RESEARCHING CHEMISTRY Flashcards
what is stoichiometry
Stoichiometry is the** study of mole relationships** involved in chemical reactions.
percentage by mass
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%
Percentage by volume
Percentage by volume is the number of cm3 of solute made up to 100 cm3 of solution.
ppm meaning
The unit ppm stands for **parts per million **and refers to 1 mg per kg or 1 mg per litre
10^6
how can percentage yield be reduced
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
gravimetric analysis - precipitation conversion
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
gravimetric analysis - volatilisation conversion
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
what is gravimetric analysis
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.
weighing by difference
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. **
weighing accurately approximately
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.
heating to constant mass
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
volumetric analysis skills
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
standard solution definition
A solution of accurately known concentration is known as a standard solution.
preparation of a standard solution
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
requirements for primary standards
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
examples of primary standards
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
why sodium hydroxide isnt a primary standard
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.
what is EDTA
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)
when is a back titration useful
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
colorimetry techniques
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.
use of distillation
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
heating under reflux
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
vacuum filtration
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.
use of recrystallisation
technique of recrystallisation to purify an
impure solid
