Osmosis Pre lab
Turn in a brief mini-report (an outline) of your Osmosis Lab here. Be sure to include your hypothesis, a brief description of the osmosis experiment setup, results (including table 2 of % change vs time, graph 1, table 3 of the rate of change vs NaCl concentrations, graph 2, and determined isotonic point), and your conclusions/discussion (What was the isotonic point? Was your hypothesis supported by the results?). No references needed unless you use them.
EXERCISE 3
OSMOSIS AND THE SCIENTIFIC METHOD
OBJECTIVES
Upon the completion of this exercise, the student should be able to:
1. formulate a scientific hypothesis.
2. design and understand a basic experiment.
3. distinguish between dependent and independent variables.
4. measure the mass of an object.
5. calculate volumes needed to dilute a solution and make the dilutions.
6. express data in tabular form, graph data, and calculate the slope of a line to determine rate.
7. determine the tonicity in living cells (isotonic point).
8. explain the importance of tonicity in living cells and organisms.
A REVIEW OF THE SCIENTIFIC METHOD
Scientists study natural phenomena by following the scientific method. For example, after observing a fruit fly, a biologist might use the steps in the scientific method as follows1:
Observation:
Observe something in the natural world
The life cycle of a fruit fly is 30 days at 29 °C.
Question:
Ask a question about how it works
How do changes in temperature affect the life cycle of a fruit fly?
Hypothesis:
A possible explanation for the observation(s); Testable and falsifiable
A decrease in temperature to 18 °C will increase the time it takes the fruit fly to complete its life cycle.
Prediction
:
If my hypothesis is correct
then this will happen. (“if…..then” statement)
If I decrease the temperature of a fruit fly’s environment to 18 °C then the time it takes the fly to complete its life cycle will increase.
Experiment:
Design a controlled way to test the hypothesis
Place 100 flies at 18 °C for one generation, and place 100 flies at 29 °C for one generation. Compare how long it takes each group to complete their life cycle.
Analysis of Results:
Support or reject the hypothesis based on the results of the experiment.
Fruit flies placed at 18 °C have a longer life cycle than those placed at 29 °C.
Conclusions:
What you conclude from your results
Decreasing the temperature of a fruit fly’s environment to 18 °C will increase the time it takes the fruit fly to complete its life cycle.
1Harris, Dr. Katherine, Hartnell College Biology Tutorials: Scientific Method Tutorial.
http://www.hartnell.edu/tutorials/biology/scimethod.html
Developing a hypothesis:
1. A hypothesis must be specific.
2. A hypothesis is a statement and is not written in the form of a question.
3. A good hypothesis should clearly state how the dependent variable is expected to be affected by the independent variable. There should only be one independent variable, and what it is should be stated in the hypothesis.
4. The hypothesis must be testable.
a. You must be able to perform an experiment to test your hypothesis.
b. You must be able to make observations or measurements to determine how the dependent variable changes as a result of the independent variable.
5. The hypothesis must be falsifiable. In other words, it must be possible for the hypothesis to be incorrect.
6. A hypothesis is written in language that is clear and simple. It should be obvious to the reader exactly what you are testing.
Designing the Experiment
A well-designed experiment should include the following:
Independent Variable – The factor that is changed or manipulated by the experimenter to see what effect occurs. Temperature is the independent variable in the example above.
Dependent Variable – The factor that is measured/assessed during the experiment, because it is expected to change due to changes in the independent variable. Time it takes the fruit fly to complete its life cycle is the dependent variable in the example above.
Experimental Group(s) – The group(s) where the independent variable is being tested. Fruit flies placed at 18 °C are the experimental group in the example above.
Control Group(s) – The group(s) that the experimental group(s) is compared to. Fruit flies placed at 29 °C are the control group in the example above.
In some cases, positive and/or negative controls are used to show that an experiment is working correctly.
Positive control – a sample, etc. that is treated in the same manner as the experimental group and is designed/expected) to give a positive result.
