Practical Chemistry

Question 1

Outline the method of quenching.

Answer 1

1. Remove a sample using a pipette at timed intervals (every 5 minutes)
2. Run the sample through a large volume of cold solvent in acini all flask. (This slows down or stops the reaction)
3. Titrate the sample quickly with a standard or know concn of NaOH in a butette.

Question 2

Reasons for a low % yield

Answer 2

• the reaction didn’t go to completion/ incomplete reaction
• a loss of solids in transfer between apparatus
• side reactions take place
OR
• by-products are formed

Question 3

Indication of an impure sample

Answer 3

• sample melts over a range of temperatures below the melting point

Question 4

Describe and explain the steps in purification of an organic solid by recrystallisation

Answer 4

• dissolve the organic solid in a minimum of hot solvent
• (use vacuum filtration so that the solution remains hot)
• filter the hot solution (to remove any insoluble impurities)
• cool the solution
• re-filter the crystals that are formed using vacuum filtration and allow soluble impurities to go through in solution
• wash the crystals with cold solvent (to ensure that any soluble impurities left are washed through)
• allow the crystals to dry (either air dry or oven dry @ a temperature lower than the mp. of the solid

Question 5

What is a monobasic acid?

Answer 5

An acid that only has one H+ ion to donate to a base in a acid-base reaction.

Question 6

How to calculate % uncertainty

Answer 6

Absolute uncertainty
——————— x100
Measured value

Question 7

Factors that could cause a higher than usual titre

Answer 7

• solution may have not been inverted therefore the solution will be more concn @ the bottom/more dilute @ the top