To measure the relative molecular mass of a volatile liquid

Question 1

What does the term volatile mean?

Answer 1

Liquid with a low boiling point (evaporates easily)

Question 2

Apart from the liquid you used in the experiment, give one other example of a suitable liquid

Answer 2

Propanone, cyclohexane, propanal, hexane

Question 3

What does the term s.t.p represent?

Answer 3

Standard temperature and pressure (s,t -> 273K and s.p _> 1x 10^5)

Question 4

Why must a very small hole be punctured in the aluminium foil?

Answer 4

To allow excess vapour to escape from the flask

Question 5

What units are normally used to measure atmospheric pressure?

Answer 5

Pascals

Question 6

What must be done to ensure that all of the volatile liquid has vaporised fully?

Answer 6

The flask should be left in the boiling water for several minutes

Question 7

Identify three sources of error in this experiment

Answer 7

• forgetting to ensure that the inside of the flask is clean and dry
• measuring volume of conical flask inaccurately
• not leaving flask in boiling water for sufficient length of time
• forgetting to dry the outside of the flask when re-weighing
• there may be bubbles of air in the hypodermic syringe

Question 8

Why does the liquid have to be volatile?

Answer 8

When conical flask is placed in boiling water the liquid will be completely changed into a gas and fill the conical flask

Question 9

Describe how the
i) mass
ii) volume
of vapour is determined

Answer 9

i) weigh flask and heat until all liquid is gone and then cool, dry and reweigh and find different for mass
ii) Fill flask with water and empty into graduated cylinder

Question 10

Explain why the pressure of the vapour is the same as atmospheric pressure

Answer 10

The pinhole means vapour is exposed to atmosphere

Question 11

Why is this method unsuitable for liquids that are non-volatile?

Answer 11

do not vaporise easily as boiling points too high

Question 12

The vapour of 0.63 g of a pure liquid occupies a volume of 330 cm3
at a temperature
of 100 °C and at a pressure of 101 kPa. Calculate the number of moles of vapour and hence calculate the relative molecular mass

Answer 12

PV = nRT     PV/RT = n
101 x 1000 x 330 x 10-6
8.31 x 373 
=.01076
MR = .63 / .01076 = 58.55 (moles x mr = g)

Question 13

What modern instrumental technique could be used as a more accurate method to measure
the relative molecular masses of volatile and non-volatile liquids as well as of solid and gaseous
substances?

Answer 13

Mass spectrometer

Question 14

How is the volatile liquid allowed to vaporise?

Answer 14

Placing in container surrounded by hot water [conical flask]

Question 15

Give the procedure

Answer 15

Heat beaker with water to almost boiling with Bunsen burner
Cut circle in aluminium foil large enough to cover mouth of conical flask
Find total mass of clean dry conical flask, aluminium foil and rubber band
Using dropping pipette, add volatile liquid to flask
Cover mouth of flask with the foil and hold tightly with rubber band
With pin, prick one small hole in the centre of aluminium foil, attach clamp to neck of the flask
Immserse flask to boiling water and all liquid vaporises and some will escape out the hole in cap until pressure inside flask is equal to atmospheric pressure
When flask appears empty, remove flask from beaker and record exact temp of hot water
Record value of atm pressure using barometer
Allow flask to cool and find mass of flask, cap, rubber band and contents
Remove cap and rubbber band and find the volume of the flask by filling with water and transferring to graduated cylinders
Record volume of liquid transferred

Question 16

Why is a rubber band used

Answer 16

So no vapour can escape between the foil and the glass

Question 17

If there is small quantity of liquid found in the flask after it cools, what is it

Answer 17

It is the volatile liquid which has cooled and condensed

Question 18

APPARATUS

Answer 18

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