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 calculationsIf 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 experimentDiameter of watch glass = 4.9cmMonolayer = 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.

Questions1. Calculate the

mass of 0.10 mol of benzene (C6H6)

2. Calculate the mass of 0.75 mol of ammonium nitrate (NH4NO3)

3. What mole quantity of iron (III) oxide is present in a 1.0 kg sample?

4. What mole quantity of cobalt (II) chloride (CoCl2) is present in a 2.40 g sample?

5. 8.8 moles of a compound has a mass of 1.41 kg. Calculate its molar mass.

6. 0.010 mol of an oxide of hydrogen has a mass 0.340 g. Deduce it’s formula.

Key Points

massmolar

massmoles

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