Potato Osmolarity Lab: DP Biology
Name:__________________
Background-Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic
solutions [Textbook p 41]
Osmosis is due to solutes that form bonds with water. These solutes are osmotically active. Glucose,
sodium ions, potassium ions and chloride ions are all osmotically active and solutions of them are often
used in osmosis experiments. Cells contain many different osmotically active solutions.
The osmolarity of a solution is the total concentration of osmotically active solutes. The units for
measuring it are osmoles or milliosmoles (mOsm). The normal osmolarity of human tissue is about 300
mOsm.
An isotonic solution has the same osmolarity as a tissue. A hypertonic solution has a higher
osmolarity and a hypotonic solution has a lower osmolarity. If samples of a tissue are bathed in
hypertonic and hypotonic solutions, and measurements are taken to find out whether water enters or
leaves the tissue, it is possible to deduce what concentration of solution would be isotonic and therefore
find out the osmolarity of the tissue.
Instructions
Reference:
1. https://www.youtube.com/watch?v=jTDATlaBV-o
2. https://www.youtube.com/watch?v=M4LugywewAU&list=PLAd0MSIZBSsECWUTkX5b
rcDmfp0MN4zod
Materials:
1. 250 ml beakers: 6
2. NaCl Salt: ~30 g
3. Digital Scale x 1
4. Paper towels
5. Cutting board
6. Potato cutting material
7. Potatoes and Sweet Potatoes: enough to make 6 (1cm x 1 cm x 5 cm) strips.
8. Distilled Water: ~600 ml
Solution preparations: Distilled water (100 ml), 1 M NaCl 100 ml, 0.8 M NaCl 100 ml, 0.6 M Na Cl,
100 ml, 0.4 M NaCl 100 ml, 0.2 M NaCl 100 ml
Note: To prepare 100 ml of 1 M (1 mol/L) NaCl, do the following calculation:
Molecular Weight of NaCl = 23.0 g/mol + 35.5 g/mol = 58.5 g/mol
𝑔
Molarity (M) = moles/liter; so 1 𝑀 𝑁𝑎𝐶𝑙 𝑛𝑒𝑒𝑑𝑠 58.5 𝑚𝑜𝑙 𝑥1 𝑚𝑜𝑙 = 58.5 𝑔 𝑖𝑛 1 𝑙𝑖𝑡𝑒𝑟 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟
=𝑜𝑟 5.9 𝑔 𝑁𝑎𝐶𝑙 𝑖𝑛 0.1 𝐿(100 𝑚𝑙) 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟
The other solutions can be made by using ratios:
5.9 𝑔 𝑁𝑎𝐶𝑙
𝑥 𝑔𝑁𝑎𝐶𝑙
= 0.8 𝑀
1𝑀
Potato preparations: Carefully cut six 1.0 cm x 1.0 cm x 5.0 cm rectangular strips of potatoes and
sweet potatoes. We use rectangular shaped potatoes cut by knife since we do not have tools to cut core
samples. Use rulers to measure accurately your samples. Remember to use cutting boards so as not to
cut up the lab tables. Weight your samples carefully and record weight of each individual piece ready
for specific water samples. Might be good to label your beaker instead of each individual potato piece.
You only have 30 minutes to complete this first part preparations.
Note: Ensure that the surface of the potato samples is dry when finding their mass, both at the start and
end of the experiment.
Soaking period: Soak your potato and sweet potato samples in the six samples solutions for 25
minutes. Make sure all samples are ready first and then soak samples all at the same time. Make sure
also water solution cover over the entirely of your potato pieces.
Wrap-up: At the end of 25 minutes, you will measure weight of each potato piece again.
Lab Writeup
Make a clear, easy-to-read table to record your results.
Graph your results with Y-Axis (Mass Change %) and X Axis (NaCl concentration M)
Include answers to these concepts in your lab report.
1. State whether water moved into or out of the potatoes at 0.0 mol/dm-3 NaCl solution.
2. State whether water moved into or out of the potatoes at 1.0 mol/dm-3 NaCl solution.
3. Deduce which potato had the lowest solute concentration in its cytoplasm. Include how you
reached your conclusion in your answer.
4. Suggest reasons for the differences in solute concentration between the potatoes.
5. Explain the reasons for using percentage mass change rather than the actual mass change in
grams in this type of experiment.