chemistry - chromotography Flashcards
(10 cards)
what is an Rf value?
An Rf value is the ratio between the distance travelled by the dissolved substance (the solute) and the distance travelled by the solvent
how can you use chromatography to see if a certain substance is present within a mixture?
- run a pure sample of the substance alongside the unknown mixture
- if the Rf value of the pure substance is the same as the Rf values of one of the spots in the mixture, it is likely to be present
how to calculate an Rf value?
Rf value = distance travelled by substance / distance travelled by solvent
how could you use paper chromatography to separate a mixture of food colourings?
- draw a pencil (horizontal) line 2cm from the bottom of the paper
- mark 4 pencil mark on the line
- on each mark, place one dot of 3 known food colourings and one dot of an unknown mixture, label each dot
- place the paper in a beaker containing 1cm of a solvent (like water)
- wait for the solvent to travel up most of the paper
- remove from the beaker and draw a line where the solvent reached
- hang the paper to dry
- calculate each Rf value and compare
why is pencil used to draw the line?
it will not affect the experiment as it is insoluble in the solvent
why should the solvent be no deeper than 1cm?
if it is any deeper it will touch the substances on the paper, causing them to dissolve before moving up the paper
what is the mobile and stationary phase?
mobile phase - the solvent
stationary phase - the chromatography paper
what two factors affect how long molecules spend in the mobile and stationary phase?
- how soluble they are
- how much they are attracted to the chromatography paper
what is the Rf value dependent on?
- it is dependent on the solvent
- if you repeat the experiment using different solvents, the Rf value will change
why should you use a lid on the beaker when carrying out the experiment?
to prevent the solvent evaporating
