practical skills

Question 1

Describe steps in making a 100cm^3 0.1M standard solution.

Answer 1

1. Calculate the mass of solid required to prepare 100cm^3 of 0.1M standard solution: Mass=Mr*moles
2. weigh solid accurately with scales
3. dissolve in a beaker of less distilled water
4. transfer solution into a volumetric flask- remains of solution rinsed into flask with distilled water
5. fill flask to graduation line by adding distilled water (until bottom of meniscus line reaches it)
6. slowly and careful invert volumetric flask to mix solution thoroughly

Question 2

When making a standard solution, what is the result of filling the flask above the graduation line? how is the titre affected?

Answer 2

The concentration of the solution will be less than calculated/ it will be too dilute; the titre value will be greater

Question 3

When making a standard solution, what occurs when the volumetric flask is not inverted? how is the titre affected?

Answer 3

The solution will not be mixed thoroughly; titration results will be inconsistent.

Question 4

Describe how to carry out a titration.

Answer 4

1. Add a measured volume of one solution to a conical flask using a pipette and the other solution into a burette. Record initial burette reading to the nearest 0.05cm^3
2. add a few drops of indicator to the solution in the conical flask
3. run solution from burette into conical flask; indicator will change colour at end point.
4. record final burette reading; rough titre
5. repeat but more accurately; add dropwise as endpoint is approached.
6. further titrations are carried out until results are concordant- within 0.1cm^3

Question 5

Describe recrystallisation.

Answer 5

1. pour chosen solvent into a conical flask- if flammable, warm solvent over a water bath. if water, set up beaker/tripod
2. tip impure solid into a second conical flask
3. add solvent to impure sample until it dissolves-once dissolved allow to cool
4. crystals should form
5. filter the crystals under reduced pressure to obtain dry crystalline solid.

Question 6

Describe apparatus for heating under reflux.

Answer 6

• round bottom/pear shaped flask
• rubber tubing
• stand and clamp
• heat source
• vertical liebing condenser