Determining the molar concentration in carrots
In order to successfully complete this assignment, you will have to previously have some research on osmosis, water potential, molarity.
Read the instructions provided before the labs and prepare your own lab design. Ask questions before, not during the lab.
Once you have done some research, continue with procedures.
Procedures
1. Prepare 100 ml of distilled water, 0.2 M, 0.4M, 0.6M, 0.8M, and 1.0M sucrose solution using steps 2-3.
2. Use the following formula to determine the moles of sucrose needed for each molar solution. Then convert moles to grams.
1) Determine moles of sucrose needed using the Molarity Formula: i. Molarity= moles of solute/Liters of Solution or M = mol/L
• Solve for moles. (Don’t forget to convert 100 ml to L first)
2) Convert moles of sucrose to grams of sucrose: i. Multiply the molar mass of sucrose (342.30g) by the moles of sucrose calculated above. This total is the mass (g) of sucrose that will be needed to make 100 ml of solution.
(____mol x 342.30 g/mol = ___grams of sucrose)
3. Add appropriate mass of sucrose to a beaker or measuring cup, then fill with distilled water up to the 100 ml mark and stir until all sucrose is dissolved. Pour solution into one 3 oz. cup until cup is full. Label the molarity and discard the excess solution.
4. Repeat steps 2-3 for each solution.
5. Carefully cut the ends off 4 baby carrots for each cup (24 total). You should cut enough off each end so that the resulting length is 2 cm. Keep carrot pieces in a separate covered cup until you are ready to mass them.
6. Using a food scale, determine the mass of the four carrot pieces. Record this mass on your data table. Place these pieces into their respective cup of solution. Write the initial mass of the carrots on the cup. Repeat this step for each cup of solution.
7. Cover each cup and let them stand for 24 hours.
8. Pour off the solution in each cup, blot the carrot pieces with a paper towel and take their final mass.
9. Record the final mass in the data table below. Calculate the percentage change using the formula below. (Note: If final mass is less than initial, you will have a negative % change in mass)
Write down your recordings in a data table
10. Graph your individual data to show Molarity of Solution vs. Percentage Change in Mass using Excel
11. Determine the molar concentration of the carrots. This would be the sucrose molarity in which the mass of the carrot pieces does not change. To find this, draw the straight line on the graph that best fits your data. The point at which this line crosses the x-axis represents the molar concentration of sucrose with a water potential that is
equal to the carrot tissue water potential. At this concentration there is no net gain or loss of water from the tissue.
Indicate this concentration of sucrose in your report.
Molar concentration of sucrose = ____________ M

Part 2--‐
CALCULATION OF WATER POTENTIAL FROM EXPERIMENTAL DATA
1. Calculate the solute potential of this sucrose solution using the following formula:
ψs = -iCRT i = Ionization constant (for sucrose this is 1.0 because sucrose does not ionize in water)
C = Molar concentration of solution resulting in 0% change in mass of carrots
R = Pressure constant (R = 0.0831 liter bars/mole K)
T = Use thermometer to find Temperature in K (273 + oC of the solution)
The units of measure will cancel as in the following example:
2. The pressure potential of this sucrose solution is 0, since it is an open container.
3. Calculating the solute potential of the solution (ψs) and knowing that the pressure potential of the solution is zero
(ψp = 0) allows you to calculate the water potential of the solution.
ψ = 0 + ψs
ψ = ψs
4. The water potential of the solution at equilibrium (point at which there is a 0% change in carrot mass) will be
equal to the water potential of the carrots. What is the water potential of the carrot cells from Part 1?
Show your work.
Lab Discussion Questions
1. Describe what happened when the carrot samples were immersed in hypotonic solutions?
• Initially, where was water potential the highest, in the carrot cells or in the hypotonic solution?
• Which solutions were hypotonic?
2. Describe what happened to the carrot samples when immersed in hypertonic solutions?
• Initially, where was water potential the highest, in the carrot cells or in the hypertonic solution?
• Which solutions were hypertonic?
3. Explain the relationship between the molarity of the solutions and the % change in mass of the carrots.
4. Explain how you used the percent change in mass of the carrot samples to determine the water potential of the carrot cells.
5. What would happen to the water potential of the carrot cells if the carrots were allowed to dehydrate. Why?.
6. Name one other way, other than solute concentration, in which you could change this experiment. Give a brief description of your procedures.
Practical Situation Questions
Using what you’ve learned about water potential, hypertonic, hypotonic, and isotonic solutions, discuss the following are common practices in everyday life:
1. Why do grocery stores spray their produce with water on a regular basis even though they are separated from the rest of the plant?
2. How does salt or sugar curing meat prevent the growth of bacteria and fungus?
3. Why is it important to determine the water potential of human cells before developing standard IV solution concentrations. What might happen if human cells are immersed in extremely hypotonic and hypertonic solutions?
Adapted from Lab One: Diffusion and Osmosis, College Board, 2001, Biology Lab Manual.