Negative control – a sample, etc. that is treated in the same manner as the experimental group and is designed/expected to give a negative result
To interpret the results, each experimental group can be compared to the positive and negative controls.
DIFFUSION, TONICITY, AND OSMOSIS
Cells must move materials through membranes and throughout the cytoplasm in order to maintain homeostasis. The movement is regulated because cellular membranes, including the plasma membrane and organelle membranes, are selectively permeable. Membranes are phospholipid bilayers containing embedded proteins; the hydrophobic characteristics of the phospholipid fatty acids limit the movement of water, polar solutes, and charged solutes across the membrane.
All matter above absolute zero (0 Kelvin; -213°C) has kinetic energy, which is displayed as random molecular motion. Although individual molecules move about at random, a net directional movement may occur in response to inequalities in concentration, pressure, or temperature. Molecules will move from a region of higher concentration, higher pressure, or higher temperature to regions of lower concentration, lower pressure, or lower temperature until an equal distribution of molecules is achieved. This passive movement is called diffusion; it does not require energy input. The difference between the higher concentration, pressure, or temperature and the lower concentration, pressure, or temperature is referred to as a gradient.
The cellular environment is aqueous, meaning that the solvent in which the solutes, such as salts and organic molecules, are dissolved is water. Water moves through membranes by a special form of diffusion called osmosis (osmosis is the movement of solvent, but in a biological system the solvent is water). Water is able to pass slowly through the membrane or more rapidly through specialized protein channels called aquaporins. Most other substances, such as ions, move through protein channels, while larger molecules, including carbohydrates, move via transport proteins or bulk transport.
Like solutes, water moves down its concentration gradient. Water moves from areas of high free water concentration (low number of solute particles) to areas of low free water concentration (high number of solute particles). Again, solutes decrease the concentration of free water, since water molecules surround the solute molecules.
Tonicity is a measure of the ability of a solution to cause water to move. It is influenced only by solutes that cannot cross the membrane. The terms hypertonic, hypotonic, and isotonic are used to describe solutions separated by selectively permeable membranes and refer to the relative concentration of the solute (not water). A solution with greater solute concentration as compared to the solution on the other side of the membrane is called a hypertonic solution; therefore, water will move into the hypertonic solution through the membrane by osmosis. A hypotonic solution has a lower solute concentration than the solution on the other side of the membrane; water will move down its concentration gradient into the more concentrated solution. Isotonic solutions have equal solute concentration on either side of the membrane. Water still moves back and forth across the membrane, but there is no overall or net change in the concentration on either side.
In non-walled cells, such as animal cells, the movement of water into and out of a cell is affected by the relative solute concentration on either side of the plasma membrane. As water moves out of the cell, the cell shrinks; if water moves into the cell, it swells and may eventually burst. In walled cells, including fungal and plant cells, osmosis is affected not only by the solute concentration, but also by the resistance to water movement in the cell by the cell wall. This resistance is called turgor pressure. The presence of a cell wall prevents the cells from bursting as water enters; however, pressure builds up inside the cell and affects the rate of osmosis. Turgor pressure is responsible for the support in many plants and for the ability for plant roots to draw water from the soil. Walled cells become turgid in hypotonic environments. In hypertonic environments, walled cells lose water, which causes the plasma membrane to separate from the cell wall, causing plasmolysis. In effect, plasmolysis is shrinkage of the cytoplasm, but not the cell itself.
Please watch this video on diffusion and osmosis and how potatoes can be used to determine the isotonic point of a solution.
https://edpuzzle.com/media/5ed0f7994f0c493f982e1dc1
Pre-lab Assignment: Introduction to Osmosis and Tonicity Practice Problems
In this pre-lab assignment, each dialysis bag represents a cell. Like the plasma membrane, the dialysis tubing is selectively permeable. The solutions are aqueous solutions, which means a solute (such as NaCl or sucrose) is in the solvent water. Water can move into or out of the dialysis bag or “model cell”, but the solute cannot. In this model system, the environment is the solution in the beaker surrounding the dialysis bag.
