Water Potential Calculator

This calculator works out water potential the way biology courses define it, as the solute potential plus the pressure potential. Enter the ionisation constant, the molar concentration, the temperature and any pressure potential, and it returns the solute potential from -iCRT along with the total, in bars and in megapascals. A second mode compares two solutions, such as a cell and the bath it sits in, and says which way water moves and by how much. A third turns a known solute potential back into the molar concentration that produced it, which is what the potato core experiment asks for. The pressure constant is an input, so a course working in megapascals can set it to 0.00831.

Calculator Type

Solute potential from -iCRT, plus the pressure potential. Leave the pressure potential at zero for an open solution.

Particles per formula unit: 1 for sucrose or glucose, 2 for NaCl or KCl, 3 for CaCl2 or Na2SO4. Use a measured value such as 1.8 for NaCl if your course gives one.
Converted to kelvin as degrees C plus 273, which is the convention on the AP Biology formula sheet.
Zero for an open solution in a beaker. Positive inside a turgid cell pushing against its wall, negative for water under tension in xylem.
Leave this at 0.0831 unless your course uses another figure. Results are shown in bars and in megapascals either way.
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Two solutions at the same temperature, such as a cell and the bath it is sitting in. Give each one its molarity and its pressure potential.

Converted to kelvin as degrees C plus 273, which is the convention on the AP Biology formula sheet.
Leave this at 0.0831 unless your course uses another figure. Results are shown in bars and in megapascals either way.
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The other direction, and the one the potato core lab asks for: you found the molarity at which the tissue neither gained nor lost mass, so its solute potential matches that solution. Turn a solute potential back into a molarity.

Enter it as a negative number. If your figure is in megapascals, multiply by 10 first.
Particles per formula unit: 1 for sucrose or glucose, 2 for NaCl or KCl, 3 for CaCl2 or Na2SO4. Use a measured value such as 1.8 for NaCl if your course gives one.
Converted to kelvin as degrees C plus 273, which is the convention on the AP Biology formula sheet.
Leave this at 0.0831 unless your course uses another figure. Results are shown in bars and in megapascals either way.
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