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Osmosis Lab Report

Calculating the Percent Concentration of Starch in Two Solutions Through the Use of Osmosis

By

Vaishali Krishnamoorthy

Sowmya Taketapalli

Neha Rao

Jane Wang

AP Biology

Days A, B, D

Mr. Resch

February 16th, 2009

Abstract:

This experiment was performed in order to determine the percent concentrations of starch in two different solutions. It taught us through the method used, which was osmosis (the diffusion of water). Osmosis obviously occurred because there was a change in mass for both the dialysis tubing filled with the unknown solution and the beaker of sucrose and water. This proved that diffusion of water had occurred across the semi-permeable dialysis tubing. This is verified because the weight of the dialysis tubing and the beaker both changed. The results showed us that diffusion occurred differently for the two solutions. For solution A, the dialysis tubing weighed less after osmosis. For solution B, the dialysis tubing weighed more after osmosis. It was concluded that water diffused out of the dialysis tubing/into the beaker for solution A in contrast to the water diffusing out of the beaker/into the dialysis tubing for solution B.

Introduction:

This experiment was performed to demonstrate the process of osmosis and to show visible as well as quantitative evidence proving that osmosis occurred. Through the tasks of determining the percent concentrations in two different solutions, we were studying the process of osmosis. Osmosis is the best way to perform this experiment because as we went through the experiment, the weight of the beaker/dialysis tubing changed and the only logical explanation was that diffusion of water had occurred. Osmosis is the diffusion of water. Depending on which was heavier (the beaker or the dialysis tubing) after the experiment was performed, the direction of water diffusion was apparent. If the beaker was heavier, then that implies that the water diffused from the dialysis tubing to the beaker. In contrast to this process, if the dialysis tubing was heavier after the experiment, then the water would have diffused from the beaker to the dialysis tubing. We saw both these processes of osmosis occur in our experiments because we had two different solutions.

According to the seventh edition of Campbell/Reece’s Biology textbook,  Osmosis is, “the diffusion of water across a selectively permeable membrane”(glossary Campbell). Because diffusion is the fundamental process, the characteristics of diffusion apply to osmosis. Diffusion has to do with shifting particles until different sides have a similar number of particles. A major factor is difference in concentration. Diffusion and osmosis occur because the concentrations should somewhat equal out (Sheppard 1). The main difference is that osmosis is the diffusion of water. Water molecules often diffuse across cell membranes, so the process is important. However, the amount of water in the cell does not usually change because there is basically an equal amount of water entering and exiting the cell (Bowen 1). Because osmosis occurs across a “selectively permeable membrane”, the definition of this term must be acknowledged. As stated in an article regarding osmosis, “A selectively permeable membrane is one that allows unrestricted passage of water, but not solute molecules or ions”(Bowen 1). It is widely apparent that osmosis is an extremely significant process to be studied and was the best choice for our experiment, which provided us with logical results through the study of osmosis process.

Materials and Methods:

Our lab procedure. Materials we used include dialysis bag, water, sucrose, and beakers

1. We filled the dialysis bag with approximately 10 grams of unknown solution.

2. We filled the beaker with about 200 grams of water and 2 grams of sucrose.

3. Then we completely submerged the dialysis bag into the beaker with solution and it remained in there for twenty minutes.

4. Next we removed the dialysis bag from the beaker, dried it off, and measured its mass.

5. We used the mass of the dialysis bag that was obtained, and determined the grams of sucrose and water present in the beaker. We did this by calculating the concentration of the unknown solution, and set up a proportion whereby x was equivalent to the grams of sucrose originally in the dialysis bag.

Set the equation as: X/(weight of solution in bag after waiting) = 2/(grams of solution in beaker after waiting)The original percent concentration is x/10.

RESULTS:

Table 1: Before and after weights of water, starch, and solution both inside the dialysis bag and in the beaker for unknown solution A.

 Mass of Starch in Beaker (g) Mass of Solution in Beaker (g) Mass of Solution in Dialysis Tubing (g) Before Osmosis 2.00 200.02 10.19 After Osmosis 2.00 200.84 9.37

Table 2: Before and after weights of water, starch, and solution both inside the dialysis bag and in the beaker for unknown solution B.

