In this exercise you will be calculating the osmolarity of a carrot or a potato, which ever one you might have sitting around the house. You will need one potato or one carrot. It must be fresh, not one of those rubbery things that you find in the back of your refrigerator when you clean it out. You will be making seven dilutions of a NaCl solution and placing a section of vegetable each one. By precisely measuring the volume of each section before and after the incubation period, and graphing the percent change (increase or decrease) in volume, we can determine the osmolarity. Do you remember the idea of tonicity? This is what we are looking at. If the solution is hypertonic, water will leave the cells, the cells will shrink, and the vegetable will get shorter and thinner. If the solution is hypotonic, water will enter the cells, the cells will swell, and the vegetable will get longer and thicker. At some dilution point a solution will be isotonic to the vegetable. That is to say no water will go into the cells or come out of the cells, and the volume of the vegetable will be the same before and after incubation. You will have seven data points to graph. You may not have a solution that will be exactly isotonic to the vegetable cells, but when you make your graph, the osmolarity of the solution that would have been exactly isotonic will be obvious. I will leave it up to you to figure out why.
MATERIALS
These are the materials you will need for this experiment:
Tap water
Table salt (NaCl)
One carrot, or one potato.
A cutting board.
A sharp knife.
A millimeter rule.
Measuring containers: 1 cup, 3/4 cup, 2/3 cup, 1/2 cup, 1/3 cup, 1/4 cup.
7 cups, glasses, or small bowls
PROCEDURE.
1. We will start by making a 10% NaCl solution, and then diluting it to make other percent solutions. If we were doing this in the laboratory, we would be using an accurate scale that could measure to .1 g and a volumetric flask that would be a very precise measurement of the water volume. We can do this at home without all of the fancy equipment and be pretty accurate. Here’s what we need to know:
3 tablespoons of NaCl is very close to 50 g
500 ml of water is about 4 1/4 cups, actually just a little under 500 ml.
You will recall from your laboratory exercise on molar and percent solutions, that the proper way to make these solutions is to put the solute into a container and add water up to a certain point. If we start out with just a little bit less water than the final volume of solution, we will be pretty close to the percent solution that we want to get. In this case we are adding 50 g NaCl to 500ml H2O, giving us a 10% solution. We can make our graph using whatever units we wish, percent, molarity or osmolarity. The graph will look the same. At the end of the exercise, for extra credit, you can take the percent solution that is isotonic to the vegetable and convert percent to osmolarity.
2. Measure out the water and add the salt. To be sure the salt dissolves completely.
3. Marx seven containers 1 through 7. Prepare them as follows:1 1 cup NaCl solution. = 10%2 3/4 cup NaCl solution + 1/4 cup H2O = 7.5%.
3 2/3 cup NaCl solution + 1/3 cup H2O = 6.7%.
4 1/2 cup NaCl solution + 1/2 cup H2O = 5.0%
5 1/3 cup NaCl solution + 2/3 cup H2O = 3.3%
6 1/4 cup NaCl solution + 3/4 cup H2O = 2.5%.
7 1 cup H2O = 0%
4. Choose your vegetable. Peel it. Cut 10 pieces about 1 cm X 1 cm X 5 cm. The exact dimensions are not important, but the seven pieces should be as close to the same size as possible. Now make a small notch near one end of each piece. The notch is so that you can be measuring exactly the same side. Before and after. No matter how careful your measurements, one side of the piece is going to be shorter than the other, even if only by a fraction of a millimeter. So once you have the notch, figure out how you want to orient the piece that you measure it. For example, the notch can be facing up on the right side, or whatever.
5. Now measure each piece of vegetable. Measure both dimensions of one end, height and width. Now measure one side, length. Remember which and you used to measure height and width. Remember which side you used to measure length. Measure to within 1 mm accuracy. Now multiply height X width X length. That gives you volume, which will be in mm 2. As you measure each piece, place it next to a bowl and write down the volume of the vegetable piece with the number of the bowl. After you have made all of the measurements, place the vegetable pieces in their respective bowls and leave them there for exactly three hours. Now go clean your house and, if you can, stop by every half-hour or so and stir the water a little bit in each of the bowls.
6. At the end of three hours and, remove the vegetables and measure them again. Note the before and after measurements and calculate the percent change in volume.
Change in volume = (volume change/initial volume) X 100.
Where volume change = final volume – initial volume.
7. Graph the data. The independent variable (X axis) is the percent solution of NaCl. The dependent variable (y-axis) is the percent change in volume. Be sure to keep your units accurate. In other words the distance on the x-axis between 0 and 2.5 is not the same as the distance between 2.5 and 3.7. Set up the graph to allow for negative values on the Y axis. Once you have plotted all seven data points, draw a straight line to more or less connect the points. Do not connect the points, because you will not get a straight line. Rather draw a straight line that comes as close as possible to all the dots. The line may go through all or some or maybe not even any of the points. Actually if your measurements are extremely precise, you should get a straight line.
If you don’t have a piece of graph paper, you can get one online. Just Google graph paper.
8. Figure out the percent NaCl solution that is isotonic to the vegetable. This is the trick. You need to figure out how to do that. You may need to do some googling! Or discuss it with the rest of the class in the discussion section.
9. For extra credit (five points) calculate the osmolarity of this solution.
10. Go to the dropbox and report:
1. initial volume, final volume and percent volume change (including + or -) for each
piece
2. Percent solution that is isotonic to the vegetable.
3. Osmolarity, if you chose to decide to do it. If you decided to try this, you must show me your work if you want the extra credit.