4.1 researching chemistry

Question 1

what is stoichiometry

Answer 1

the study of mole relationships involved in chemical reactions

Question 2

what is percentage by mass

Answer 2

the mass of solute made up to 100cm3 of solution

Question 3

what is percentage by volume

Answer 3

the number of cm3 of solute made up to 100cm3 of solution

Question 4

what is gravimetric analysis used to determine

Answer 4

the mass of an element or compound in a substance

Question 5

what occurs in precipitation conversion

Answer 5

this is when 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 6

what occurs in volatilisation conversion

Answer 6

the substance is heated and any volatile products are evaporated. the substance is heated to constant mass and the final mass is recorded

Question 7

what is a standard solution

Answer 7

a solution of accurately known concentration

Question 8

what must a primary standard be

Answer 8

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

Question 9

what are examples of primary standards

Answer 9

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

Question 10

why is sodium hydroxide not a primary standard

Answer 10

it has a low gfm, is unstable as a solid and as a solution. it must be standardised before being used in a volumetric analysis

Question 11

what can EDTA be used to determine

Answer 11

the concentration of metal ions in solution

Question 12

describe a back titration

Answer 12

• 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 used in the reaction can be determined.
• the initial number of moles of the substance being analysed can then be calculated.

Question 13

when is a back titration useful

Answer 13

when trying to wok out the quantity of substance in a solid with a low solubility

Question 14

what does a colorimeter use the relationship between

Answer 14

colour intensity of a solution and the concentration of the coloured species present

Question 15

what is a colorimeter used for

Answer 15

measuring absorbance of light of a series of standard solutions, and this data can be used to plot a calibration graph

Question 16

what is distillation used for

Answer 16

identifiction and purification of organic compounds

Question 17

what does heating under reflux allow

Answer 17

heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping

Question 18

what does heating under reflux consist of

Answer 18

placing the reaction mixture in a round bottomed flask with anti bumping granules and fitted with a condensor. the flask is then heated using an appropriate source of heat

Question 19

what is vaccuum filtration

Answer 19

this technique involves carrying out filtration under reduced pressure and provides a faster means of separating a precipitate from a filtrate

Question 20

what does recrystallisation involve

Answer 20

• dissolving an impure solid gently in a minimum volume of hot solvent
• hot filtration of the resulting mixture to remove any insoluble impurities
• cooling the filtrate slowly to allow crystals of the pure compound to form, leaving soluble impurities dissolved in the solvent
• filtering, washing, and drying the pure crystals

Question 21

how is the solvent for recrystallisation chosen

Answer 21

so that the compound being purified is completely soluble at high temperatures and only sparingly soluble at lower temperatures

Question 22

what does solvent extraction involve

Answer 22

isolating a solute from a liquid mixture or a solution by extraction using an immisible solvent in which the solute is soluble

Question 23

describe the steps to a solvent extraction

Answer 23

• two immiscible solvents form two layers in the separating funnel
• the solute dissolves in both solvents and an equilibrium establishes between the two layers
• the ratio of solute dissolved in each layer is determined by the equilibrium constant
• the lower layer is run off into a container and the upper layer is poured into a second container
• this process is repeated to maximise the quantity of solute extracted

Question 24

how can you maximise the quantity of solute extracted

Answer 24

using a number of extractions using smaller volumes of solvent rater than a single extraction using a large volume of solvent

Question 25

what can the determination of the melting point of a compound indicate

Answer 25

the purity of the compound

Question 26

what is chromatography

Answer 26

a technique used to separate the components present within a mixture. it separates substances by making use of difference in their polarity of molecular size

Question 27

what does the distance travelled in chromatography depend on

Answer 27

how soluble the compounds are in the chosen solvent and how well they adhere to the plate. a developing agent or ultraviolet light is normally required to visualize the spots on the chromatogram

Question 28

how does thin layer chromatography assess the purity of substances

Answer 28

a pure substance when spotted and developed on a TLC plate should appear as a single spot. the presence of more than one spot shows that impurities are present