Student’s Name Instructor
PRE-LAB ASSIGNMENT #1: OSMOSIS AND TONICITY PRACTICE PROBLEMS
To be completed before beginning week one of the Osmosis and the Scientific Method Lab.
Part I: Dialysis bags (“cells”) are placed in three different beakers. The cells, as well as the beakers, contain different aqueous solutions of NaCl. For each of the beakers, answer the questions below.
Beaker # 1
Cell # 1
Beaker # 2
Cell # 2
Beaker # 3
Cell # 3
Cell # 1 contains 15% NaCl Beaker contains 35% NaCl
Cell # 1 contains 30% NaCl Beaker contains 30% NaCl
Cell # 1 contains 50% NaCl Beaker contains 25% NaCl
a) The solution inside the cell contains 15% NaCl and % water.
b) The beaker contains 35% NaCl and % water.
c) The solution inside cell #1 is
tonic to the solution outside cell #1.
d) The solution outside cell #1 is
tonic to the solution inside cell #1.
e) In which direction will water move? (circle one below)
Into the cell Out of the cell No net movement of water
f) Draw an arrow or arrows on Beaker #1 above to show the movement of water.
g) Explain how you determined the movement of water.
h) Over time, what will happen to cell #1? (circle one below)
Shrinks Swells Remains the same size
Beaker #2:
a) The solution inside the cell contains 30% NaCl and % water.
b) The beaker contains 30% NaCl and % water.
c) The solution inside cell #2 is
tonic to the solution outside cell #2.
d) The solution outside cell #2 is
tonic to the solution inside cell #2.
e) In which direction will water move? (circle one below)
Into the cell Out of the cell No net movement of water
f) Draw an arrow or arrows on Beaker #2 above to show the movement of water.
g) Explain how you determined the movement of water.
h) Over time, what will happen to cell #2? (circle one below)
Shrinks Swells Remains the same size
Beaker #3:
a) The solution inside the cell contains 50% NaCl and % water.
b) The beaker contains 25% NaCl and % water.
c) The solution inside cell #3 is
tonic to the solution outside cell #3.
d) The solution outside cell #3 is
tonic to the solution inside cell #3.
e) In which direction will water move? (circle one below)
Into the cell Out of the cell No net movement of water
f) Draw an arrow or arrows on Beaker #3 above to show the movement of water.
g) Explain how you determined the movement of water.
h) Over time, what will happen to cell #3? (circle one below)
Shrinks Swells Remains the same size
Part II
: Dialysis bags or model “cells” (A, B, C and D) are placed in beakers containing a 20% sucrose solution. Each model “cell” contains an unknown concentration of sucrose solution. Every 15 minutes, the model “cells” are weighed and the data entered in the table below.
Table: Mass of Model “Cells” Containing Different Concentrations of Sucrose
Time (minutes)
“Cell”A
“Cell” B
“Cell” C
“Cell” D
Mass (grams)
Mass (grams)
Mass (grams)
Mass (grams)
0
11.4
12.5
10.8
10.8
15
11.3
13
9.6
10.2
30
11.2
13.5
8.2
9.5
45
11.3
14
7.2
9.2
Movement of Water
% Sucrose in “Cell”
Tonicity in “Cell”
a) Use the data in the table to determine the movement of water. Choose from “Into the cell”, “Out of the cell” or “No net movement” Write your answer in the row labeled “Movement of Water”.
b) Based on the movement of water, you can determine the concentration inside each model “cell” A-D. Choose from 40%, 20%, 15% or 10%. Write your answer in the row labeled “% Sucrose in Cell”.
c) Determine whether each sucrose solution in the “cell” is “hypertonic”, “isotonic” or “hypotonic” relative to the solution outside the “cell”. Write your answer in the row labeled “Tonicity in Cell”.
Answer the questions below about Part II of this exercise:
1) A. Explain why there was little or no mass change for the model “cell” containing solution A.
