Title: Lesson 4 Determining Molar Mass and
Moles of Water of Crystallisation
Learning Objectives:
• Use data from Stearic Acid monolayer experiment to try and
determine Avogadro's number.
• Determine Molar Mass experimentally.
Doing the calculations
If we drop a small amount of stearic acid onto water, it will spread out to form a
monolayer and we can measure the area this occupies. The monolayer will be
circular (it is formed in a circular watch glass) so:
If we know the cross-sectional (end-on) area of a molecule of stearic acid (this is
approximately 2.1x10-15 cm2) we can calculate the number of molecules in our
monolayer:
If we know the mass of stearic acid used and the relative molecular mass we can
work out the number of moles we have:
Finally we can combine the number of molecules and the number of moles to
work out Avogadro’s constant:
Ms Easton’s Results
Data from experiment
Diameter of watch glass = 4.9cm
Monolayer = 8 drops
Mass of 8 drops = 9.630 (drops +
cylinder) – 9.443 (cylinder) = 0.187g
Molecular formula Stearic Acid =
C18H3602
Analysis
•How does your calculated value of Avogadro’s constant
compare to the literature value?
Mine was too low!
•What do you think are the main sources of error in this
experiment? How could they be overcome?
Not cleaning the watch glass properly – this affected how the
monolayer was formed. Pasteur pipettes not
accurate/calibrated correctly – drop sizes were not the same.
•What assumptions does our method make? Are these valid
assumptions?
That the monolayer was 1 layer thick, that we could accurately
measure the mass of Stearic Acid used. Not valid assumptions
because we couldn’t prove it by determining the correct value
for Avogadro’s number.
Molar Mass, Mm
• This is the mass of one mole of something.
• To calulate Mm, simply stick ‘g’ for grams on the end of Mr.
• For example:
– Mr(H2O) = 18.02
– Mm(H2O) = 18.02 g
• Note: This is why the value of L was chosen to be what it
was.
Relating ‘mass’ and ‘molar mass’
• You need to be able to solve problems like:
– How many moles of Y is mass X?
– What is the mass of X moles of Y?
– X moles of Y has a mass of Z, what is it’s molar
mass?
• Use this equation:
NOTE: Molar Mass is measured in the unit (g mol-1
or g/mol)
For example:

How many moles of water are
present in 27.03g?



Calc Mm(H2O):
 Mm(H2O)= 2x1.01 + 16.00
= 18.02
Find n(H2O):
 n(H2O) = M / Mm
= 27.03 / 18.02
= 1.50 mol

What is the mass of 4.40 mol of
iron (III) oxide (Fe2O3)?


Calc Mm(Fe2O3):
 Mm(H2O)= 2x55.85 + 3x16.00
= 159.70 g
Find M(Fe2O3):
 M(Fe2O3) = n x Mm
= 4.40 x 159.70
= 703 g
1.30 moles of an unknown compound has a mass of 20.9g, what is it’s
molar mass?

Mm(unknown)= M / n
= 20.9 / 1.30
= 16.1 g/mol
Homework Task: Complete the test yourself
questions on page 11 of your student textbook.
Check your answers on page 558.
Questions
1. Calculate the
mass of 0.10
mol of
benzene
(C6H6)
3.
What mole
quantity of iron
(III) oxide is
present in a 1.0
kg sample?
5.
8.8 moles of a
compound has
a mass of 1.41
kg. Calculate
its molar mass.
2. Calculate the
mass of 0.75
mol of
ammonium
nitrate
(NH4NO3)
4.
What mole
quantity of
cobalt (II)
chloride (CoCl2)
is present in a
2.40 g sample?
6.
0.010 mol of
an oxide of
hydrogen has a
mass 0.340 g.
Deduce it’s
formula.
Key Points
mass
moles 
molar mass
Moles of Water of Crystallisation
• Many compounds can incorporate water into their crystal
structure, this is called the water of crystallisation.
• CoCl2 is blue
CoCl2.6H2O is pink
• The ‘.’ means the water is ‘associated’ with the CoCl2
– It is loosely bonded, but exactly how is unimportant
• In this experiment you will calculate the moles of water of
crystallisation of a compound