Here are some of the answers to our lab exercise:
1. What happened to the erythrocytes placed in:
a. 0.9% Nacl? No change.
Why? Equal concentration of solvent on both sides of the semipermeable membrane.
b. Distilled water? Swell and burst.
Why? Concentration of water is higher outside the RBC than on the inside.
c. 3% NaCl? Shrivel.
Why? Since the water in the NaCl solution is lower then that of the red blood cells it will move out of the red blood cells via osmosis.
d. Benzene? Lysis of RBC.
Why? RBCs have a phospholipid membrane like other cells of the body. Lipids are soluble in alcohols like benzene, so the membrane falls apart.
e. 2% Urea?
0.033mol per 250mL of water, is the same as 0.033 x 4 = 0.133 mol per litre of water.
So the molality is 0.133 M.
'milli' is 10^-3. For example, a millimeter is 1/1000 (which is 10^-3) of a meter. So the millimolality of the solution will be 133 mM. Are you following?
osmolality is slightly more complex. It takes into account that a molecule may dissociate (split apart) in solution.
For the first solution, the milliosmolality is simply the millimolality (133mM) multiplied by 2 (since NaCl dissociates into 2 ions in solution - Na+ and Cl-).
Glucose however does not dissociate, so in the case of solution 2 the millimolality and milliosmolality will be the same.