RESEARCHING CHEMISTRY

Question 1

what is stoichiometry

Answer 1

Stoichiometry is the** study of mole relationships** involved in chemical reactions.

Question 2

percentage by mass

Answer 2

Percentage by mass is the mass of solute made up to 100 cm3 of solution.

% mass = (mass of solute ÷ mass of solution) × 100

Example

100 g of salt solution has 30 g of salt in it.
% mass = (30 ÷ 100) × 100 = 30%

Question 3

Percentage by volume

Answer 3

Percentage by volume is the number of cm3 of solute made up to 100 cm3 of solution.

Question 4

ppm meaning

Answer 4

The unit ppm stands for **parts per million **and refers to 1 mg per kg or 1 mg per litre

10^6

Question 5

how can percentage yield be reduced

Answer 5

Theoretical yields can be calculated and compared with actual yields, leading to determining the percentage yield.

The percentage yield is reduced by:
♦ mass transfer or mechanical losses
♦ purification of product
♦ side reactions
♦ equilibrium position

Question 6

gravimetric analysis - precipitation conversion

Answer 6

In precipitation conversion the substance undergoes a precipitation reaction.

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

so weighing precipitate after collecting it via filtration

Question 7

gravimetric analysis - volatilisation conversion

Answer 7

In volatilisation conversion the substance is heated and any volatile products (often water) are evaporated.

The substance is heated to constant mass and the final mass recorded.

so getting rid of liquids by evaporating, then weighing the solid

Question 8

what is gravimetric analysis

Answer 8

Gravimetric analysis is 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.

The conversion can occur either through** precipitation** or volatilisation.

Question 9

weighing by difference

Answer 9

weighing by difference is used to accurately measure the mass of substance.

The mass of an empty weighing bottle and stopper is measured and the chemical is added to the bottle and reweighed. The **difference is the mass of the substance in the bottle. **

Question 10

weighing accurately approximately

Answer 10

weigh out as close as possible to a certain desired mass but ensure that the actual mass given on the balance is recorded and used in calculations.

Question 11

heating to constant mass

Answer 11

heating to constant mass:
— 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 12

volumetric analysis skills

Answer 12

Candidates must be familiar with use of the technique of volumetric analysis, including:
♦ preparing a standard solution
♦ accurate dilution
♦ standardising solutions to determine accurate concentration
♦ titrating to obtain concordancy using burettes, pipettes and volumetric flasks
♦ choosing an appropriate indicator

Question 13

standard solution definition

Answer 13

A solution of accurately known concentration is known as a standard solution.

Question 14

preparation of a standard solution

Answer 14

A standard solution can be prepared by:
♦ weighing a primary standard accurately
♦ **dissolving in a small volume of solvent **(usually deionised or distilled water) in a beaker
♦ transferring the solution and rinsings into a volumetric flask
♦ making up to the graduation mark with solvent
♦ stoppering and inverting

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 15

requirements for primary standards

Answer 15

A primary standard must:
♦ be available in a high state of purity
♦ be stable when solid and in solution
♦ be soluble
♦ have a reasonably high GFM

Question 16

examples of primary standards

Answer 16

Examples of primary standards include:
♦** sodium carbonate**, Na2CO3
♦ hydrated oxalic acid, H2C2O4 ·2H2O
♦ potassium hydrogen phthalate, KH(C8H4O4)
♦ silver nitrate, AgNO3
♦ potassium iodate, KIO3
♦ potassium dichromate, K2Cr2O7

Question 17

why sodium hydroxide isnt a primary standard

Answer 17

Sodium hydroxide is not a primary standard as:

• it has a** relatively low GFM**
• is unstable as a solid (absorbs moisture)
• unstable as a solution.

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

Question 18

what is EDTA

Answer 18

EDTA is an important complexometric reagent and can be used to determine the concentration of metal ions in solution (1:1 ratio for EDTA and metal ions)

Question 19

when is a back titration useful

Answer 19

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

basically measuring quantities of everything other than desired one

Question 20

colorimetry techniques

Answer 20

Candidates must be familiar with use of the technique of colorimetry, including:
♦ preparing a series of standard solutions of appropriate concentration
♦ choosing an appropriate colour or wavelength of filter complementary to the colour of
the species being tested
♦ using a blank
♦ preparing a calibration graph

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

A colorimeter or a spectrophotometer is** used to measure the absorbance of light of a series of standard solutions**, and this data is used to plot a calibration graph.

The concentration of the solution being tested is determined from its absorbance and by referring to the calibration curve.

The concentration of coloured species in the solution being tested must lie in the straight line section of the calibration graph.

note: standard solutions of the chemical being investigated.

Question 21

use of distillation

Answer 21

Distillation is used for the** identification and purification of organic compounds. **

The boiling point of a compound, determined by distillation, is one of the physical properties that can be used to confirm its identity.