B. This system is said to be at equilibrium. Explain what equilibrium means in terms of the movement of water across the plasma membrane of a cell.
2) The model “cell” containing solution B gained weight. Therefore, the water moved____________ the “cell”. Explain why the water moved as it did in this model system.
3) Model “cells” C and D both lost weight. Therefore, the water moved the “cell”. Explain why the water moved as it did in these systems.
4) A. “Cell” C lost more weight than “cell” D over the same amount of time (60 min). Therefore, “cell” C lost water at a
slower or faster rate than “cell” D. (circle one)
B. Explain the difference in the rate of water loss for “cell” C and D.
5) Do you see a pattern of how water flows into or out of the cell based on the concentration of solute?
Water always moves from a ____________tonic solution to a ___________tonic solution.
6) In the experiment your group will design, you will be using a piece of potato composed of many living cells. Based upon what you have learned about osmosis and tonicity, predict whether a potato core would gain weight, lose weight or stay about the same weight if you placed it in each of the following solutions:
Hypertonic:
Hypotonic:
Isotonic:
· END OF PRELAB –
PART A: OSMOSIS AT HOME USING POTATO CUBES
This is an easy, mostly qualitative lab, for you to see how water moves in and out of potatoes at different salt (NaCl) concentrations. You will be measuring the length of the side of potato cubes before and after soaking them in various salt solutions. You will then use the data collected here in Part B of your lab, where you will draw and test a hypothesis about the isotonic concentration of NaCl in potatoes.
Potatoes are modified underground stems of a potato plant, which contain large amounts of starch, protein, vitamins and minerals. During peak growth of the potato plant, 75-85% of the sugar from photosynthesis is stored underground in the cells of the potato. The ions, proteins and carbohydrates stored in the potato do not generally cross the plasma membrane unless specifically transported across (otherwise they would be lost into the soil around the plant).
Sodium Chloride (NaCl) is an ionic compound. It is a crystal of sodium and chloride ions present in a 1:1 ratio. NaCl provides humans with the sodium and chloride ions needed for metabolism and it also maintains the salinity of the extracellular matrix.
Materials Needed:
· For this experiment, you will need 5 glasses/mugs to hold 1 cup of salt water and potato cube. Coffee mugs worked for fine for me and I had plenty but you could use glasses, jars or plastic storage containers as well. Try to find ones that are fairly similar in shape just to make sure that the cube stays fully covered the whole time.
· Table salt (NaCl)
· Water
· Measuring spoon(s): ¼ teaspoon, ½ teaspoon (helpful but not required), 1 teaspoon
· Measuring cup: be able to measure 1 cup of water
· 2 medium to large potatoes
· Knife and cutting board
· You will need to be able to photograph and upload photos of your set up and results to show that you completed the home experiment.
This experiment takes about 15-20 minutes to set up, you then leave it for 8-10 hours and you will need about 15 minutes to record your results and clean up.
Procedure:
1. Label 5 containers with 0M, 0.16M, 0.31M, 0.47M, 0.63M
2. Add water: in each container place exactly 1 cup of tap water (approximately room temperature is fine)
3. Add salt:
· In the 0.16M container place ¼ teaspoon of salt (make sure it is a level measurement for accuracy).
· In the 0.31M container place ½ teaspoon of salt (make sure it is a level measurement for accuracy).
· In the 0.47M container place 3/4 teaspoon of salt (make sure it is a level measurement for accuracy).
· In the 0.63M container place 1 teaspoon of salt (make sure it is a level measurement for accuracy).
· NOTE: A cup of water is approximately 240 ml and ¼ teaspoon of salt weighs 1.5g. Molarity (M) of the solutions was calculated based on these approximations.
4. Stir to completely dissolve the salt in each mug (ideally start from the lowest concentration and work up so you don’t contaminate you solutions alternatively rinse off the spoon between containers)
5. Cut potato cubes:
· I suggest trying to cut all 5 potato cubes to 2 cm cubes (or 1 inch cubes if you don’t have a metric ruler).
