If we made a range of different sugar solutions with a different concentration of sugar and work out their Osmotic Pressure, we could use these solutions to find the Osmotic Pressure needed to prevent osmosis in potatoes.
We did some preliminary work earlier this year. We put three potato pieces in three different strengths of sugar solutions; 1M, 0.5M and distilled water. Before they were placed in the solutions they were weighed and their weight was recorded. Then the pieces were left over night in the solutions and then weighed again to calculate the difference in mass.
This preliminary test will help us find the breakeven point, when the OP of the cell is the same as the water around it. This is because the point must be between distilled water and 1M of solution. We know this because distilled water gains weight and 1M loses weight.
Method
The apparatus will be set up as shown.
Equipment
- 20 pieces of a potato cut by a potato cutter
- 20 test tubes
- 2 beakers to keep 10 test tubes in each
- Distilled water
- 1M solution
- 0.5M solution
- 10ml measuring tube
- Weighing scales
I will begin by:
- Making 10 serial dilutions, twice
- Measuring 10ml of 1M solution and putting it into test tube 1.
- Removing 5ml of this solution and adding equal amounts of distilled water to produce an M/2 in test tube 2
- Removing 5ml of M/2 solution and adding it to 5ml of distilled water to produce an M/4 in test tube 3
I will then repeat the above steps until I reach test tube 9, which will be M/256 solution, test tube 10 will be 5ml of distilled water.
I will then:
- Cut 10 equal sized pieces of potato, using the potato cutter and then weighing each piece and record the weights
- Put each piece into a different test tube and leave them over night
- Note the changes in the potato’s mass, whether it increased or decreased
- Repeat the experiment to find the average results. This will ensure more accurate results.
I will keep the experiment a fair test by measuring the amounts of each solution and putting the potato pieces in and out of each solution simultaneously. Each potato piece is cut down to a similar weight using the cutting board and scalpel.
I predict that the potato pieces which have been placed in the solution with the lower concentration of sugar, due to water moving out of the cell the mass of the potato will decrease. I also think that a piece of potato in the solution with a higher concentration of sugar, due to water moving into the cell, the mass will increase. Finally, I predict that around the middle of the concentrations, there will be no change in the weight of a piece of potato because the Osmotic Pressure in the solution will be equal to that in the potato.
Analysis
Using my graph, I can see there is a pattern in my results: When the osmotic pressure of the solution increases, the percentage change decreases. In conclusion, I found that as the osmotic pressure increases, the solution in the potato will be a less concentrated sugar solution. This means that the water will move out of the potato cell into the stronger sugar solution in the beaker, making the potato cell decrease in mass. When the osmotic pressure is low, the concentration of sugar solution in the potato cell is higher than the concentration outside of the cell. The water will move into the potato cell increasing it’s mass.
Using my graph, I can find where there is no change in the mass of potato because there is no water movement, the osmotic pressure in and around the potato cell are equal. The reading taken will be the osmotic pressure of the potato cell. I found from my graph, that the osmotic pressure was 45kPa.
Evaluation
Using my graph I can see some of my results were accurate, yet I had a few very inaccurate anomalies. These were the points which didn’t touch the line of best fit: -8kPa at +10.5%, 31kPa at +4%, 2Kpa at +7% and 4kPa at +7%. These anomalies may have occurred due to inaccurate testing mentioned in how I cold improve the experiment.
Errors which may have happened during the experiment could have had an affect on my results. When we made the serial dilutions, we took 5ml from a previous dilution and added 5ml of distilled water to it. The 5ml of solution we removed might not have had equal amounts of water and solution in it. If this was the case, then the osmotic pressure would have been inaccurate due to an error in the serial dilutions.
To improve the experiment, the experiment could have been repeated again. Having a third set of results can produce much more accurate averages and could be compared to the 2 tests before to show which test had the errors. However, only repeating the experiment once gave some fairly accurate results.
To extend the experiment, I could take more readings and make more serial dilutions around the point 40kPa and 50kPa to find an accurate result of the osmotic pressure of a potato cell. Doing this could be successful as the osmotic pressure was found where the line of best fit crosses the x-axis and if readings were took above and below this point, a more precise osmotic pressure can be found.
