Volumetric Analysis

Question 1

What is a solution of accurately known concentration

Answer 1

A solution of accurately known concentration is a standard solution

Question 2

A standard solution can be prepared by:

Answer 2

♦ weighing a primary standard accurately
♦ dissolving in a small volume of solvent (usually deionised or distilled water) in abeaker
♦ transferring the solution and rinsings into a volumetric flask
♦ making up to the graduation mark with solvent
♦ stoppering and inverting

Question 3

Accurate dilution

Answer 3

Standard solutions can also be prepared by accurate dilution 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 4

A primary standard must be :

Answer 4

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

Question 5

Examples of primary standards include

Answer 5

♦ sodium carbonate, Na2CO3
♦ hydrated oxalic acid, H2C2O4·2H2O
♦ potassium hydrogen phthalate, KH(C8H4O4 )
♦ silver nitrate, AgNO3
♦ potassium iodate, KIO3
♦ potassium dichromate, K2Cr2O7

Question 6

Why is sodium hydroxide not a primary standard

Answer 6

Sodium hydroxide is not a primary standard as it has a relatively low GFM, is unstable as a solid (absorbs moisture) and unstable as a solution.

Question 7

What must happen to sodium hydroxide before being used in volumetric analysis

Answer 7

Sodium hydroxide solution must be standardised before being used in volumetric analysis

Question 8

Back titrations

Answer 8

back titrations used to find the number of moles of a substance by reacting it with an
excess volume of a reactant of known concentration.

Question 9

When are back titrations used

Answer 9

• the substance being analysed is insoluble eg aspirin calcium carbonate
• one of the reactants is volatile eg ammonia

Question 10

When is a back titration useful

Answer 10

A back titration is useful when trying to work out the quantity of substance
in a solid with a low solubility.

Question 11

What happens to the resulting mixture

Answer 11

The resulting mixture is then titrated to work out the number of moles of the reactant in excess

Question 12

What can be determined from the initial number of moles of that reactant

Answer 12

From the initial number of moles of that reactant, the number of moles used in the reaction can be determined. The initial number of moles of the substance being analysed can then be calculated