· Exact size is unimportant but
ALL must be the same dimensions at the start
.
· NOTE: I found it easiest to roughly cut the 2 cm cubes and then line them up and trim them to be the same size.
TAKE A PICTURE OF YOUR POTATO CUBES AND CONTAINERS AT THIS POINT. Be sure to include a ruler for scale in the picture.
6. Once all potatoes cubes are cut to the same size place them in the various solutions (1 cube per container) and leave them for 8-10 hours. Be sure to record your start and end times.
7. At the end of the time, remove the potatoes and line them up again in the order of concentration of the solutions: 0M, 0.16M, 0.31M, 0.47M, 0.63M. They should be different sizes and textures at this point.
Record the length of and texture changes. take a picture of the cubes to indicate size differences. Be sure to include a ruler for scale in the picture.
8. Clean up: the potato cubes can go in the trash and the salt water can go down the drain. Wash dishes as normal after that.
9. Fill in the report and submit it to your instructor.
What to include in Osmosis Assignment (see pp. 17-18)
· Picture of set up, including container, salt type used, and potato cubes aligned to show they are identical in size (with ruler visible)
· Time experiment started:
· Time experiment ended:
· Total time of incubation of potato cubes in various solutions:
· Picture of potato cubes BEFORE and AFTER incubation in salt solution (in order of concentration of solution: 0M, 0.16M, 0.31M, 0.47M, 0.63M with ruler visible)
· Description of size and texture changes for each cube:
Change in size of cube
Change in texture of cube
0M
0.16M
0.31M
0.47M
0.63M
The following are pictures illustrating how you can set up this lab at home and the type of pictures you should take.
Before incubation: 2 cm cubes
Various salt solutions and potato cubes at start of experiment:
After incubation in various salt concentrations for over 8 hours.
You can see that the lower concentration solutions increased in size and the higher concentration solutions decreased in size.
How to dilute a stock solution
In science, it is very common to make up a concentrated stock solution and then dilute it as needed for various experiments. The idea is similar to buying orange juice (OJ) as a frozen concentrate since it is smaller and can be stored easily for long periods. When needed, you remove a can from the freezer and add the amount of water needed to dilute the OJ to the concentration (flavor) of your choice.
Concentration is the ratio of the amount of solute over the amount of solution. Concentration can be expressed in many ways, but the two most common are: molarity (M) (moles/liter) and % (g/100 ml of solution).
Watch the following video on dilutions to help you understand how to do it.
https://expl.ai/TXPNQSU
Your experiment will likely be testing solutions of differing molarity (concentration). The stock solutions will all be 0.5 M. In your protocol you must include a table describing the
components and quantities to combine to make the various solutions you will be testing. Include your calculations as well so your instructor can confirm your results. See the section below on how to do the calculations for dilutions.
Diluting a stock solution practice problems
If you have a stock solution and you need to make a series of solutions of lower concentrations you can use the formula:
Concentration 1 x Volume 1 = Concentration 2 x Volume 2
C1V1= C2V2
What this means is that the initial concentration of the stock solution (C1) multiplied by the volume used of that solution (V1) will equal the final concentration of the diluted solution (C2) multiplied by the final volume of the diluted solution (V2).
For example: How much of a 20% sucrose solution do you need to mix with water to get 50 ml of a 5% sucrose solution?
· C1 is the stock solution concentration = 20% sucrose
· C2 is the diluted solution concentration = 5% sucrose
· V1 is the volume of stock solution needed = unknown
· V2 is the final volume of the diluted solution = 50 ml
C1V1 = C2V2
(20%)(V1) = (5%)(50 ml)
Now divide both sides by 20% to get the V1 alone
V1 = 12.5 ml
The answer is 12.5 ml of 20% sucrose is needed to make 50 ml of a 5% sucrose solution.
When you solve for this you will see that the units of % for the solution will cancel and the answer will be in milliliters (ml) of stock solution. The same is true when using units of molarity (M). NOTE: there is no need to move the decimal when using percent since the units cancel out.
