How can the results shown below the procedures be explained? Liquid in Solid. Get a petri dish containing 'gulaman' gel. Use a Pasteur pipet and fill the four holes with equal drops of these...

How can the results shown below the procedures be explained?


Liquid in Solid. Get a petri dish containing 'gulaman' gel. Use a Pasteur pipet and fill the four holes with equal drops of these two compounds of varying molecular weights (MWs): 1 M (molar) methylene blue (MW = 374 g/mole) and 1 M potassium permanganate (KMnO4) (MW = 158 g/mole). Observe how the liquid solutions of three compounds diffuse outward from the holes. [N.B.: Once you have filled the holes, do not transfer the plates to another place. This might cause spill - over of the liquid solutions.] Record your observations. Measure the diameters (in mm) of the zones of diffusion using a mm ruler.

Results:
Diffusion of different compounds (of varying molecular weights) in the 'gulaman' gel.

1) Compound: 1 M methylene blue
Molecular Weight (g/mole): 374
Diameter of diffusion zone (in mm): 8 mm
2) Compound: 1 M KMnO4
Molecular Weight (g/mole): 158
Diameter of diffusion zone (in mm): 10 mm

Expert Answers
ndnordic eNotes educator| Certified Educator

There are a variety of factors involved in the diffusion of these two compounds through the gel.  First you have the difference in molecular weight, with the larger molecule moving more slowly through the gel. Based on that you might conclude that the larger the molecule the slower it moves.  However,  secondly, you are comparing an organic molecule (methylene blue) to an inorganic molecule. Since "like dissolves like" generally and the gel is an organic, sugar based polymer the similarity between the methylene blue and the gel may have helped the methylene blue move relatively faster than if you were comparing two similar molecules where just the molecular weight differed.  Finally, there is a big difference in the ionic character of the two compounds.  Methylene blue is primarily a covalently bonded compound with one quarternary ammonium group which gives it some polarity. The potassium permanganate is largely an ionic compound.  This big difference in ionic character will also affect how the two molecules move through the gel.

In summary, if you are trying to demonstrate the effect of molecular weight on diffusion rate you need to have two molecules that are more similar in type and degree of ionic character.