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ETD 25: A. Draw + Explain how to calculate a potato's molarity using the graph from the lab. B. Explain: How would you then calculate the Water Potential of that Potato?
November 12, 2013 at 4:30pm


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Brett Babbel
November 13, 2013 at 8:13pm

Mom, sister, and brother in law: Learned that the sweet potato has more sugar in it natural so it took a greater amount of concentration in the solution to make the sweet potato reach equilibrium. Learned that the more solutes in the solution the more negative the water potential is. So the greater concentration of solutes in the solution equals a lower negative water potential so the high the concentration the more water will be lost from the potato because the difference between the potatoes water potential and the solutions water potential is greater.

Student: I learned how important water potential is and how the concentration of solutes in the solutions effect the water potential by making it more negative. I also realized that because the sweet potato has more sugars in it then the other potatoes it takes a greater concentration from the solution to make the potato reach dynamic equilibrium.

Sophie Wulfing
November 13, 2013 at 8:31pm

Molarity is the amount of substance(moles) in a liter of water.  Experiment is learning how a substance transfers through a membrane.  Water potential is waters ability to move through a membrane (to do work). Water potential is inversely related to solute potential, and also related to pressure (but held constant in this experiment).  Experiment is starting with different molarities of water and looking at change in potato mass over time.  Negative slope graph; if change is >0, potato is receiving water, so molarity is greater in the potato than the water solution, i.e. hypo-tonic.  If change is < 0, potato is giving up water, so molarity is less in the potato, i.e. hyper tonic.  Zero change implies potato and water have the same molarity.  Sugars (sucrose) can't go through the membrane, but water can.


Sophie- I had to look up a hypertonic membrane vs, a hypotonic membrane. I didn't mention that when a membrane is hypertonic, the solution around it is hypotonic and vice versa. I also didn't explain solute potential =iCRT (i being the number of solution particles, C being concentration which is found with the graph, P meaning .0831 which is the pressure constant, and Temperature in Kelvin). I kept forgetting how each part of that equation cancels out, leaving you with the unit Bars.

Kelli Vetter
November 19, 2013 at 9:40pm

Parent: Looking at the graph, it was apparent that the sweet potato reached dynamic equilibrium when the molarity of the outer solution was higher(she showed me that equilibrium occurred when y=0 because the mass of the potato didn't change). I didn't know why, so Kelli explained to me that the sweet potato had more solutes than the other potatoes resulting in a higher equilibrium molarity concentration. She explained that a solution with less solutes is called a hypotonic solution, while a hypertonic solution has more solutes in the solution. *Hypo follows hyper and water follows solute*

Student: When I started explaining to my mom why the graph was the way it was, my mom gave me a look like she didn't understand. So I grabbed a blank piece of paper and drew out a membrane that showed the 1.0 molarity solution and the sweet potato. I drew out triangles to represent the solutes in both solutions. I showed my mom that the potato was hypotonic because it had less solutes than the solution outside of the potato. I then taught her that water follows solute (water potential moves from high to low). I then explained that when the potato was a .5 molarity because that is when the graph hit y=0, therefore showing dynamic equilibrium. Although I didn't discuss about water potential to my mom in depth, I didn't fully understand why water potential must be negative. I looked it up and realized that pure water is 0, so it makes sense because if pure water is 0 (no solutes) then if solutes are added, the water potential will be negative.

Kevin Turek
December 31, 2013 at 9:18am

Parent: My son showed me the graph he drew in his book. He said that you can tell the molarity of the potato by seeing where the graph crosses the x-axis. He explained that in the lab all the groups had different potatoes and that they dunked them in different molarity of sugar water and found the averages from that. He said they know it's molarity at the x axis because this meant it reached dynamic equilibrium.

Student: Basically to describe this one I just showed my mom the graph that we drew inside our ETD's.  I showed her that the potatoes reached dynamic equilibrium when they hit the x axis. She didn't quite understand that, so I told her that when we find its dynamic equilibrium we have found it's molarity. 

Madison Boggan
November 22, 2014 at 3:38pm

Parent: The potato's molarity is determined by where it's line based off of molarity of the solution and the percent change in mass crosses the x axis. Once this is determined it can be plugged into a simple formula to calculate water potential that Maddie explained to me. Water potential is negative then multiplied by the ionization constant, the pressure constant, the concentration molarity and the temperature. This sounds like a process that Madison enjoys as her brain works well with formulas. 

Student: Explaining the formula to my mom helped me to process the formula verbally rather than just the letters or the numbers that would be filled into each one. It helps me to understand why the formula is the way that it is rather than just memorizing it. 

