- Created by: Katie
- Created on: 04-04-11 20:14
Qualitative Analysis tells you what a sample contains
It doesn't tell you how much of each - that's Quantitive analysis
Quantitive Analysis tells you how much it contains
Used to work out the molecular formula. If you had a sample containing carbon and hydrogen you'd no it's a hydrocarbon but without quantitive, you won't know if it's methane, butane etc.
Chemical Analysis is carried out on samples
Difficult to test all of the material -test a small bit
If something goes wrong, you can go back for another sample and try again
Must represent the bulk of the material bein tested
Samples are analyised in solution
Solution - dissolving the sample in a solvent - aqueous and non-aqueous
An aqueous solution = solvent = water. Non aqueous = Not water, eg; ethanol
Standard Procedures mean everyone does thngs the same way
Instructions, safest effective ways, most accurate way.
Analysis - Chromatogrpahy
Chromatography uses two phases, it's an analysis method used to separate the substances in a mixture - identify individual substances
Mobile Phase - Molecules can move, always a liquid or gas,Stationary Phase - Molecules can't move, always a solid or thick liquid - How quickly the chemical moves depends on how it ditributes itself between the two phases - different chemicals seperate and end at different points.
The molecules constantly move between the mobile and stationary phases
- said to reach a dynamic equilibrium In paper chromatography - the stationary phase = paper A spot of the substance is put on the baseline of the paper
The bottom of the paper is placed in a beaker containing a solvent eg water, this is then the mobile phase. The solvent moves up the paper, the chemicals dissolve int the solvent and move between it and the paper - sets up a equilibrium between solvent and paper Before it reaches the top, the paper is removed from the beaker. Different samples form serparate spots on the paper, chemicals which spend more time in the mobile phase are further up the paper
Depends on - How soluble they are, how attracted they are
Thin Layer chromatography - stationary = thin layer of solid - silica gel spread onto a glass plate, mobile phase - the solvent such as ethanol
Analysis - Chromatography 2
The result of chromatography analysis is called a chromatogram, some of the spots on here may be colourless, so you use a locating agent to show them.
R = distance travelled by substance divided by the distance travelled by solvent
R, value is the ratio between the distance travlelled by the disolved substance and distance. Chromatography is usually carried out to see a certain substance is present - run a pure, known sample alongside the unknown substance, which may be the same. Chemists use substances called 'standard referance materials' to check the identities, these have carefully controlled concentrations and purities.
Gas Chromatography = more high tech
Used to analyse unknown substances too, if they're not gases, they have to be vapourised. Mobile phase - unreactive gas such as nitrogen. Stationary Phase - a viscious (thick) liquid - oil
Unknown mixture is injected into a long tube coated with the stationary phase
Mixture moves along tube with the mobile phase until it comes out the other end
Time taken for a chemical to travel through the tube = retention time = different to each chemical. The chromatogram is a graph each peak = a different chemical
Along the x axis = retention time. Area under the peak = how much of the chemical was used
Analysis - Solution Concentrations
Concentration = Mass divided by Volume
The concentration is measured in grams per decimetre cubed
EXAMPLE: 25g copper sulfate is dissolved in 500cm cubed of water. What's the concentration in g/dm cubed?
25g divided by 0.5 dm cubed = 50g/dm cubed
(convert the volume by dividing by 1000)
A standard solution has a known concentration
This needs careful measuring and maths;
EXAMPLE: Make 250cm cubed of a 314g/dm cubed solution of sodium chloride
Mass = concentration x volume (314g/dm cubed x 0.25dm cubed = 78.5g)
Remember to convert cm into dm by dividing by 1000
- Weigh out the mass of the solute - weigh beaker first and make note of weight
- Add a small amount of distilled water to the beaker and stir until dissolved
- Tip it into a volumetric flask using a funnel
- Rinse beaker and stirring rod with distilled water and add to flask
- Top the flask up with the correct volume (250cm cubed) with distilled water
- Stopper the bottle and make sure it's all mixed.
Analysis - Titration
You need several consistent readings
You record the volume added of acid or alkali added from a burette to neutralise the alkali or acid.
The first titration should be the 'rough' one
Repeat carefully a few times making sure you're within 0.2cm cubed of the same answer
A mean value can then be calculated from repeats, but ignore anomalous results