Now you must calculate how much water to add to 12.5ml of sucrose stock solution to make a total volume of 50ml.
The amount of water you need will be the difference between 50 ml (final solution volume) and 12.5 ml (the volume of 20% sucrose you calculated from above). This equation will work just as well if you use the units of Molarity instead of % or liters instead of milliliters.
50ml (final volume) minus 12.5ml stock sucrose = 37.5 ml water.
Thus, to make 50 ml of a 5% sucrose solution using a 20% sucrose stock, take 12.5 ml of 20% sucrose and add to 37.5 ml water.
Now it is your turn to practice this.
Fill out the chart below and have your instructor check it:
Assume that you have a 20 M glycerol stock solution. You need to make 5 different solutions for your experiment: 0 M or no glycerol, 2.5 M glycerol, 5 M glycerol, 7.5 M glycerol and 10 M glycerol. In each experiment, you will need to make 50 ml of the diluted solution.
Final Conc. of Solution (C2)
Amount (V1) of 20 M Glycerol Stock Solution Needed
Amount of Water (V2-V1)
Final Volume of the Solution (V2)
0 M Glycerol
2.5 Glycerol
5 M Glycerol
7.5 M Glycerol
10 M Glycerol
Some students find it helpful to organize their calculations for each solution:
0 M glycerol 7.5 glycerol
C1 = C1 =
V1 = V1 =
C2 = C2 =
V2 = V2 =
Volume of 20 M glycerol needed? Volume of 20 M glycerol needed?
Volume of H2O needed? Volume of H2O needed?
2.5 M glycerol 10 M glycerol
C1 = C1 =
V1 = V1 =
C2 = C2 =
V2 = V2 =
Volume of 20 M glycerol needed? Volume of 20 M glycerol needed?
Volume of H2O needed? Volume of H2O needed?
5 M glycerol
C1 =
V1 =
C2 =
V2 =
Volume of 20 M glycerol needed?
Volume of H2O needed?
PART B: DETERMINING THE ISOTONIC POINT OF POTATO CORES IN NaCl
In this part of the exercise, you will write a question and a testable hypothesis, regarding the isotonic point of potatoes in NaCl. Your hypothesis should be based on the evidence you collected in Part A, your “Osmosis at Home” lab.
You will determine the amount of mass lost or gained by potato plugs, graph the results and calculate the estimated concentration of NaCl in potato cells (isotonic point).
Suggestions for running the experiment in a wet lab (this was done for you in an online lab).
1. Prepare solutions of various concentrations of your solute of interest, and place them in beakers. Solutions will be prepared using a concentrated stock solution and making dilutions.
2. Construct a data table to record potato mass measurements.
3. Record the mass of each piece of potato at time point zero, and then place in the appropriate beaker. Make sure the potato cores are submerged in solution. Note the time you started the experiment.
4. For each time point, carefully removed each potato, dry it slightly on a paper towel and then record its mass on the table you constructed. Immediately return the potato to its beaker before weighing the next piece.
5. Collecting data every 10 minutes for a total of 60 minutes works fairly well.
6. At the end of the experiment the potato can be discarded in the regular trash, the solutions discarded in the sink, the beakers rinsed and placed in the appropriate location and your lab benches wiped down with a damp paper towel. Make sure any leftover pieces of potato are discarded and not left in your preparation area.
Your Osmosis Assignment should include the following components (see posted assignment)
· Introduction to Osmosis, tonicity. It should also include:
· The question
· The hypothesis
· The independent and dependent variables
· Results of Part A/Home Osmosis Lab (see pp. 9-13)
· Include completed table on p. 11 and pictures of setup and results (potato cubes before and after overnight incubation in salt solutions)
· Data Analysis
· TABLE 1: mass of potato plugs per time point. (this is provided for you for an online lab, see below)
· TABLE 2: percent change in mass per time point for each potato plug (you will need to prepare this as part of the assignment)
· GRAPH 1: percent change vs. time for each potato plug (you will need to prepare this as part of the assignment)
· TABLE 3: RATE of percent change (slope of each line) (you will need to prepare this as part of the assignment)
· GRAPH 2: RATE of percent change vs. solution molarity for each sample. (you will need to prepare this as part of the assignment)
· Summary of results
· Conclusion
For help with data analysis, tables and graphs, please watch this video!
