Supposed you have two solutions separated by a semi-permeable membrane in a flask. Solution A has 100 molecules of glucose per ml while Solution B has 100 molecules of NaCl per ml. How will water move across a semi-permeable membrane?
a. There will be no net movement of water since the concentration of solute in each solution is equal
b. There will be a net movement of water from Solution A to Solution B until both solutions have equal concentrations of solute
c. There will be no net movement of water since the concentration of solute in each solution is not equal
d. There will be a net movement of water from Solution B to Solution A until both solutions have equal concentrations of solute
Answers
Introduction: Human blood, at 0.9% salt concentration, is a little less salty than seawater, which has a salt concentration of about 35 parts per thousand (3.5%). If we take seawater as an example of a solution, the salt is called the solute (the particles that are dissolved) and the water is the solvent (the liquid that dissolves the particles). Osmosis is the movement of a solvent across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration. The water (the solvent) can move across the membrane but the dissolved solutes (the sodium and chloride ions that form salt) cannot. In such situations, water will move across the membrane to balance the concentration of the solutes on both sides. Cells tend to lose water (their solvent) in hypertonic environments (where there are more solutes outside than inside the cell) and gain water in hypotonic environments (where there are fewer solutes outside than inside the cell). When solute concentrations are the same on both sides of the cell, there is no net water movement, and the cell is said to be in an isotonic environment. In this lab we will test samples of potato tissue to see how much water they absorb or release in salt solutions of varying concentrations. This gives us an indirect way to measure the osmotic concentration within living cells.
Hypo=under, iso=equal, hyper=over
osmosis
Compare initial and final states. Which way did the water move? Why?
Osmosis Lab Setup
Osmosis Lab Setup
Materials:
electronic balance (0.01 g range)
metric ruler with mm scale
metric measuring cups
6 cereal bowls or shallow pans
a small piece of raw potato to cut into six ~5 mm cubes 5 millimeter cube (this square is 5 x 5 mm)
single edged razor or knife
paper towels
watch or clock
table salt, distilled or tap water
6 beakers (250 ml or larger) or cups
Methods:
Pre-mix 6 beakers of salt solutions (0%, 0.1%, 0.5%, 1%, 2.5%, 5%) in distilled water. You can use this solution calculator to help you make your solutions. Just enter the water volume of your container and the percentage of salt you want and it will tell you how many grams of salt to add. A 1% salt solution is 1 part salt to 100 parts water. To make a 1% salt solution, you could use a 100 ml bottle, add exactly 1 gram of salt (use your electronic balance) to your bottle, and bring the water volume up to 100 ml. To make a 0.1% solution, add 1 gram of salt to 1000 ml of water (or add 0.1 g salt to 100 ml of water). If you have more water than you need, just stir well and then discard the excess.
Prepare six small potato cubes with no skin that are all about equal in size (approximately 5 millimeters in length, width and height) and blot them dry on a paper towel. (Blot means just gently remove the surface water; no need to squeeze them!)
Mass (weigh) each to the nearest 0.01 grams, keeping them separate, and record each initial mass in Table 1. Don't wait too long before putting them into the solutions, as evaporation will occur.
Fill each bowl with one of the 6 stock solutions, keeping track of which is which! Label them. You won't be able to tell the salinity just by looking. Note which potato piece went into which bowl.
Leave one of the potato slices in each of the salt solutions for up to 24 hours so that they may gain (or lose) water by osmosis. (Keep them all in the salt water the same amount of time--leaving them overnight is likely to give the best results).
Remove the slices, blot them dry on a paper towel, carefully re-weigh them and record in the data table as final mass.
