Water Potential Calculator
Calculate solute potential and total water potential using the exact formula from the College Board AP Biology exam reference sheet.
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Water potential is always ≤ 0 for a solution (pure water in an open, unpressurized container is exactly 0). Water moves from higher (less negative) to lower (more negative) water potential.
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How Water Potential Is Calculated
Water potential measures the tendency of water to move from one area to another, combining solute concentration and physical pressure — governed by the official AP Biology exam reference formula.
How to Calculate Water Potential
Solute Potential (Ψs) = −iCRT, where i is the ionization constant, C is molar concentration, R is 0.0831 L·bar/mol·K, and T is temperature in Kelvin. Total Water Potential (Ψ) = Ψs + Ψp (pressure potential).
Water Potential Calculator Formula
- Temperature (K) = Temperature (°C) + 273
- Solute Potential = −i × C × 0.0831 × Temperature (K)
- Water Potential = Solute Potential + Pressure Potential
Why Is Water Potential Always Negative or Zero?
Pure water in an open, unpressurized container has a water potential of exactly 0 — the highest possible value. Adding any solute always lowers the solute potential component, so any actual solution has a water potential at or below 0.
Water Potential Examples
0.3M sucrose solution, 22°C, pressure potential 0: Solute potential = −1 × 0.3 × 0.0831 × 295 ≈ −7.35 bars. Total water potential = −7.35 bars.
Common Water Potential Calculation Mistakes
Forgetting to convert Celsius to Kelvin before applying the formula. Using the wrong ionization constant for ionic solutes — non-ionizing solutes like sucrose use i=1, while dissociating ionic compounds use a higher value. Ignoring pressure potential in contexts (like turgid plant cells) where it's not actually zero.
How to Use This Calculator
Enter the ionization constant (1.0 for non-ionizing solutes like sucrose), molar concentration, temperature in Celsius, and pressure potential (0 for an open, unpressurized system).
Formula & Methodology
Solute potential: Ψs = −iCRT, where i is the ionization constant, C is molar concentration, R is the pressure constant (0.0831 L·bar/mol·K), and T is temperature in Kelvin. Total water potential: Ψ = Ψs + Ψp.
Example: 0.3 M sucrose solution, 22°C, pressure potential 0
T = 22 + 273 = 295 K. Ψs = −1 × 0.3 × 0.0831 × 295 ≈ −7.35 bars. Ψ = −7.35 + 0 = −7.35 bars.
Uses the simplified two-term formula (Ψ = Ψs + Ψp), the standard form used in AP Biology and most introductory contexts. It omits gravitational potential (Ψg) and matric potential (Ψm), which are typically negligible for the cell/solution-level problems this formula is designed for, but become relevant in whole-plant or soil-water contexts.
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Frequently Asked Questions
- What is the water potential formula?
- Ψ = Ψs + Ψp, where Ψs (solute potential) = −iCRT, using i = ionization constant, C = molar concentration, R = 0.0831 L·bar/mol·K, and T = temperature in Kelvin.
- Why is water potential always negative or zero?
- Pure water in an open, unpressurized container has a water potential of exactly 0 — the highest possible value. Adding solute always lowers (makes more negative) the solute potential component, so any solution has a water potential at or below 0.
- What ionization constant should I use?
- Use 1.0 for solutes that don't ionize in water, like sucrose. For ionic solutes, use the number of particles the compound dissociates into — for example, roughly 2 for a fully dissociating 1:1 salt.
- Which direction does water move?
- Water moves from an area of higher (less negative) water potential to an area of lower (more negative) water potential — the same principle whether you're comparing two solutions or a cell to its environment.
Last updated 7/12/2026