Determines The RMM Of A Volatile Liquid

Question 1

What equipment is used?

Answer 1

• clean, dry conical flask
• hot plate
  -graduated cylinder
  -retort stand and clamp
  -barometer
  -thermometer
  -tinfoil
  -Elastic band
  -cyclohexane or other volatile liquid
  -large beaker
  -water
  -electronic balance

Question 2

Explain how to prepare for this experiment

Answer 2

1. Record the mass of the conical flask, tinfoil and elastic band. (M1)
2. Pour 10cm^3 of cyclohexane into the conical flask
3. Place foil over neck of the flask and secure with an elastic band
4. Make a small pinhole in the Center of the foil
5. Turn on hotplate and place beaker full on water onto it
6. Wait for water to boil

Question 3

Explain how to evaporate the volatile liquid

Answer 3

1. Clamp conical flask onto a retort stand and slowly lower into the beaker of boiling water on a hot plate.
2. Ensure water covers as much of the flask as possible
3. Leave for approx 10 min to ensure all of volatile liquid has vaporised
4. Once all of the liquid has vaporised, remove flask from beaker and allow to cool for 5 minutes

Question 4

How do we find the temperature

Answer 4

By measure the temperature of the water with a thermometer, as the temp of the water is the same as the temp inside the flask

Question 5

How do we measure the pressure

Answer 5

Using a barometer

Question 6

Why is the pressure of the room the same as the pressure in the flask?

Answer 6

The pinhole allows excess pressure to escape so it becomes equalised with the pressure outside

Question 7

How do I find the mass of the volatile liquid?

Answer 7

1. Dry the outside of the flask
2. Weigh the flask, liquid, elastic band + foil (m2)
3. Subtract the initial mass of the flask, elastic band + tinfoil (M1) from the mass just found
   M2-M1= volume of volatile liquid

Question 8

How to find the volume of the flask

Answer 8

Fill conical flask to brim with water
Decant water into graduated cylinder until empty
Record the amount of times you fill the graduated cylinder and add all of them up = volume of flask

Question 9

How do we find the volume of the volatile liquid?

Answer 9

Using ideal gas equation: PV= nRT rearranged to V=nRT/P

Question 10

Why may you not get exactly the right answer for the RMM of the volatile liquid?

Answer 10

1. Mass balance may have an inaccuracy of one decimal place
2. A more accurate volume if the water inside the conical flask may be used for example a Burette
3. It is difficult to cover the whole conical flask with boiling water so some of the vapour at the top of the flask may not be the same temperature as the boiling water
4. The flask may have had some residual water on the outside when we recorded M2

Question 11

What units should all parts of PV=nRT be in?

Answer 11

P- pa
V-m^3
T- Kelvin