Osmosis—Potato Lab Student's name________________________________-
OBJECTIVES

Be able to:

1. Determine the effect of molecular mass on the diffusion rate of particles through a media.

2. Measure the osmotic pressure in cells of a potato using a gradient of solutions.

Materials: 5 salt solutions: 0.3%, 0.6%, 0.9%, 1.2%, 2.0%n (or close to it), potatoes, test tubes and cork borers

PROCEDURE

I. Diffusion of material through living cells

a. Using a cork borer cut 7 (seven) cylinders from a single potato (the cuts are made parallel).

b. A razor blade is used to cut the ends of the potato cylinders square (all 7 cylinders are equal in length). A length of about 30 mm gives good data.

c: All 7 cylinders of potato must be equal in length, width, and appearance.

d. Measure and record the length of each potato cylinder. All measures must be similar.

e. Place a potato cylinder in 6 test tubes. The 7th cylinder is placed in the "unknown" solution in a numbered test tube
.f. Each of the test tubes is labeled and filled about 2/3 full with a different one of the salt solutions
.g. After about 2 hours (up to 24 hours) remove the potato cylinders from the test tubes. Measure length and record data on hand-in sheet in the form of a graph.

h. Answer question about the unknown solution on the hand-in

1. Define:

diffusion

osmosis

solute

solvent

solution

hypotonic solution

hypertonic solution

2. As potato cells are placed in solutions that are increasingly hypotonic, what tends to happen?

3. 20.0 gm of salt in 600.0 gm of solution is a what percent solution?

4. How many gm of salt and how many gm of water are needed to make 300.0 gm of a 4.0 percent solution?

5. 250.0 gm of a 2.0 percent salt solution contains how many gm of:
gm of solute

gm. of solvent

gm of solution

8. 20.0 gm of salt added to 200.0 gm of water is a what percent solution?
Comparing two solutions with free diffusion.

a. QUIZ number of unknown solution---____________.unknown solution is HYPER/ ISO/ HYPO tonic (circle one)

b. Solution isotonic to potato is____________________________ %. Where graph crosses the original length value for the potato, make a 90-degree angle and read value on the X axis.

c. If your results in this experiment differ from the expected, explain what went wrong.

Osmosis

The rate of water movement in osmosis can be observed with an osmometer (see figure below). A starch solution in the thistle funnel is separated from the water in the beaker by a dialysis membrane that allows water to pass through but is impermeable to starch. (The starch solution has had food coloring added to it so that you can track any movement of the solution in the thistle funnel.) What do you expect will happen over time in this type of setup

Figure 1. A Simple Osmometer

Early in the lab, measure the height of the column of fluid in the thistle funnel. At intervals of about 20 minutes during the lab, repeat the measurement. Record the time and height of the fluid column in the table below.

Time

Elapse Time

Height of Fluid Column

Describe what is happening to both the starch and water molecules in the osmometer.

Over time do you expect that the rate of water movement will increase, decrease, or remain the same? Why?