Salt Concentration of a Potato Lab

Osmosis Lab

Background:

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 or carrot 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

Problem/Purpose: What is the concentration of s in a potato cell?

Hypothesis:

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 (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: General Hints

Pre-mix beakers of salt solutions 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.

What size potato slices will you use?

What will you measure and when?

How many potato slices will you test?

What will the potato slices be placed in?

What solutions do you need to make?

How much salt and water will you place in your solutions?

How long will you let the potato slices remain in the solutions?

Results:

1. Record your actual results in a table like this one:

2. Prepare a graph showing change in mass as a function of % salt. Scale the x-axis of your graph in units of 0.5 percent. The y-axis has a zero line half way up, indicating whether the samples lost or gained weight. You will have to scale the y-axis according to your greatest and smallest changes in mass.

Download this Excel spreadsheet or Link to this Google Document if you need help making a graph.

When completed, use a ruler to draw a straight line of best fit through your six data points, or use the computer to graph your data and calculate the line of best fit. Where the line of best fit crosses the horizontal zero line, draw a vertical line down to the x-axis. This is the point at which the potato is isotonic with its surroundings, and is therefore the estimated salt concentration of the potato.

Discussion

1. Accept or Reject your hypothesis using specific data from your results for support. Be sure to include your final conclusion regarding the true salt concentration of a potato cell.

2. Discuss a minimum of two experimental errors or methodology errors. (These are changes you would make to ?improve the accuracy of your results.)

Final Adjustments to Your Lab:

Make sure you have altered your method to fit any changes you made.

Are your data tables accurate: Titles, Units of Measure

Self Evaluate your lab and make changes.

Have another group Peer Evaluate your lab.

Check over the rubric on your lab document to make sure you have completed everything that is expected.

Thank You!!!! Credits: Entire Lab with Minimal Edits Larry MacPhee Arizona State University (http://www2.nau.edu/lrm22/lessons/osmosis/osmosis.html)