Partially Permeable Membrane
I will draw a graph of the result (sucrose concentration against change in mass of potato cylinders). The point where the line meets the x-axis (sucrose concentration) is the water potential of the potato.
The Pilot: this is the experiment that I will carry out to get a general idea of the area in the graph that I will focus on to find the exact value of the water potential of the potato.
First I will use the 2 molar sucrose solution to make different concentrations of water (different water potentials). I will do this by diluting the sucrose solution. To make five different concentrations of sucrose solutions I will use the following compositions of water and sucrose solution:
- 10 cm3 of sucrose + 10 cm3 of water = 1 mole of sucrose solution;
- 10 cm3 of sucrose + 20 cm3 of water = 0.5 mole of sucrose solution;
- 5 cm3 of sucrose + 15 cm3 of water = 0.25 mole of sucrose solution;
- 5 cm3 of sucrose + 20 cm3 of water = 0.125 mole of sucrose solution;
- 5 cm3 of sucrose + 25 cm of water = 0.0625 mole of sucrose solution.
I will then extract cylinders from a potato using a cork borer to ensure a fixed shape of the cylinders. I will use the cork borer carefully and put the potato on a cutting board to avoid injuries. Finally I will put the 2 potato cylinders in each beaker after I weigh them and record the weight. I will put all the beakers in one room, so that the temperature is the same. Temperature can affect the result because the movement of molecules increases with the increase of temperature allowing water to diffuse quicker and thus more weight loss or gain. By doing this I will keep the shape of potato, its surface area and temperature constant and vary the concentration of the sugar solution.
I will collect the results after 24 hours to allow enough change to take place.
I predict that the potato cylinders in the beaker that contains 0.0625 molar solution will gain weight, because the solution is nearly pure water. This means that water will move from the beaker (higher water potential) into the potato tissues that contain lower water potential and that causes the increase in weight.
After I collect my results I will plot them on a graph paper (concentration of sugar against change in mass). I will look at the point where the line crosses the x-axis, which is the water potential of the potato tissues.
THE ACTUAL EXPERIMENT:
The table below shows the results of the pilot experiment:
From this table I noticed that all potato cylinders have lost weight except the ones in the beaker that contains 0.0625 molar solution. This means that the point where the line crosses the x-axis is somewhere between 0.125 and 0.0625 molar solutions.
I have decided to resolve the two solutions above into 5 solutions, so that I can get a more accurate result. This would also give a more exact position of the line when it crosses the x-axis (concentration of sugar solution).
I will use the same procedure that I used for the pilot study. However, I will use different concentrations of sugar solutions. The concentrations that I will use are:
- 5 cm3 of sucrose solution + 20 cm3 of water = 0.125 mole of sucrose solution;
- 5 cm3 of sucrose solution + 21.25 cm3 of water = 0.109375 mole of sucrose solution;
- 5 cm3 of sucrose solution + 22.5 cm3 of water = 0.09375 mole of sucrose solution;
- 5 cm3 of sucrose solution + 23.75 cm3 of water = 0.078125 mole of sucrose solution;
- 5 cm3 of sucrose solution + 25 cm3 of water = 0.0625 mole of sucrose solution.
To get the exact volumes of water I will use a burette. I will put 2 potato cylinders in each beaker after weigh them and record their weight. I will collect the results after 24 hours.
To draw the graph I will work out the change in mass as a percentage of the original mass (their mass before I put them in the beakers) and plot them against the concentration of sugar solution.
I will use the
Hopefully this strategy would give accurate and reliable results.
The table below shows the weights and the change in weights of the potato cylinders:
From the table above I noticed that all the potato cylinders have lost weight, unlike what I expected to happen. This might be due to a mistake in the composition of the solutions or the pilot might have given me the wrong idea about where to concentrate.
I will use both the pilot result and the actual experiment’s results. I will plot them on separate graphs to work out the approximate position of the line of best fit.
The table below shows the pilot’s results that I will use:
The graph of the results above suggests that the line of best fit crosses the x-axis at 0.09375 mole of sucrose solution. This means that the concentration of solute suggested inside the potato from this graph is 0.09375. Another observation is that the more concentrated the sugar solution is, the bigger the change in mass loss takes place.
For the second set of results I will extend the line of best fit backwards in order to get an approximate position of the point of crossing. This is not very reliable or accurate, but will give me a rough indication whether the first graph is reliable or not.
The second graph shows that line of best fit crosses the x-axis at less than 0.0625 mole of sugar solution. However, I will use the pilot’s results, because I can not assume that the line of best fit of the actual experiment’s results will continue in the same way that I drew it. The second experiment’s results are not reliable so I should not use it.
The sample size is 2 potato cylinders in each beaker and I used 5 beakers. The total number of potato cylinders is 2*5= 10.
To calculate the mean weight of potato cylinders before I put them in the beakers:
(3.85 + 3.72 + 3.85 + 3.94 + 3.81) / 10 = 1.917 g (sample 1)
The mean weight of potato cylinders after 24 hours in the beakers:
(2.35 + 2.68 + 3.39 + 3.80 + 3.96) / 10 = 1.618 g (sample 2)
Variance for sample 1 :
(Σx2/n) – mean2 = [(14.8 + 13.8 + 14.8 + 15.5 + 14.5) /10] – 3.7 = 3.64
Variance for sample 2:
(54.3/10) – 1.618 = 3.82