Table 1: mass of potato cores in various NaCl solutions
NaCl Concentration (M)
Mass at various time points (g)
0 min
10 min
20 min
30 min
40 min
50 min
60 min
0
5.30
5.50
5.53
5.60
5.67
5.77
5.73
0.063
5.37
5.50
5.55
5.67
5.70
5.80
5.80
0.13
5.27
5.33
5.33
5.40
5.40
5.43
5.47
0.25
5.47
5.47
5.40
5.40
5.33
5.37
5.37
0.63
5.53
5.40
5.23
5.13
5.07
4.97
4.83
ANALYZING THE DATA
Calculating percent change in mass of potato cores:
Since each potato piece starts at a slightly different mass we can’t directly compare mass lost or gained fairly so we need to calculate what percent of the original mass was lost or gained and that percent change can be fairly compared between pieces.
Percent Change in Mass = x 100
1. Construct a table (Table 2) to record the percent change in mass for each solution/potato sample you are testing.
2. Calculate the cumulative percent change in mass for each time point, for each potato piece, and complete Table 2.
3. Construct a graph (Graph 1) using TAILS. Each solution tested should have a line on the graph, so 5 lines. Each line should have 7 points, one for each time point tested starting with zero. You can do your graph by hand, on graph paper, or on Excel. If you are doing this graph on Excel, display the equations of the lines of best fit on the graph. Insert the graph (or a picture) in your assignment.
Now you can complete Table 2 and Graph 1 in your assignment.
Calculating the RATE or SLOPE of percent change in mass of the potato piece:
Each potato core will have a rate at which it gained or lost mass in each NaCl concentration. Calculating the slope of the line for percent change in mass (Graph 1) gives you the rate.
Calculating the slope of a line:
The slope of a line is defined as the change in Y values divided by the change in X values between two points over a region of a graph where the line is linear.
Slope = where Δ means “change in”
Slope = = Rate of percent change in mass per minute
1. To determine the slope, you can do it one of two ways:
a. Manually: find two points on line of best fit of Graph 1 and use the equation above to find slope.
i. The point at an earlier time is point 1 [coordinates X1 and Y1] and the one at later time is point 2 [coordinates X2 and Y2] (you can also think of this as final – initial points).
ii. Write the X and Y coordinates next to the two points you chose on the graph.
iii. Find the difference between the Y coordinates and divide with the difference between the X coordinates to find the slope
b. Using Excel: using the equation of the line of best fit: y = mx + b, where m = slope
2. Do this for each of the 5 lines on your graph of cumulative percent change.
3. Construct a table (Table 3) to record the
rate of percent change in mass for each sample tested. You should have 5 calculated rates, one for each concentration.
4. Graph the rate of percent change for each sample (Graph 2) versus the concentration of NaCl solution. You can do the graph by hand, on graph paper, and insert a picture in your assignment, or do it on Excel.
a. Draw a line of best fit through your data (either by hand or by choosing “add trendline” on Excel).
b. Where the line crosses the X axis is where the solution is isotonic with the potato, i.e. they have the same solute concentration. In other words, when the rate of change in mass is zero, there is no gain or loss of water by the potato, suggesting that the NaCl concentration inside the potato is equal to the concentration of the solution. The cells are in equilibrium with their environment.
c. If you did Graph 2 on Excel, use the equation of the line to find the isotonic concentration: set y=0 in the equation of the line (y = mx + b) and solve for x.
Now you can complete Table 3 and Graph 2 in your assignment as well as find the isotonic point of the potato for NaCl.
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