10.1 To measure the relative molecular mass of a volatile liquid

Question 1

Define the term relative molecular mass?

Answer 1

Is defined as:

1. The average of the mass numbers of the isotopes of the element,
2. As they occur naturally.
3. Taking their abundances into account and
4. expressed on a scale in which the atoms of the carbon-12 isotope have a mass of exactly 12 units.

Question 2

In this experiment, a volatile liquid is used. What does the term volatile mean?

Answer 2

A volatile liquid is one with a low boiling point (evaporates easily).

Question 3

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

Answer 3

Examples of suitable liquids are propanone, cyclohexane, hexane and ethanaol.

Question 4

What does the term s.t.p. represent?

Answer 4

Standard temperature and pressure. Standard temperature is 273 K and standard pressure is 1 × 105pascals.

Question 5

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

Answer 5

The small hole is necessary to allow the excess vapour to escape from the flask.

Question 6

What units are normally used to measure atmospheric pressure?

Answer 6

Pascals (newtons per metre squared) or symbols Pa (N m-2).

Question 7

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

Answer 7

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

Question 8

Indicate one source of error in this experiment. (9)

Answer 8

Sources of error:
• Forgetting to ensure that the inside of the flask is clean and dry.

• Measuring the volume of the conical flask inaccurately.
• Not leaving the flask in the boiling water for a sufficient length of time.
• Forgetting to dry the outside of the flask when re-weighing.
• Not using a balance accurate enough to 2 decimal places.
• Forgetting to measure the temperature of the boiling water (it may not be exactly 373 K).
• Lack of accuracy in the measuring instruments, e.g. the graduated cylinder and the gas syringe can only read to 0.5 cm3.
• There may be bubbles of air in the hypodermic syringe.
• Parallax errors may be involved in taking readings.

Question 9

What does R represent in the equation pV = nRT?

Answer 9

R is the universal gas constant. (The value of R is 8.31 J mol ^-1 K ^-1.)

Question 10

Why does the liquid have to be volatile?

Answer 10

The liquid must be volatile so that when the conical flask is placed in the boiling water the liquid will be completely changed into a gas and fill the conical flask.