Researching Chemistry

Question 1

Gravimetric analysis

Answer 1

Used to determine the mass of an element or compound in a substance. The substance is converted into another substance of known chemical composition, which can be readily isolated and purified.

Question 2

Precipitation conversion

Answer 2

The substance undergoes a precipitation reaction. The precipitate is separated from the filtrate and the filtrate is tested to ensure the reaction has gone to completion. The precipitate is washed, dried to constant mass and then weighed.

Question 3

Volatilisation conversion

Answer 3

The substance is heated and any volatile products (often water) are evaporated. The substance is heated to constant mass and the final mass recorded.

Question 4

Heating to constant mass process

Answer 4

• heating a substance
• allowing to cool in a desiccator to prevent absorption of water
• weighing
• repeating the steps of heating, cooling and weighing until no further changes in mass are observed

Question 5

Standard solution

Answer 5

A solution of accurately known concentration.

Question 6

How to prepare a standard solution

Answer 6

• weighing a primary standard accurately
• dissolving in a small volume of solvent in a beaker
• transferring the solution and rinsings into a volumetric flask
• making up to the graduation mark with solvent
• stoppering and inverting

Or

By pipetting an appropriate volume of a standard solution into a volumetric flask, making up to the graduation mark with solvent, stoppering and inverting.

Question 7

A primary standard requirements

Answer 7

• be available in a high state of purity
• be stable when solid and din solution
• be soluble
• have a reasonably high GFM

Question 8

Examples of primary standards

Answer 8

• sodium carbonate
• hydrated oxalic acid
• potassium hydrogen phthalate
• silver nitrate
• potassium iodate
• potassium dichromate

Question 9

Why is sodium hydroxide not a primary standard?

Answer 9

• it has a relatively low GFM
• is unstable as a solid (absorbs moisture)
• unstable as a solution
• it must be standardised before being used in volumetric analysis

Question 10

4 types of titration

Answer 10

• acid/base titrations
• redox titrations
• complexometric titrations
• back titrations

Question 11

Colorimetry

Answer 11

Uses the relationship between colour intensity of a solution and the concentration of the coloured species present.

Question 12

Redox titrations

Answer 12

Based on reactions between oxidising and reducing agents.

Question 13

Complexometric titrations

Answer 13

Based on reactions in which complexes are formed - EDTA is an important complexometric reagent and can be used to determine the concentration of metal ions in solution.

Question 14

Back titrations

Answer 14

Used to find the number of moles of a substance by reacting it with an excess volume of a reactant of known concentration. The resulting mixture is then titrated to work out the number of moles of the reactant in excess. From the initial number of moles of that reactant, the number of moles use dint eh reaction can be determined. The initial number of moles of the substance being analysed can then be calculated.