PASCO ME-6940 Diffusion-Osmosis Apparatus - Manual (Page 11)

M o d e l N o . M E - 6 9 4 0 A d d i t i o n a l E x p e r i m e n t s
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3. Calculate the final molarity of the solution in column #2 at the end of the experiment. Record in Table 3.
Answer: First, determine the number of moles in column #2 at the beginning of the experiment. The initial volume of
fluid in column #2 was 40 ml (0.04 L) and the initial molarity was 2.0 mol/L.
• 2.0 mol/L * 0.04 L = 0.08 moles
The final volume of solution in column #2 was 50 mL (0.05 L). Since sucrose could not cross the membrane, we
know that we still have 0.08 moles of sucrose in the solution, but the volume has changed. Find the molarity of a solu-
tion that contains 0.08 moles in 0.05 L.
• 0.08 mol / 0.05 L = 1.6 mol/L
The solution in column #2 was diluted from 2.0 mol/L to 1.6 mol/L after 24 hours.
Additional Experiments
• Simply observe the osmotic movement of water (without observing pressure changes) by filling the columns
with solutions of varying solute concentration, as long as the solute does not cross the membrane, you will
observe the volume of change. Examples of substances that are too large to cross the membrane: disaccha-
rides, ex. Sucrose; polysaccharides ex. Starch; proteins. Examples of substances that are small enough to cross
the membrane: monosaccharides, ex. Glucose; ions, ex. Na+, Cl-, protons; Gases, ex. CO2, O2.
• Explore the concept of semi-permeability by observing the movement of solutes across the membrane. Fill
one column with distilled water and the other column with pickle juice. Test the initial conductivity, pH and
blue Absorbance of the water and pickle juice. After 45-60 minutes, measure the final conductivity, pH and
Absorbance of the fluids to determine whether salts, acetic acid and yellow dye diffused across the membrane.
• Determine the size of solute molecules by comparing their rate of diffusion across the membrane.
• Add a few grams of dry yeast and a few grams of sugar to 50 mL of distilled water to create an activated yeast
solution. Allow the yeast 15-20 minutes to become active. Make a 10% glucose solution by dissolving 10 g of
glucose in 100 mL of water. (Remember table sugar is not glucose! It is sucrose!) Fill column #1 with 30 mL
of yeast solution and column #2 with 30 mL of glucose solution. Place the CO2 gas sensor into column #1 and
the O2 gas sensor into column #2. Measure the increase in CO2 and the decrease in O2 as the glucose diffuses
across the membrane and is used in the respiration of the yeast.
• Chemistry and physical science classes can use the appartus to study the gas laws. Explore the effects of vol-
ume on pressure changes within the columns.
• Find the molecular mass (M
2
) of an unknown solute in solution. Create a solution containing an "unknown"
solute that cannot cross the membrane. Give the student the mass of the solute in the solution and the volume
of the solution in the columns. Fill column #1 with the protein solution and column #2 with distilled water.
Measure the pressure changes for 24 hours. Use the equation below to find the molecular mass of the
unknown substance.
where M
2
= molar mass (g), C
2
= molarity (mass of the solute in the solution (g) / volume of the solution (L)),
R = 0.0083145 L kPa/ mol K, T = 298 K, and P = Final Absolute pressure #1.
AP® Biology Lab 1: Diffusion and Osmosis*
• Part one: Make a 15% glucose/1% starch solution. Using glucose test strips and iodine test for the initial
presence of sucrose and starch. Fill column #1 with distilled water and column #2 with the glucose/starch
solution. Do not put the caps on the apparatus, leave it open to the environment. After 45-60 minutes, test
M
2
C
2
RT
P
------- =