Can I solve for different variables?

Yes! This calculator supports reverse solving. You can solve for: Concentration (c), Molecular weight (M), Number of ions (n), Osmotic coefficient (Φ), Osmotic pressure (π), Solute mass (m), Temperature (T), Volume of solution (V).

Osmotic pressure

Solve osmotic pressure relationships quickly using core chemistry formulas.

Formula shown7 inputs definedUpdated Nov 2025By Automated Chemistry GeneratorFree, no sign-up

Quick Summary

Use this when you need:

Estimate the osmotic pressure exerted by intravenous solutions
Determine how concentration or temperature changes Π in desalination
Back-calculate solute molarity from a measured osmotic pressure

You’ll need:Molecular weight (M), Number of ions (n), Osmotic coefficient (Φ), Osmotic pressure (π), and 3 more

You’ll get:Concentration (c)

How this is calculated

Formula:

Π = i M R T

In plain language:

The van ’t Hoff equation states that osmotic pressure Π equals the van ’t Hoff factor i times molarity M times the gas constant R and absolute temperature T, mirroring the ideal gas law for solutions.

The formula is always visible so you can verify the math and explain your numbers to anyone who asks.

What these terms mean

Concentration (c)

Concentration (c) used in the calculation.

Molecular weight (M)

Molecular weight (M) used in the calculation.

Number of ions (n)

Number of ions (n) used in the calculation.

Osmotic coefficient (Φ)

Osmotic coefficient (Φ) used in the calculation.

Osmotic pressure (π)

Osmotic pressure (π) used in the calculation.

Solute mass (m)

Solute mass (m) used in the calculation.

Temperature (T)

Temperature (T) used in the calculation.

Volume of solution (V)

Volume of solution (V) used in the calculation.

When to use this calculator

Estimate the osmotic pressure exerted by intravenous solutions
Determine how concentration or temperature changes Π in desalination
Back-calculate solute molarity from a measured osmotic pressure

Last updated: November 19, 2025

Osmotic pressure calculator explained

Osmotic pressure ( $\\Pi$ ) describes the pressure required to stop solvent flow across a semipermeable membrane. For dilute solutions it follows the van't Hoff equation $\\Pi = i \\Phi M R T$ , where $i$ is the van't Hoff factor, $\\Phi$ is the osmotic coefficient, $M$ is molarity, $R$ is the gas constant, and $T$ is absolute temperature. This calculator computes concentration {concentration} from solute mass {solute_mass}, molar mass {molecular_weight}, and solution volume {volume}, then multiplies by {number_of_particles}, {osmotic_coefficient}, and $R T$ to report osmotic pressure {osmotic_pressure}.

Use it to size reverse-osmosis systems, verify intravenous solution tonicity, or back-calculate solute molar masses from osmotic pressure measurements.

How the conversion works

Steps:

Compute molarity $M = \\frac{m}{M_w V}$ .
Apply $\\Pi = i \\Phi M R T$ with $R = 0.08314$ L bar mol $^{-1}$ K $^{-1}$ (or the unit version you prefer).

The calculator exposes $i$ as {number_of_particles} (number of ions) and the non-ideality correction as {osmotic_coefficient}.

Units and conversions

Quantity	Units	Notes
Solute mass $m$	g	Convert mg to g before entering.
Molecular weight $M_w$	g/mol	Use the apparent molar mass for polymers.
Volume $V$	L	Solution volume after dilution.
$i$	dimensionless	1 for nonelectrolytes, ~2 for NaCl, etc.
$\\Phi$	dimensionless	Accounts for deviations from ideality.
Temperature $T$	K	Convert deg C to K.
$R$	L bar mol $^{-1}$ K $^{-1}$	Built in as 0.08314.
Osmotic pressure $\\Pi$	bar (or the unit implied by your $R$ )	Convert to kPa by multiplying by 100.

Worked examples

Physiological saline
0.15 M NaCl ( $i \\approx 2$ , $\\Phi = 0.93$ ) at 310 K.

\\Pi = 2 \\times 0.93 \\times 0.15 \\times 0.08314 \\times 310 = 7.2\\ \\text{bar}

Equivalent to 720 kPa, matching the osmotic pressure of blood plasma.

Polymer molar mass from osmotic pressure
Dissolve 0.250 g of an unknown polymer in 100 mL solution. At 298 K the osmotic pressure is 0.450 bar ( $i = 1$ , $\\Phi = 1$ ).

