Expert: Dana Krempels, Ph.D. - 9/16/2009

Question
QUESTION: Hey,

I did an experiment on Osmosis/Diffusion for my course and although I understand osmosis and diffusion I still couldn't figure this out.

I had 2 thistles with a semi-permeable membranes attached on the top (see pic link below) put into one 3 Litre (101 Oz) beaker with 15mL (0.5oz) of glucose solution inside each thistle.

no. 1 = 20 % concentrated
no. 2 = 40 % concentrated

The results were (after 24hrs) no.1 contained 37mL liquid and no.2 contained 36mL liquid.


Why isn't there a significant difference in the results? I thought the 40% would gain more to reach an equilibrium? If so would it be the same if I had 91% glucose solution in another thistle?

Your help would be GREATLY appreciated...

Regards,
Phil.




- Thistle pic
http://upload.wikimedia.org/wikipedia/commons/2/21/ThistleTube.jpg

ANSWER: Dear Phil,

Yes, you might expect to have more dramatic results, but it's possible that the difference in concentration wasn't sufficient to produce a significantly different result.  I would suggest you replicate the experiment to rule out manipulation errror, and then try with solutions that have a greater difference in concentration.  It could just be that you'd need more time in the beakers to show a significant difference between the osmosis rates in solutions that are only 20% different in concentration.

Hope this helps.

Dana

---------- FOLLOW-UP ----------

QUESTION: Thank you did help a lot, just a quick follow up.

So you would it be expected that the 40% would have gained more water, if so why is this the case?

Thanks again.

Answer
Dear Phil,

Now you know I don't do homework.  

But you should know that the water potential is lower in the 40% solution (i.e., there are fewer water molecules in that solution per unit volume than in the 20% solution).  

Using what you know about movement of water molecules along a potential gradient, you should now be able to answer this question.

Dana