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Calculating Rf Values in Chromatography
Background Information and Keywords
Background Information and Keywords
Keywords
Keywords
- Chromatography - A separation technique used to work out what the different components are in a mixture
- Chromatogram - A finished chromatography when the mixtures have separated and formed dots on the paper
- Stationary Phase - The substance the sample is placed on. It doesn't move. - This is the paper in paper chromatography
- Mobile Phase - The substance which travels along the stationary phase and carries the sample. It does move. - This is the solvent in paper chromatography
- Solvent - A substance which the solute/sample is soluble in
- Soluble - When a substance can be dissolved in the solvent
- Insoluble - When a substance cannot be dissolved in the solvent
- Rf Value - A value describing how attracted to the mobile or stationary phase a chemical is (i.e. how high up the paper it travels) An Rf value is always between 0 and 1.
Background Information
Background Information
This practical is all about the ideas and use of chromatography.
Chromatography is a seperation technique used to help us identify what the different substances are in a mixture.
In GCSE we only worry about paper chromatography which is set up a bit like this:
Paper chromatography is simple and cheap and can be done quickly to help work out what a sample is made of. Is it pure or a mixture? Is it made of two chemicals or three? What are these chemicals?
Samples in chromatography will travel different distances up the chromatography paper depending on what solvent is used (e.g. if we use water compared to alcohol)
We can calculate Rf values for samples which can be compared to databases to work out what the sample is.
Other things you need to be able to do
Other things you need to be able to do
To do the practical
To do the practical
Be able to follow written instructions carefully
Be able to set up a practical from a diagram
Be able to measure distances with a ruler accurately
To handle the data and make conclusions
To handle the data and make conclusions
To explain why different samples travel different distances up the paper
To be able to round answers to a suitable number of significant figures
To know the reason why each step is done when setting up a chromatogram
"How To Do It" Video
"How To Do It" Video
Watch this video to remind yourself how this practical is carried out.
What are the key steps to this practical?
What are the pieces of equipment called?
Why are these pieces of equipment used?
Diagrams
Diagrams
Key Steps
Key Steps
Draw a pencil line 1-2cm above the bottom of the chromatography paper with a ruler
Place small dots of your samples and unknown on the pencil line
Place the chromatography paper on a support and lower into a beaker with solvent in it. DO NOT let the samples touch the solvent
Allow the solvent to travel up the paper
The samples will move up the paper with the solvent
When the solvent nears the top of the paper, take the paper out of the solvent
Mark the highest point the solvent traveled. This is the solvent front
Compare your samples to the unknown for similarities and differences and calculate Rf values
Key Questions from this Practical
Key Questions from this Practical
Why must the start line be drawn in pencil, not pen?
What must the dots of sample not be allowed to be beneath the level of the solvent?
Why might a lid sometimes be needed for the beaker during chromatography?
How would you know from a chromatogram that a sample is pure substance?
How would you know from a chromatogram that a sample is a mixture of substances?
How would you be able to work out what the unknown sample is made from using the chromatogram?
How would you work out an Rf value for a spot?
What numbers must an Rf value always be between?
If a sample travels high up the paper, how attracted is it to the mobile/stationary phase?
If a sample doesn't travel far up the paper, how attracted is it to the mobile/stationary phase?
Question Practice and Exam Questions
Question Practice and Exam Questions
Required Practicals Mastery Booklet - PAG 6 - Chromatography.pdfPAG 6: Mastery Question Booklet
PAG 6: Mastery Question Booklet
This Mastery Booklet is filled with questions and exam questions to help you master this core practical. All answers and mark schemes are attached at the end.
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