First find molarity using $M_w$ as the unknown:

M = \\frac{0.250}{M_w \\times 0.100}

Plug into $\\Pi$ :

0.450 = 1 \\times 1 \\times \\frac{0.250}{M_w \\times 0.100} \\times 0.08314 \\times 298

M_w = \\frac{0.250 \\times 0.08314 \\times 298}{0.450 \\times 0.100} = 13{,}800\\ \\text{g}\\,\\text{mol}^{-1}

Entering the same numbers in the calculator reproduces the result.

Tips and pitfalls

Use absolute temperature; a 10 K error changes $\\Pi$ by the same percentage.
Adjust $i$ for partial dissociation or pairing when dealing with multivalent electrolytes.
Osmotic coefficients deviate from 1 when ionic strength exceeds about 0.1 M; use tabulated values for accuracy.
Ensure solution volume reflects final volume, not solvent volume alone, when computing molarity.

References and further reading

Frequently Asked Questions

Was this calculator helpful?

Your feedback helps us improve calculators for everyone.

Found an error?

Report incorrect formulas, unit conversions, or worked examples.

Report an issue

Calculator info

Version:: v1.0.0
Last reviewed:: Nov 15, 2025
Sensitivity:: low
References:: 1

Last updated: November 19, 2025

Osmotic pressure calculator explained

Use it to size reverse-osmosis systems, verify intravenous solution tonicity, or back-calculate solute molar masses from osmotic pressure measurements.

How the conversion works

Steps:

Compute molarity $M = \\frac{m}{M_w V}$ .
Apply $\\Pi = i \\Phi M R T$ with $R = 0.08314$ L bar mol $^{-1}$ K $^{-1}$ (or the unit version you prefer).

The calculator exposes $i$ as {number_of_particles} (number of ions) and the non-ideality correction as {osmotic_coefficient}.

Units and conversions

Quantity	Units	Notes
Solute mass $m$	g	Convert mg to g before entering.
Molecular weight $M_w$	g/mol	Use the apparent molar mass for polymers.
Volume $V$	L	Solution volume after dilution.
$i$	dimensionless	1 for nonelectrolytes, ~2 for NaCl, etc.
$\\Phi$	dimensionless	Accounts for deviations from ideality.
Temperature $T$	K	Convert deg C to K.
$R$	L bar mol $^{-1}$ K $^{-1}$	Built in as 0.08314.
Osmotic pressure $\\Pi$	bar (or the unit implied by your $R$ )	Convert to kPa by multiplying by 100.

Worked examples

Physiological saline
0.15 M NaCl ( $i \\approx 2$ , $\\Phi = 0.93$ ) at 310 K.

\\Pi = 2 \\times 0.93 \\times 0.15 \\times 0.08314 \\times 310 = 7.2\\ \\text{bar}

Equivalent to 720 kPa, matching the osmotic pressure of blood plasma.

Polymer molar mass from osmotic pressure
Dissolve 0.250 g of an unknown polymer in 100 mL solution. At 298 K the osmotic pressure is 0.450 bar ( $i = 1$ , $\\Phi = 1$ ).

First find molarity using $M_w$ as the unknown:

M = \\frac{0.250}{M_w \\times 0.100}

Plug into $\\Pi$ :

0.450 = 1 \\times 1 \\times \\frac{0.250}{M_w \\times 0.100} \\times 0.08314 \\times 298

M_w = \\frac{0.250 \\times 0.08314 \\times 298}{0.450 \\times 0.100} = 13{,}800\\ \\text{g}\\,\\text{mol}^{-1}

Entering the same numbers in the calculator reproduces the result.

Tips and pitfalls

Use absolute temperature; a 10 K error changes $\\Pi$ by the same percentage.
Adjust $i$ for partial dissociation or pairing when dealing with multivalent electrolytes.
Osmotic coefficients deviate from 1 when ionic strength exceeds about 0.1 M; use tabulated values for accuracy.
Ensure solution volume reflects final volume, not solvent volume alone, when computing molarity.

Quick Summary

Calculator Tool

How this is calculated

What these terms mean

Concentration (c)

Molecular weight (M)

Number of ions (n)

Osmotic coefficient (Φ)

Osmotic pressure (π)

Solute mass (m)

Temperature (T)

Volume of solution (V)

When to use this calculator

Osmotic pressure calculator explained

How the conversion works

Units and conversions

Worked examples

Tips and pitfalls

References and further reading

Frequently Asked Questions

Related Calculators

Molarity

Neutralization

Reconstitution

Resuspension

Calibration curve

Detention time

Was this calculator helpful?

Found an error?

Calculator info

Osmotic pressure calculator explained

How the conversion works

Units and conversions

Worked examples

Tips and pitfalls

References and further reading

Frequently Asked Questions

Related Calculators

Molarity

Neutralization

Reconstitution

Resuspension

Calibration curve

Detention time

Was this calculator helpful?

Found an error?

Calculator info