Distillation can be used to purify a compound by separating it from less volatile substances in the mixture.

using differences in boiling point to separate a mixture

Question 22

heating under reflux

Answer 22

Heating under reflux allows heat energy to be applied to a chemical reaction mixture over an** extended period of time without volatile substances escaping.**

When carrying out heating under reflux, the **reaction mixture is placed in a round-bottomed flask with anti-bumping granules **and the flask is fitted with a condenser. The flask is then heated using an appropriate source of heat.

water goes in end of condenser nearest round bottom flask

heating mantle for flammable liquids

Question 23

vacuum filtration

Answer 23

Vacuum filtration involves carrying out a** filtration under reduced pressure** and **provides a faster means of separating a precipitate from a filtrate. **

A Büchner, Hirsch or sintered glass funnel can be used during vacuum filtration.

Question 24

use of recrystallisation

Answer 24

technique of recrystallisation to purify an
impure solid

Answer 25

Candidates must be familiar with use of the technique of recrystallisation to purify an impure solid involving:
♦ dissolving an impure solid gently in a** minimum volume** of a 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 26

recrystallisation solvent requirements

Answer 26

The solvent for recrystallisation is chosen so that the **compound being purified is completely soluble at high temperatures **and only sparingly soluble at lower temperatures.

Question 27

use of solvent extraction

Answer 27

Solvent extraction involves isolating a solute from a liquid mixture or solution by extraction using an immiscible solvent in which the solute is soluble.

Question 28

solvent extraction steps

Answer 28

When carrying out a solvent extraction, the 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, K .

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 29

perk of doing multiple solvent extractions

Answer 29

The quantity of solute extracted is greater if a number of extractions using smaller volumes of solvent are carried out rather than a single extraction using a large volume of solvent.

Question 30

solvent requirements for solvent extraction

Answer 30

The solvent used should be:
♦** immiscible with the liquid mixture or solution** (usually water)
♦ one in which the** solute is more soluble in than the liquid mixture or solution**
(usually water)
♦ volatile to allow the solute to be obtained by evaporation of the solvent
♦ unreactive with the solute

selection of a suitable solvent is crucial to achieve a high concentration of the solute after the extraction

if solute more soluble in new solvent then equilibrium shifted to allow a greater proportion of the solute to exist in the solvent. (otherwise no change in concentration of solvent in each liquid kinda)

Question 31

melting point definition

Answer 31

The melting point of a substance is **the temperature range over which the solid first starts to melt, to when all of the solid has melted. **

Question 32

use of mixed melting point analysis

Answer 32

The identity of a pure compound can be confirmed by melting point analysis and a comparison of the experimentally determined melting point with a literature or known melting point value.

Determination of the melting point of a compound can give an indication of the purity of a compound.

The presence of impurities in the compound lowers the melting point and broadens its melting temperature range due to the** disruption in intermolecular bonding in the crystal lattice**.

Question 33

why do impure compounds have lower melting points

Answer 33

The presence of impurities in the compound lowers the melting point and broadens its melting temperature range **due to the disruption in intermolecular bonding in
the crystal lattice. **

Question 34

mixed melting point steps

Answer 34

Determination of a mixed melting point involves mixing a small quantity of the product with some of the pure compound and determining the melting point.

The melting point value and the range of the melting temperature can be used to determine if the product and the pure compound are the same substance.

Question 35

what is chromatography

Answer 35

chromatography is a technique used to separate the comonents present within a mixture

substances are separated by making use of differences in their polarity or molecular size.

Question 36

thin-layer chromatography steps

Answer 36

Thin-layer chromatography (TLC) uses a fine film of silica or **aluminium oxide spread over glass, aluminium foil or plastic. **

A **small sample of the mixture being tested is spotted onto the base (pencil) line of the chromatogram. **

A solvent dissolves the compounds in the spot and **carries the compounds up the chromatogram. **

How far the compounds are carried depends on how soluble the compounds are in the chosen solvent and how well they adhere to the plate. (polarity)

A developing agent or ultraviolet light is normally required to visualise the spots on the chromatogram.

Question 37

how to calculate Rf value

Answer 37

Rf = distance travelled by the sample/distance travelled by the solvent

Under the **same conditions **(temperature, solvent, and saturation levels) a compound always has the same Rf value (within experimental error).

Question 38

how to confirm the identity of a compound by TLC

Answer 38

The identity of a compound can be confirmed by:
♦ comparing the experimentally determined Rf values with a literature or known value determined under the same conditions
♦ making a direct comparison on a TLC plate between the compound being tested and the pure substance — a co-spot could be used

Question 39

use of thin-layer chromatography

Answer 39

TLC is used to assess the purity of substances.

A pure substance, when spotted and developed on a TLC plate, should appear as a single spot (some impurities may not be visible by TLC analysis).

The** presence of more than one spot shows that impurities are present.**

Question 40

Techniques for purifying

Answer 40

distillation
recrystallisation

Question 41

techniques for finding quantitative measurements of substances

Answer 41

gravimetric analysis (accurate mass)
volumetric analysis (accurate volumes)
colorimetry (concentration)

Question 42

techniques for identifying substances

Answer 42

distillation
melting point analysis (and comparison with literature values)
mixed melting point analysis
chromatography (comparison to literature Rf values, co-spots)

Question 43

techniques for separating and isolating substances

Answer 43

precipitate conversion (precipitation reaction, then filtration)
volatilisation conversion (heated to evaporate off other stuff)
vacuum filtration
solvent extraction

Question 44

techniques for assessing purity of products

Answer 44

melting point analysis
mixed melting point analysis
thin-layer chromatography