ChemBench

Osmotic Pressure Calculator

The pressure that drives osmosis.

Van 't Hoff factor (i)

Molarity (mol/L)

Temperature (K)

Osmotic pressure

2.447 atm

What you entered

Van 't Hoff factor (i): 1Molarity (mol/L): 0.1Temperature (K): 298

1
Π = iMRT
1 × 0.1 × 0.0821 × 298= 2.4466 atm

Result

Osmotic pressure: 2.447 atm

This solution's osmotic pressure is 2.447 atm.

Osmotic pressure of common solutions at 25 C and body temperature

Each row applies the van 't Hoff equation the calculator uses, Pi = i x M x R x T, with R = 0.0821 L atm per mol per K. Temperatures are 298 K for 25 C bench conditions and 310 K for 37 C body temperature. The van 't Hoff factor i counts the particles each formula unit releases in solution.

Solution	van 't Hoff factor (i)	Molarity (M)	Temperature (K)	Osmotic pressure (atm)
Glucose, 0.10 M	1	0.10	298	2.45
Urea, 0.20 M	1	0.20	298	4.89
5% dextrose IV fluid	1	0.278	310	7.08
Sucrose, 0.50 M	1	0.50	298	12.23
Sodium chloride, 0.10 M	2	0.10	298	4.89
Normal saline, 0.9% NaCl	2	0.154	310	7.84
Potassium chloride, 0.15 M	2	0.15	310	7.64
Sodium chloride, 0.30 M	2	0.30	310	15.27
Calcium chloride, 0.10 M	3	0.10	298	7.34
Seawater, 0.60 M NaCl equivalent	2	0.60	298	29.36

Osmotic pressure is a colligative property: it counts dissolved particles, not what they are. That is why 0.10 M glucose and 0.10 M NaCl differ by exactly a factor of two, and why 0.10 M CaCl2 is three times the glucose figure. It also explains why the two clinically isotonic rows, 5% dextrose and 0.9% saline, land within a whisker of each other near 7.1 to 7.8 atm despite one being a sugar and the other a salt. The seawater row is why reverse-osmosis desalination has to push well past about 30 atm to move any water at all. These are ideal-solution values: real electrolytes fall slightly short of the whole-number van 't Hoff factor because of ion pairing, so measured pressures for the salt rows run a few percent below the table. Treat the clinical rows as chemistry illustrations, not as guidance on preparing or administering anything.

The pressure that opposes water's natural flow

Osmotic pressure is the exact pressure that would need to be applied to a solution to stop water from flowing into it across a semi-permeable membrane — it depends on how many dissolved particles are present, not what they are.

Why it matters biologically

Osmotic pressure governs how cells gain or lose water in different solutions — IV fluids are carefully formulated to match blood's osmotic pressure so they don't cause red blood cells to swell or shrink.

Frequently asked questions

A 0.30 M NaCl solution at 37°C — what is the osmotic pressure?

Π = iMRT = 2 × 0.30 × 0.08206 × 310 = 15.3 atm. That is roughly the osmotic pressure of blood plasma.

How is osmotic pressure related to boiling point elevation?

Both are colligative properties that depend on the number of dissolved particles, not their identity. Osmotic pressure uses molarity and is measured in atm, while boiling point elevation uses molality and gives a temperature change. Use the boiling point elevation calculator for temperature effects.

What is the van 't Hoff factor for glucose vs NaCl?

Glucose does not dissociate, so i = 1. NaCl splits into Na+ and Cl-, so i = 2. CaCl2 gives i = 3. The higher the van 't Hoff factor, the greater the osmotic pressure for the same molar concentration.