EMPA questions

Question 1

Outline a method to find out x in CuSO4.xH2O

Answer 1

1. set up crucible on tripod
2. measure mass of crystals by difference
3. heat crystals to remove water of crystallisation
   4 heat crystals to constant mass then use n = mass/Mr to find out how many moles of water have been removed

Question 2

Outline a calorimetry method of finding enthalpy change

Answer 2

1. burn fuel or allow reaction to occur
2. measure temp change water
3. work out q
4. enthalpy change = q(kj)/n

Question 3

Why is heating under reflux a good way of ensuring that all of the aldehydes are reacted

Answer 3

1. most organic molecule are volatile
2. reflux allows us to heat the volatile reagents without losing them
3. reflux means to boil - evaporate - condense over and over again

Question 4

Name 5 purification and separation techniques

Answer 4

1. fractional distililation
2. steam distillation
3. filtration
4. buchner filtration
   5 chromatography

Question 5

Outline 2 main identification techniques and their types

Answer 5

1. melting point - pure substances have fixed melting and boiling points
2. spectroscopic techniques -
   a. infrared: detect bonds
   b. mass spec: yields, Mr and thus molecular formula
   c. NMR: yields structure
   d. UV-VISIBLE
   e. colorimetry: determines concentration as A prptnal C (via graph)

Question 6

Name the 5 main sources of uncertainty in apparatus

Answer 6

1. measuring flasks
2. burettes
3. pipettes
4. timing devices
5. balances

Question 7

How can error in apparatus be calculated?

Answer 7

+_ 1/2 of the smallest graduation e.g. if measures to 50cm3 accurately to 0.1 cm3 then error is +_0.05 cm3

Question 8

How can percentage in total apparatus be calculated?

Answer 8

error in instrument/ capacity of instrument x100

add together for all apparatus

Question 9

Outline 4 ways of evaluating uncertainty and improving reliability

Answer 9

1. discuss repetition - reduces uncertainty in averaged results
2. discuss variables that could affect results and discuss the importance of controlling them
3. discuss the fact that errors are cumulative (more apparatus = more errors)
4. reduce errors by:
   a) extending the number and range of the results
   b) using greater volumes of reactants