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Osmosis Across Dialysis Tubing

Investigating osmotic pressure and osmosis, we found a linear correlation between sucrose concentration in a water solution and osmotic pressure (0.71 bars per percent) as well as a linear correlation between percent mass gained through osmosis and concentration (2.70e-4 percent per percent by mass and 5.13e-4). Our procedure remained relatively error free and we were able to determine the second unknown solution as 6.7% (0.5%) and the first one as 4.8% (1.9%).


We hypothesized that the higher concentration sucrose bags would expand more than the lower concentration bags. The lab's purpose includes learning lab technique, investigating osmotic pressure, and observing "hands on" osmosis. The formula for osmotic pressure, similar in many respects to the ideal gas law, was give as P = -iCRT, where i is the ionization constant, C is the molarity, R is the ideal gas constant, and T is the ambient temperature. Osmosis, or the diffusion of water, is the tendency of substances in fluidic media to travel from higher concentrations to lower concentrations.

Materials and Methods

Our procedure was crafted to allow compensation for error as well as identify any anomalous readings. By making each solution (5%, 10%, and 20%) from the solid solute and water, we changed a systematic potential for error into a random one. Each cup was massed to create a baseline for future measurements. Water was added to the cup and the system was then remassed. Dialysis tubing was massed by placing it in another cup, one with a known mass and a known quantity of water. Pipettes transferred 5.0 mL (0.05mL) solution to the dialysis tubing. After a little more than 24 hours, the system of bag, cup, and water, was massed, the bag massed, the cup with the bag massed, and the volume of the solution determined. All cups were labeled with letters instead of percentages so that experimental biases would play less of a role during the execution of the procedure.

Due to a lab setup error, our first unknown sample is not reliable, but has been included in the analysis for comparison. Another unknown sample was provided accompanied by only basic measurements. Averaging values such as cup mass and bag mass filled in the gaps, but the computations induce some uncertainty about the results from that bag.

The knots of the bags are not known to be of any specific permeable. The ends of the bags were originally kept out of the solution but in the process of handling, the ends slipped in, potentially allowing some of the sucrose to leak out. This could have been tested with a sucrose indicator.


Results are found in the Appendix. All gray table squares in the sample were derived from other readings. The sample letters are out of order to allow for easier plotting by the computer. This is particularly true of the second unknown sample. The "bag" calculation subtracted the "h2o w/bag" from the "h2o w/o bag." The "h2o" signifies the use of a container with some water which allows the bag to stay wet while being massed. Day 1 solution calculations come from the difference between the mass of the bag with solution and the mass of the bag calculated earlier. Day 1 water calculation is the difference between the cup with the water and the cup alone. Day 2 calculations were similarly done.

The change in the amount of solution when correlated with the concentration seems fairly linear. The y-intercept of 35.7% change in mass suggests that there is some random exchange of fluids or contaminants -- on the order of 5% concentration in the bag -- systematically throughout the experiment. The volumetric best fit line suggests a -3.2% change in volume. This disparity can be corrected by simply taking more data. Four points is insufficient to determine if the relationship is linear, exponential or logrithmatic (even though it seems obviously linear, we should not be biased). The lack of data on the second unknown bag is disturbing. The experiment's control can no longer guarantee that the results are accurate because of the timing differences.

The unknown solutions (2nd: 6.7% 0.5% 1st: 4.8% 1.9%) were determined by a linear fit with a constant term. Error margins were derived from the differences between the volumetric and mass best fit lines. The lines fit very well (.996 and .976 correlation coefficients).


Further investigation into the change of osmotic pressure with depth and the speed at which osmosis occurs would give better insight into how fast cells explode and what role osmosis plays with biochemical processes of deep sea animals.

  1. See Appendix
  2. See Appendix
  3. See Appendix
    This graph is the same except for scaling and a flip along the x-axis. Comparing slopes makes absolutely no sense whatsoever because of the disparity between the units.
  4. Because the osmotic pressure difference would be reduced, the membrane bags would not expand. The bags (5 and 10%) with lower molarity solutions would lose water to the outer 0.4M solution. Only the 20% solution would gain marginal amounts of water.
  5. Osmosis and diffusion are important for distributing nutrients and water in many biological systems. They are important for oxygen distribution, hormone distribution, and waste gas removal. In the roots of trees, a semi-permeable membrane facilitates osmosis sustaining the tree with water for such operations as the Kreb Cycle. In the human body, negative feedback regulation relies upon diffusion of hormones to control process such as body temperature and reproductive capabilities.
  6. A protozoan is often in a hypertonic environment. Osmosis makes water pour into the critter and the cell will explode unless its contractile vacuole is working, pumping water out of the cell.
  7. The water potential of dehydrated potato is much higher than that of a hydrated one because the potato cells are utilizing little of their potential to hold water. The concentration of the aqueous compounds is higher in the dehydrated one; thus its ability to hold water has increased.
  8. The lab went pretty well, although adding better evaporation protections may have helped reduce error. The lab could benefit from avoidance of a late second unknown sample. In future, use of a sucrose indicator to determine the presence of bag leaks might prevent egregious errors. The y-intercept best fit line and our control indicates some osmosis occurring in a "water-water" setup. This should not happen and probably indicates systematic contamination. Future procedures should wash the bags, cups, and other apparatus to reduce possibilities of contamination. Additional samples should get us a better best fit line.