Researching Chemistry

Question 1

weighing by difference

Answer 1

mass of the crucible is measured before adding the substance and the final mass of the substance is determined by subtracting the mass of the crucible from the original mass

Question 2

heating to constant mass

Answer 2

process of heating the substance, allowing it to cool in a desiccator (dry atmosphere) and then reweighing it all in a crucible

Question 3

gravimetric analysis

Answer 3

1. mass of the analyte present in a substance is determined by changing that chemical substance into solid by precipitation of known chemical composition and formula
2. precipitate needs to be readily isolated, purified and
   weighed, easier to remove the analyte by evaporation
3. final product has to be dried completely which is done by “heating to constant mass”- crucible lid should be left partially off to allow the water to escape, blue flame should be used to avoid build up of soot on the outside of the crucible which could affect the mass, heating should be started off gently and then strongly
4. sample is weighed by difference
5. process of heating, drying and weighing is repeated until a constant mass is obtained- shows that all the water has been driven off

Question 4

characteristics of primary standard solution

Answer 4

• high state of purity
• stability in air and water
• solubility
• high GFM

Question 5

Preparation of standard solution

Answer 5

1. calculate the mass of the primary standard required to make the concentration of solution required in the appropriate volume of solution
2. make sure glassware is clean
3. place beaker on 2 d.p. balance and tare it, accurately weigh out and record approximately the mass of solid requred
4. add distilled water to beaker and stir until all of the solid dissolves and transfer solution to a volumetric flask using a funnel
5. take 2/3 rinsings of the beaker and funnel to ensure all of the chemical is transferred into the volumetric flask
6. add distilled water up to 2cm3 short of graduation mark on flask and top up until bottom of meniscus is touching mark using wash bottle
7. stopper and invert flask 2/3 times to completely mix

Question 6

controls

Answer 6

validates a technique and may consist of carrying out a determination on a solution of known concentration

Question 7

Complexometric titration

Answer 7

form of volumetric analysis in which the formation of a coloured complex is used to indicate the end point of a titration, particularly useful for the determination of a mixture of different metal ions in solution

Question 8

back titrations are used if

Answer 8

• analyte is insoluble
• reaction between analyte and standard solution is slow
• end point of direct titration is unclear or there is no suitable indicator

Question 9

back titration process

Answer 9

known excess amount of standard solution is added to the analyte until it all reacts, excess amount can then be titrated with another standard solution to work out how much hadn’t reacted, knowing how much was added and how much is left unreacted we can work out how much reacted with the analyte

Question 10

colorimetry

Answer 10

used to determine conc of coloured substances, the more concentrated the solution, the greater the absorbance of light

Question 11

construction of calibration curve

Answer 11

preparing solutions of various concentrations (standard solutions) and recording their absorbance values, straight line section of calibration curve should cover the dilution range likely to be used in determination

Question 12

distillation

Answer 12

process of heating a liquid or mixture until boiling and then collecting and cooling the resultant vapours

Question 13

uses of distillation

Answer 13

to purify a compound by separating it from a non-volatile or less volatile material
to identify compounds because the boiling point is a well defined physical property

Question 14

steam distillation

Answer 14

distillation for temperature sensitive materials like

natural aromatic compounds

Question 15

fractional distillation

Answer 15

separation of a mixture into its component parts (fractions)

Question 16

refluxing

Answer 16

technique used to apply heat energy to a chemical reaction mixture over an extended period of time without volatile reactants/ products escaping into the atmosphere

Question 17

process of refluxing

Answer 17

The liquid reaction mixture is placed in a round-bottomed flask along with anti-bumping granules with a condenser connected at the top. The flask is heated vigorously over the course of the chemical reaction; any vapours given off are immediately returned to the reaction vessel as liquids when they reach the condenser.

Question 18

vacuum filtration

Answer 18

can be carried out using a Buchner, Hirsch or sintered glass funnel, carried out under reduced pressure and provide a faster means of separating the precipitate from the filtrate

Question 19

recrystallisation

Answer 19

laboratory technique used to purify solids based on solubility

Question 20

process of recrystallisation

Answer 20

• impure solid is dissolved in minimum volume of an appropriate hot solvent and the mixture is heated, solvent is added until solid fully dissolves
• any insoluble impurities are removed by hot filtration (filter funnel and conical flask are heated before use)- prevents solvent mixture from cooling prematurely and forming crystals during filtration
• as the solution cools down, the product crystallises out and impurities remain dissolved in solvent
• once mixture is crystallised, it is filtered

Question 21

solvent for recrystallisation must be

Answer 21

insoluble at lower temperatures but completely soluble at higher temperatures

Question 22

solvent extraction

Answer 22

method of extracting a crude product from a reaction mixture based on relative solubility of a compound

Question 23

solvent for solvent extraction requirements

Answer 23

• solute must be more soluble in it than the reaction mixture
• must not react with solute

Question 24

process of solvent extraction

Answer 24

The two solvents form two separate layers in the separating funnel and the lower layer is run off into one container and the upper layer is poured out into another container. The quantity of solute extracted depends on the partition coefficient and on the number of times that the process is repeated.

Question 25

risk of solvent extraction

Answer 25

pressure build up but can be handled by opening the stopped at the top of the funnel routinely while mixing

Question 26

thin layer chromatography

Answer 26

can be used to assess product purity- pure substances will show up as one spot, more than one spot indicates impurities
to determine identity of a substance
to separate mixtures of substances (faster than paper chromatography)

Question 27

stationary phase in TLC

Answer 27

a fine film of silica or aluminium oxide spread over glass or plastic

Question 28

process of TLC

Answer 28

When setting up a TLC plate, a pencil line is drawn, usually 1 cm up from the bottom of the plate. The sample solution is then spotted on several times to get a concentrated spot on the plate. The plate is then placed in a suitable solvent (which acts as the mobile phase) making sure that the solvent is below the level of the spot. The solvent then travels up the plate and the components of the sample separate out according to their relative attractions to the stationary phase on the plate and the mobile phase

Question 29

rf value equation

Answer 29

distance travelled by spot/ distance travelled by solvent

Question 30

why can determination of melting point give an indication of the purity of a substance?

Answer 30

the presence of impurities lowers the melting point and extends its melting temperature range

Question 31

crystalline substance melting point

Answer 31

sharp falling between narrow temperature range

Question 32

mixed melting point determination

Answer 32

used as a means of identifying the product of a reaction

Question 33

mixed melting point process

Answer 33

the product can be mixed with a pure sample of the substance and a melting point taken of the mixture
If the melting point range is lowered and widened, it means that the two are different compounds. If the melting point stays the same it means that
the two compounds are likely identical.