Ms. C. Evans
Biology Lab Report
To determine the effect of surface area on the rate of osmosis.
The main aim of the experiment was to determine the effect on the rate of osmosis by the changing surface area. The changes were observed in the masses and surface area of the chopped potatoes which were initially of different surface area but same mass. The rate of osmosis increased with increase in surface area. And this increase in the rate of osmosis was directly proportional to the surface area.
Now the rate is given by the change over time. Well the osmosis is the movement of water molecules from high concentration to a low concentration; and of course a semi-permeable membrane is required for keeping the concentration difference. So it's the movement of molecules of water along the "concentration gradient (from high to low concentration)" through the semi-permeable membrane. So the rate of osmosis is "change in concentration(moles) /time(seconds)"
Now the concentration is given by "moles/liter". And the moles are "mass(gram)/molecular mass(grams)". So the moles are directly proportional to the mass of the substance. So the rate of osmosis could even be written as "change in mass(grams)/time(seconds).
Three beaker (200 ml each)
Distilled water (at room temperature).
A tong (to take the potatoes out of water)
All the three beakers were cleaned and dried. Then the equal amount (i.e. 200 ml) of distilled water was poured in each beaker. The water in all the beakers was at room temperature. This was done so that the effect of temperature on the rate of osmosis could be negligible. Then the potatoes were chopped with the French fries maker and 3 pieces of chopped potatoes were taken. The chopped potatoes were cut out so that they had the same mass and different surface area. And hence the mass factor was also negligible.
To reduce the mass factor potatoes were named as No. 1, 2, and 3. The potato No.1 was left over as of the same dimensions. Whereas the potato No. 2 was cut so that the length of each piece of potato No.2 is 1/2 the length of the potato No.1. Now by doing that the surface areas of the potato No. 2 was made two (side surface area) more than the surface area of potato No. 1. Now the potato No. 3 was cut so that the length of each piece of resulting potato piece is ¼ the length of the potato No. 1. This was done so that the surface area of the potato was made six (side surface area) more than the surface area of potato No. 1. Now by doing that the mass was kept constant where as the surface areas were different for all the potatoes. To confirm this they were individually weighted i.e. all the pieces of No. 1(one piece only), No.2 (two pieces) and the No. 3 (four pieces). All the weights were found out to be the same.
Then each group of potatoes (i.e. No. 1, 2 and 3) were individually taken and placed in the beaker having 200 ml of distilled water which was at room temperature. After that they were kept on the table without disturbance for 10 minutes. Time was observed with the help of the wall clock.
After 10 min. the potatoes (in groups) were taken out of their individual beakers. Then they were dried with the help of brown paper and weighted again with the same weighing machine which was used before. Then the mass of each group was recorded in the observation table. The change in mass was found out and hence this change in mass is used to find the rate of osmosis.
The rate of osmosis = (the change in the mass)/ (the time taken)
Surface Area (cm^2)
Rate of osmosis
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