PAG 1 - Mole Determination (2.1.3d, 2.1.3h)

Question 1

Define

Hydrated

Anhydrous

Water of Crystallisation

Answer 1

Hydrated - water molecules integrated in the crystalline structure

Anhydrous - in the absence of water

Water of crystallisation - Water molecules that are bonded into a crystalline structure of a compound

Question 2

What is the practical method for working out the formula of a hydrated salt?

Answer 2

1. Weigh an empty crucible
2. Add the hydrated salt into the crucible and weigh both together
3. Use a pipe clay triangle, tripod and bunsen to heat the crucible
4. Leave to cool after strong heating
5. Weigh the crucible after cooling (crucible + anhydrous salt)

Question 3

When calculating the formula of a hydrated salt, what assumptions are made?

Answer 3

• You assume all water is lost, in reality you can only see the surface, some maybe left within the structure
• Solution: Heat to constant mass - repeatedly reheat
• You assume there has been no further decomposition
• It is hard to judge if this has happened if the compounds don’t change colour

Question 4

What is the practical method for identifying an unknown metal/its Mr?

Answer 4

1. Set up a conical flask and have a gas syringe on a clamp and stand
2. Weigh the metal sample and add to flask with a known concentration of acid
3. Measure the maximum volume of gas in the syringe

Question 5

What is another way to measure gas volume?

Answer 5

• Use a rubber tube and connect to an upturned measuring cylinder full of water

Question 6

What issues can be posed with using a gas syringe?

Answer 6

• Some gases are soluble in water so you will not get the true volume of gas produced
• The syringe may not be moved and get stuck
• Gases can escape before the bung is inserted