
10g
total = 250g