The following lab is based on a lab created by the Texas A&M University Physiology and Pharmacology Department. The lab was originally designed for osmolarity, however it lends itself rather well to molarity. The results were measured by submitting a sample to the lab at the Vet School. I do think that if you really wanted to, you could do a simple titration to determine concentration. I also think that you could utilize potentiometry if you had the equipment. Aside from that, the lab is a whole heck of a lotta fun! (although it is rather amazing when you find out just how good your solution can be.)
The students are given the following scenario:
Imagine that you are lost at sea and your friend is severely dehydrated. You just happen to know that you can prepare a salt solution to hydrate your friend. (teacher: you want to approximate physiologic saline that a hospital might give. Such a solution is isotonic with most body fluids like plasma, interstitial fluid, and intracellular fluid.) You remember that the appropriate solution should be approximately .15 molar (or 300 mOsm/L). Taking inventory of your meager possessions you find the following items:
some NaCl (salt packets or a container) Tongue Depressor 8" string 2 12 cc syringes (without needles) Plastic Spoon approximately 1.2 L distilled water 1 L flask some wooden sticks useful sheet containing important data (maybe include a periodic table to get them to really think!)
Using only these items and your smarts, prepare a solution that could save your friend's life.
Meaningful and Interesting Tie-ins:
1. Remind your students what isotonic, hypotonic, and hypertonic solutions are. 2. Remind them of the good Îol selectively permeable membrane (the phospholipid bilayer) and how the hydrophobic tails are responsible for making the ion gradient possible. Maintaining this ion gradient is essential to life. 3. Remind them about how water moves from higher concentrations to lower concentrations. It is essential to hydrate without disturbing the ion concentrations inside and outside of the cell. (Remember the vital sodium/ potassium pump?) 4. Include examples of what might happen when the ion gradient is disturbed (cells can shrink or explode both of which render the cell useless). You might even point out that disturbing this gradient in the digestive tract can lead to a mass exodus of water into the intestines (and you know exactly what that means!).
The following is the solution that I came up with:
1. I recalled that 1 cc water = 1 mL of water = 1 g of water. (I realize the students will not know this) 2. I did simple calculation to determine molecular weight of NaCl, the number of moles in .15 M, and the amount of NaCl in grams to make .15 M NaCl. 3. I filled up the 1 L flask almost to the line with distilled water. 4. I fashioned a scale using one syringe with approximately 8.8 mL of distilled water and used the other syringe to fill with the salt. Of course it is imperative to leave the caps on the tips of the syringes. I used the string to tie the syringes together over the tongue depressor. 5. When the "scale" was as close to balanced as I could get, I dumped the salt into the solution and carefully mixed it. I then used the water syringe to fill up to the 1 L line.
When I turned this in for analysis, I was very much skeptical of the accuracy. I was pleased when the lab reported back 299 mOsm/L (pretty darn close to the target 300!) Therefore, I am pleased with the precision and accuracy that this lab can accomplish.
I know that this will have to modified even more for high school chemistry. This lab should also be practical in Biology. I think it can be simplified or made difficult depending on the level that you teach. Please let me know how this lab turns out. I am a pre-service teacher and I will be student teaching shortly. I plan to use this lab myself if it goes smoothly in your classrooms. I don't have the practical classroom experience that you have so any and all feedback is desired!
Please contact me if you have any questions or need some more information.
Questions? Comments??
Laura Aaron