Water potential & osmosis calculator

Water potential Ψ = Ψs + Ψp, the sum of solute potential and pressure potential, measured in megapascals. Solute potential is Ψs = −iCRT, where i is the number of particles per formula unit, C is molarity, R = 0.00831 L·MPa/(mol·K), and T is the temperature in kelvin. For 0.1 M sucrose at 25 °C (298.15 K): Ψs = −1 × 0.1 × 0.00831 × 298.15 = −0.248 MPa, and with no pressure Ψ = −0.248 MPa.

Water moves by osmosis from higher water potential to lower water potential, that is, toward the more negative number. A plant cell with Ψs = −0.743 MPa and turgor pressure Ψp = +0.5 MPa has Ψ = −0.243 MPa; placed in 0.5 M NaCl (Ψ = −2.478 MPa) it loses water to the solution and plasmolyzes. Pure water has Ψ = 0 MPa, so a red blood cell dropped into it gains water until it bursts.

The ionization constant (van ’t Hoff factor) counts how many particles each formula unit releases in solution: 1 for sucrose and glucose, 2 for NaCl (Na⁺ and Cl⁻), 3 for CaCl₂. Because osmosis responds to particle numbers, 0.3 M NaCl behaves like 0.6 M dissolved particles: Ψs = −1.487 MPa at 25 °C, twice the −0.743 MPa of 0.3 M sucrose. The calculator assumes complete dissociation, which is close to true for dilute salt solutions.