Chigozie's Water Potential and Molarity Mixup

Chigozie Amonu

Mr. Hammer

AP Biology

21 November 2014

Water Potential & Molarity Mixup

            Argument One

For this lab, my lab group and I had to determine the unknown molarities of the six unlabeled solutions. The molarities of each different colored solution could be 0.0M, 0.2M, 0.4M, 0.6M, 0.8M, or 1.0M. After the conduction of experiments and compilation and analysis of data from Activity One and Two, we determined that the order of molarity of the solutions, from least to greatest (0.0M to 1.0M), was orange, dark green, light green, blue, yellow, and red. To get these results, two separate experiments were done. One was done with dialysis tubing bags, and the other was done using two vegetables, squash and parsnip. My lab group and I analyzed all of the data that we got from both experiments. We eventually decided to use the data from the experiments done with squash and parsnip because these two results proved this claim while the data from the dialysis tubing did not. Simply, the majority of our evidence supported this claim.

For the vegetable experiment, my lab group and I cut six pieces from each vegetable and weighed each piece. We then measured out 75 mL of solution to put the vegetables in. The six pieces of each vegetable were put into the six different solutions, and the cup and beakers that they were in were covered with foil and left to sit overnight. The next day in class, my lab group and I weighed the vegetables and recorded the weight in data tables. The percent change for each piece of vegetable was calculated (percent change= [final mass minus initial mass] divided by [initial mass] times 100). For the parsnip, the percent changes were -25.32% for red, 47.62% for orange, -20.96% for yellow, 10.64% for light green, 32.09% for dark green, and -13.24% for blue. For the squash, the percent changes were -45.68% for red, -6.94% for orange, -43.04% for yellow, -4.55% for light green, 20.37% for dark green, and -31.43% for blue.

Parsnip Data

Color of Solution in Which Parsnip was Placed	Initial Weight of Parsnip	Weight of Parsnip After Overnight Stay	Percent Change
Red	15.8 g	11.8 g	-25.32%
Orange	12.6 g	13.2 g	47.62%
Yellow	16.7 g	13.2 g	-20.96%
Light Green	14.1 g	15.6 g	10.64%
Dark Green	13.4 g	17.7 g	32.09%
Blue	13.6 g	11.8 g	-13.24%

Squash Data

Color of Solution in Which Squash was Placed	Initial Weight of Squash	Weight of Squash After Overnight Stay	Percent Change
Red	8.1 g	4.4 g	-45.68%
Orange	7.2 g	6.7 g	-6.94%
Yellow	7.9 g	4.5 g	-43.04%
Light Green	8.8 g	8.4 g	-4.55%
Dark Green	5.4 g	6.5 g	20.37%
Blue	7.0 g	4.8 g	-31.43%

We determined that a negative percent change indicated the shrinking of the cell. This would mean that the cell was in a hypertonic solution that had a high concentration of water in the cell and a low concentration of water outside of the cell. The water would move across the concentration gradient from high concentration to low concentration through osmosis, a type of diffusion. This information led us to believe that the most negative percent change of the vegetable equaled the highest molarity of the solution in which it is placed since there would be less water and more solute, in this case sucrose, outside of the cell (in the solution). On the other hand, a positive percent change would indicate the swelling of a cell. This would mean that the cell was in a hypotonic solution that had a low concentration of water in the cell and a high concentration of water outside of the cell. The water would move into the cell through osmosis, causing it to swell, and proving that there was a less solute concentration and higher water concentration in the solution. The most positive percent change in the vegetable equaled the lowest molarity of the solution in which the vegetable is placed.

My lab group and I then arranged these percent changes from greatest to least. For the parsnip, the order was orange, dark green, light green, blue, yellow, and red. For the squash, the order was dark green, light green, orange, blue, yellow, and red. We compared these orders and found that the blue, red, and yellow were 0.6M, 0.8M, and 1.0M, respectively, for each vegetable. The dark green, light green, and orange were in different orders on each greatest to least list. For the squash, the order was dark green, light green, orange. For the parsnip, the order was orange, dark green, light green. We decided to use the parsnip order because it seemed more logical to move one color (orange) to the top since the dark and light green were ordered consecutively on each list and would move up and down the final list as one entity. After analyzing all of our data, we concluded* that the order of molarities is orange (0.0 M), dark green (0.2M), light green (0.4M), blue (0.6M), yellow (0.8M), and red (1.0M).

As I mentioned before, we did not use the data from the dialysis tubing to support our claim. There was more room for error in this experiment. The bags of solution may not have been properly tied and could have leaked, causing a discrepancy in the dialysis tubing bag’s final mass. Also, we did not use the same scale to weigh the bags on the second day of the lab. We also concluded to not use the data from this experiment because the evidence did not support our claim. The percent changes were -6.25% for red, 19.94% for orange, 9.94% for yellow, 0.97% for light green, 0.68% for dark green, and 30.37% for blue. For this experiment, a more negative percent change would represent the shrinking of the cell, and therefore a hypertonic solution and a lower molarity. A more positive percent change would represent the swelling of the cell and therefore a hypotonic solution and a higher molarity. Based on this information, the order of molarity of the solutions would be red (0.0M), dark green (0.2M), light green (0.4M), yellow (0.6M), orange (0.8M), and blue (1.0M). All in all, this data did not support our original claim, and we determined the squash and parsnip experiment to be more accurate.

Dialysis Tubing Data

Color of Solution in Bag	Initial Weight of a Bag	Weight After One Day	Percent Change
Red	32.0 g	30.0 g	-6.25%
Orange	32.1 g	38.5 g	19.94%
Yellow	34.2 g	37.6 g	9.94%
Light Green	30.9 g	31.2 g	0.97%
Dark Green	29.5 g	29.7 g	0.68%
Blue	32.6 g	42.5 g	30.37%

Argument Two

            The water potential for parsnip is -12.29 bars, and the water potential for squash is -3.69 bars. To find this, my lab group and I had to first determine the solute and pressure potential. To calculate the solute potential, we used the formula Ѱs= -iCRT (solute potential equals the number of particles the molecule will make in water times molar concentration times pressure constant times temperature in degrees Kelvin). We had to find C, the molar concentration. To do this, we had to graph the percent changes of each vegetable from Activity Two (squash and parsnip) compared to the molarity of each solution. The x-axis is molarity, and the y-axis is percent change. The molar concentration ended up being where the graph of the percent changes hit the x-axis. These molar concentrations were 0.5M for the parsnip and 0.15M for the squash. We used room temperature (23ºC) as the temperature for the solute potential and converted it to Kelvin (273+23ºC). We then plugged these values into Ѱs= -iCRT. The equation for parsnip was –(1)(0.5)(0.0831)(296) and it equaled -12.29 bars, the solute potential. The equation for squash was –(1)(0.15)(0.0831)(296) and it equaled -3.69 bars, the solute potential. We then determined that the pressure potential was zero, and used the equation Ѱ= Ѱs+Ѱp (water potential= solute potential plus pressure potential) to calculate the water potential. The equation for parsnip would be Ѱ= -12.29+0, and would equal -12.29 bars, the water potential. The equation for squash would be Ѱ= -3.69+0, and would equal -3.69 bars, the water potential. The squash has a higher water potential than the parsnip. Because of this, water can move more freely in and out of the squash cells during osmosis.

Title: Percent Changes of Parsnip in Comparison to Molarities of Solution; x-axis represents the molarity, and the y-axis represents the percent change

Title: Percent Changes of Squash in Comparison to Molarities of Solution; x-axis represents the molarity, and the y-axis represents the percent change