- Problems[[Student version, January 17, 2003]] 255
Problems
7.1Through one’s pores
a. You are making strawberry shortcake. You cut up the strawberries, then sprinkle on some
powdered sugar. A few moments later the strawberries look juicy. What happened? Where did
this water come from?
b. One often hears the phrase “learning by osmosis.” Explain what’s technically wrong with this
phrase, and why “learning by permeation” might describe the desired idea better.
7.2Pfeffer’s experiment
van’tHoff based his theory on the experimental results of W. Pfeffer. Here are some of Pfeffer’s
original data for the pressure Π needed to stop osmotic flow between pure water and a sucrose
solution, across a copper ferrocyanide membrane atT=15◦C:
sugar concentration (g/(100gof water) Π (mmof mercury)
1 535
2 1016
2.74 1518
4 2082
6 3075
a. Convert these data to our units,m−^3 andPa(the molar mass of sucrose is about 342gmole−^1 )
and graph them. Draw some conclusions.
b. Pfeffer also measured the effect of temperature. At a fixed concentration of (1gsucrose)/(100g
water) he found:
temperature (◦C)Π(mmof mercury)
7 505
14 525
22 548
32 544
36 567
Again convert to SI units, graph, and draw conclusions.
7.3Experimental pitfalls
Youare trying to make artificial blood cells. You have managed to get pure lipid bilayers to form
spherical bags of radius 10μm,filled with hemoglobin. The first time you did this, you transferred
the “cells” into pure water and they promptly burst, spilling the contents. Eventually you found
that transferring them to a 1 millimolar salt solution prevents bursting, leaving the “cells” spherical
and full of hemoglobin and water.
a. If 1 millimolar is good then would 2 millimolar would be twice as good? What happens when you
try this?
b. Later you decide that you don’t want salt outside because it makes your solution electrically
conducting. How many moles/liter of glucose should you use instead?
7.4Osmotic estimate of molecular weight
Chapter 5 discussed the use of centrifugation to estimate macromolecular weights, but this method
is not always the most convenient.
a. The osmotic pressure of blood plasma proteins is usually expressed as about 28mmof mercury
(see Appendix A) at body temperature, 303K.The quantity of plasma proteins present has been