Osmotic Pressure Calculator

How it is calculated

The van't Hoff equation gives osmotic pressure for a dilute solution: Π = i × M × R × T. i is the van't Hoff factor (roughly the number of particles one formula unit gives, such as 2 for NaCl, 1 for glucose; blank or invalid values are treated as 1), M is molarity in mol/L, R = 0.0820573 L·atm/(mol·K) and T is the absolute temperature, calculated as °C + 273.15. The result is in atmospheres and also converted to kilopascals (× 101.325). Molarity cannot be negative. The equation is ideal-solution behaviour, so it is least accurate at high concentrations or for strongly interacting ions, where the effective i is below the ideal value.

Common mistakes

• Typing the temperature in kelvin when the field expects °C; the tool adds 273.15 itself.
• Using molality or percent concentration instead of molarity (mol/L).
• Forgetting the van't Hoff factor for salts, which makes the pressure half or a third of the real value.
• Assuming the ideal i for concentrated solutions where ions pair up.
• Mixing units by reading the answer in atmospheres when you need kPa, or vice versa.