Anne Sweeney
January 20, 2016 at 8:30pm

Parent: Anne showed me a graph and taught me that the lines represented the molarity of the potato and when the line crossed the x axis it was the percent change in mass. Using the data from the graph you can use the water potential formula, Water potential(neg) multiplied by (ionization constant)(concentration molarity)(pressure constant) (temp). Also water potential is the ability for water to do work through a membrane. Anne seems to understand the differences between hypo, hyper and isotonic very well!

Student: To explain the ETD better I drew the graph like the one we did in class.The lines helped me understand that an upward line is hypotonic, the middle line is isotonic and the downward line is hypertonic. As I drew out the lines, I realized why they are shaped that way due to their different factors. Explaining the equation for water potential helped me understand it, rather than just memorizing an equation.

Anne Sweeney
January 20, 2016 at 8:40pm

Parent: Anne showed me the graph of the different temperatures of the non.germinating vs germinating beans over time and CO2 production. This graph was an example of cellular reproduction. The four main points of cellular respiration were that an increase in temp. makes an increase in cellular respiration, plants do cellular respiration, animals have a higher rate of CR than plants and rate of metabolism is the slope (change in y/change in x). I also learned of the different paths of carbon, elections and energy creation/use  through cellular respiration. It seems like a very important process to know!

Student: I drew a graph showing the CO2 (ppm) produced over time. With each line I explained how different elements affected the slope (change in y over change in x). For example a cold-blooded animal would change its production due to the surrounding environment, while the warm-blooded animal would be producing by itself so the slope would always be steeper. his concept my mom understood but explaining the process of CR and the tracking of carbon, electrons and energy was more difficult. I definitely referred to the poster we did in class and how that process worked.

Katarina Zosel
November 26, 2017 at 11:17pm

Parent- how to determine malarity-molar concentration. seems like there was a thanksgiving abundance of potatoes. Katarina's lab group weiged and then put red potatoes into different solutions with different concentrations and then weighed them again after. The concentration in the potato vs in the solution. It gained weight in the .2 liquid, .4 and the 0- basically where the potatoe was more sugary than the water solution it was in.

Water potential- how much water can go into something through difusion. Water is 0- but if something has sugar in it... it will be  a - number. Higher water potential to a lower water potential. We had an interesting discussion about being the bath too long. Water potential = - iRCT I won't go into detail but I do feel a bit smarter than when we started.

Student- I began by describing the lab we did in class and what would cause a potato to gain or lose weight based on the concentration gradient. Then I shared the results from the lab showing how where the best fit line crossed zero the concentration on the x axis would be that of the potato as it would not change weight in that exact solution. Explaining the water potential was more difficult as I had to explain what water potential was and how it moves from a higher water potential to a lower before I could go into the equation, where I explained what each piece meant- being sure to remember r was 0.0831 and t was in kelvins.

Hitesh Boinpally
December 30, 2017 at 11:22pm

Parent: A. Hitesh explained potato lab and showed shared the graph from lab. It was interesting that this experiment proved that sweet potatoes have highest sucrose concentration. Hence, they stopped absorbing water at higher solvent sucrose concentration compared to all other potatoes. 

B. It was interesting to know how temperature unit K works as we need to add +273 to convert from Celsius.

Student: I was able to better appreciate the importance of having a semi-permeable membrane and how that allows for our experiment to properly run and helps the potato stay in a hypotonic solution like it wants to be. Furthermore, I clarified the equations for water and solute potential and was able to make sure I understood every part of the equation. Overall, this was a fairly straightforward lesson, but it was hard to explain all of the various specific terminology to my parents since they didn't have the same chemistry background that I did. However, it did help to clarify some specific parts of the calculations of molarity and the lab as a whole.

Zach Holtz
November 18, 2019 at 10:52pm

Student:   It was good review for me to go over with my mom and explain to her how to find the molarity on the graph. First you have to conduct an experiment by using different molar solutions and finding initial mass and final mass of whatever you're finding the molarity of. Once you have the initial and final mass you can find the % change and graph that. Whenever your best fit line hits the x-axis is where it tells you the molarity. Once you have the molarity you plug it in to the equation Ψ=-iCRT. Once you plug everything in you get your water potential.

Parent:   i = ionization constant (sucrose = 1); C = concentration molar (the molarity of your test specimen; R = pressure constant (0.0831 (bar*Liters/mol*Kelvin)); T = temperature (Kelvin = 273 + °Celsius).  I learned that finding molarity you have to find the x-intercept of the best fit lines.  

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