 Mass of Starch in Beaker (g) Mass of Solution in Beaker (g) Mass of Solution in Dialysis Tubing (g) Before Osmosis 2.00 201.23 12.63 After Osmosis 2.00 200.84 13.02

After determining the mass of starch and water inside the beaker and the mass of unknown solution inside the dialysis tubing both before and after osmosis, we can perform some data analysis on the results: Because we know that only water can diffuse through the membrane, the gain in mass of solution inside the dialysis tubing is exactly equal to the loss of water from the beaker. In addition, the gain is mass is due solely to the osmosis of water through the membrane and the concentrations of solution both inside and outside the dialysis tubing should be equal. Based on this, we can set up the following proportion:

X                         =                              2.00 g                    .

mass of unknown after osmosis          mass of solution in beaker after osmosis

Solving these proportions for both unknown solutions A and B, we can calculate the percent compositions of start in each solution:

 Solution A XA/9.37g = 2.00g/200.84g XA = .0933 g % CompA = .0933g/10.19g * 100% = .916 % ≈ 0.9%                 (rounded to the nearest 10th of a percent) Solution B XA/13.02g = 2.00g/200.84g XA = .130 g % CompA = .130g/12.63g * 100% =1.03 % ≈ 1.0%                 (rounded to the nearest 10th of a percent)

Thus, based on the before and after masses of solutions A (table A) and B (table B), we have that solution A originally had a percent composition by mass of 1.0% and that solution B originally had a percent composition of 0.9%. However, these results may be inaccurate due to many sources of error that will later be discussed.

DISCUSSION:

The experiment was designed to enable us to calculate the percent concentration of starch in solutions A and B.  Using a procedure involving osmosis occurring between the unknown substances and a solution of known concentration, we were able to calculate the percent composition of starch in solutions A and B.  As the results demonstrate, we determined that A originally had a percent composition by mass of 1.0% and that solution B originally had a percent composition of 0.9%.

The intent of the experiment was to study osmosis.  To that effect, the experiment was successful.  Osmosis occurred—there was a change in mass of both the dialysis tubing filled with a solution of unknown concentration and the beaker of sucrose and water, for both solutions A and B.  This indicates that water diffused across the semi-permeable dialysis tubing, changing the weight of both the beaker and the tubing.

When solution A was being tested, it was observed that the dialysis tubing filled with solution A weighed less after osmosis than it did before osmosis.  As the dialysis tubing is semi-permeable and allows only water through, we can conclude that water diffused out of the tubing and into the beaker.  This indicates that solution A had a lower concentration of starch than that of the solution in the beaker—however, this was not supported by our calculations.

When solution B was being tested, the dialysis tubing filled with solution B weighed more after osmosis than it did before osmosis.  It can be concluded, therefore, that water diffused into the tubing and out of the beaker.  This indicates that solution B had a greater concentration of starch than that of the solution in the beaker.  This was supported by the calculations made.

However, there were possible factors for error in this experiment.  The fact that the procedure was nonstandard may have had some impact; the amount of time that was needed for osmosis to occur fully was unknown.  The tubing may not have been submerged in the beaker for long enough to reach equilibrium.  Additionally, the initial mass of the unknown solution in the dialysis tubing did not conform to the stated procedure: as we were unable to pipette in the desired amount of solution, we simply filled the tubing, tied it shut, and then took its mass.  Though we presumed that this would not affect the results, as long as the concentration of the solution in the beaker was lower than that in the tubing, it is possible that this may have had a negative effect on our experiment.

Conclusion:

The word osmosis refers to the diffusion of water across a selectively permeable membrane in order to evenly distribute concentration levels on both sides of the membrane. In this lab, we have used specific materials and methods to present this process. Our hypothesis, in which we predicted that by figuring out the mass differences we would be able to figure out the percent composition of the starch in both the solutions, was correctly proven. We stimulated our process and were able to correctly form a reasonable conclusion and percent composition. Our process used the aspect of osmosis and diffusion and provided us with a great lab.

Works Cited

Bowen, R. (2000, July 2). Osmosis. Retrieved February 14, 2009, from

http://www.vivo.colostate.edu/hbooks/cmb/cells/pmemb/osmosis.html

Campbell, N. A., & Reece, J. B. (2005). Biology (7th ed.). New York: Pearson Education

Inc.

Sheppard, T. (2004). Diffusion and Osmosis. Retrieved February 14, 2009, from

This web page was produced as an assignment for an AP Biology course at Montgomery High School.

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Jane Wang,
Feb 16, 2009, 7:02 PM