Osmotic Pressure Calculator
Find the osmotic pressure of a solution.
Particles per formula unit
Results update as you type.
How it works
Osmotic pressure is the pressure that must be applied to a solution to prevent the inward flow of pure solvent across a semipermeable membrane, and this calculator finds it using the van 't Hoff equation. You enter the van 't Hoff factor, the molarity, and the absolute temperature in kelvin, and it returns the osmotic pressure in atmospheres. The equation, pressure equals the van 't Hoff factor times molarity times the gas constant times temperature, closely resembles the ideal gas law, because dissolved particles behave in some ways like gas molecules. Osmotic pressure is a colligative property, so it depends on the concentration of dissolved particles rather than their identity; the van 't Hoff factor accounts for solutes that dissociate into multiple ions. This concept is central to biology and medicine: cells rely on osmotic balance, intravenous fluids must be isotonic with blood to avoid damaging cells, and reverse osmosis, which pushes water against its osmotic pressure through a membrane, is how much of the world's drinking water is purified and desalinated. Chemists and biologists use the calculation to predict solvent flow and design solutions. Remember to use absolute temperature in kelvin. Enter the three values to compute the osmotic pressure.
Π = i · M · R · T
Osmotic pressure is the pressure needed to stop solvent from flowing across a semipermeable membrane into a solution. It equals the van 't Hoff factor times the molarity times the gas constant times the absolute temperature. Using R as 0.0821 litre-atmospheres per mole-kelvin gives the pressure in atmospheres. The equation mirrors the ideal gas law, treating dissolved particles like gas molecules exerting pressure.
Source:Chemistry (van 't Hoff equation for osmotic pressure)
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Worked example
How to use this calculator
- 1
Enter your values
Enter van 't hoff factor (i), molarity and temperature.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Osmotic pressure updates instantly as you type — no button to press.
Frequently asked questions
What is osmotic pressure?
Osmotic pressure is the pressure required to stop pure solvent from flowing through a semipermeable membrane into a solution by osmosis. It measures the tendency of solvent to move toward the more concentrated side. The greater the concentration of dissolved particles, the higher the osmotic pressure, following the van 't Hoff equation that relates it to concentration and temperature.
Why does the formula resemble the ideal gas law?
The van 't Hoff equation for osmotic pressure, pressure equals molarity times the gas constant times temperature, mirrors the ideal gas law because dilute dissolved particles behave much like gas molecules, each contributing to a pressure. This analogy, discovered by van 't Hoff, lets the same gas constant appear in both equations and makes osmotic pressure calculations parallel gas pressure calculations.
What is the van 't Hoff factor here?
The van 't Hoff factor counts how many particles each dissolved formula unit produces. A non-electrolyte like glucose stays as one particle, so its factor is one, while salt splits into two ions, doubling the osmotic pressure at the same molarity. Because osmotic pressure is colligative, this particle count directly scales the pressure the solution exerts.
How is osmotic pressure used in medicine?
Intravenous fluids and injections must have an osmotic pressure close to that of blood, making them isotonic, so that red blood cells neither swell and burst nor shrivel. Calculating osmotic pressure helps formulate safe solutions. The concept also explains how cells maintain water balance and how dehydration and fluid therapy affect the body at the cellular level.
What is reverse osmosis?
Reverse osmosis forces water from a concentrated solution through a semipermeable membrane by applying pressure greater than the osmotic pressure, leaving dissolved salts and impurities behind. It is the main technology for desalinating seawater and purifying drinking water. Knowing the osmotic pressure of the feed water tells engineers the minimum pressure the system must overcome to produce clean water.