Vapor Pressure Calculator

Have you ever wondered how scientists or engineers calculate the pressure of vapor above a liquid? Maybe you need to know the vapor pressure of water at 25°C, or pressure of propane at a certain temperature. Our Vapor Pressure Calculator is built for that exact purpose and takes away the stress of using complicated formulas manually. This tool helps students, chemists, lab workers, and even HVAC technicians find vapor pressure accurately using temperature or mole fractions.

Whether you need a water vapor pressure calculator in Celsius or Fahrenheit, or a propane vapor pressure calculator, this online tool makes it simple.

What is Vapor Pressure?

Vapor pressure is the pressure of a vapor in equilibrium with its liquid at a specific temperature. A liquid with a high vapor pressure will evaporate easily. Water has a certain vapor pressure depending on the temperature. For example, the vapor pressure of water at 25°C is 23.8 mmHg (or around 3.17 kPa).

As temperature increases, more molecules leave the liquid and enter the gas phase. This increases the vapor pressure. That is why vapor pressure and temperature are directly related.

Vapor Pressure Formula

There are two main popular formulas used in our vapor pressure calculator:

Clausius-Clapeyron Equation:

ln(P2 / P1) = ΔH / R × (1/T1 − 1/T2)

Here,

P1 = initial vapor pressure

P2 = final vapor pressure

T1 = initial temperature (K)

T2 = final temperature (K)

ΔH = enthalpy of vaporization

R = gas constant (8.3145 J/mol·K)

This formula allows you to calculate:

• Final pressure if temperatures are known
• Final temperature if pressure changes
• Enthalpy change if both sets of pressure and temperature are known

Raoult’s Law:

P_solution = P_solvent × X_solvent

Where,

P_solution = vapor pressure of solution

P_solvent = vapor pressure of pure solvent

X_solvent = mole fraction of solvent

This helps calculate saturation vapor pressure of a solution with solute particles, like sugar water or salt solution.

How to Use Our Vapor Pressure Calculator

1. Choose which formula you want to use: Clausius-Clapeyron or Raoult.
2. Enter temperature in Celsius, Kelvin, or Fahrenheit.
3. Enter pressure in Pa, atm, kPa, torr, bar, or PSI.
4. For Clausius-Clapeyron, you can choose to calculate P2, T2, or ΔH.
5. The result will appear instantly with units converted automatically.
6. You can also use it as a water vapor pressure calculator, or propane vapor pressure calculator by entering the enthalpy of that specific liquid.

Example of Calculation

Water has a vapor pressure of 102325 Pa at 280 K. Calculate the pressure at 263 K using Clausius-Clapeyron, with ΔH = 40660 J/mol.

ln(P2/102325) = (40660/8.3145) × (1/280 − 1/263)

After solving,

P2 ≈ 33090 Pa.

So the vapor pressure of water at 263 K is roughly 33090 Pa.

You can also convert this to atm or torr using our built-in pressure unit converter.

Vapor Pressure of Water at 25°C

At 25°C (which is 298.15 K), the vapor pressure of water is about 23.8 mmHg or 3.17 kPa. Our tool calculates it quickly when you input the temperature and select water’s enthalpy.

Final Verdict

Our Vapor Pressure Calculator is a powerful yet simple tool for calculating vapor pressure of liquids like water, ethanol, propane, or any mixture solution. You don’t need to remember complex formulas or worry about converting Fahrenheit to Kelvin. Everything is built in so the calculation is clean, quick, and accurate.

Whether you are on your phone or laptop, this calculator handles the math for you, including unit conversions for temperature, pressure and enthalpy.

FAQ

How do we calculate vapor pressure?

We can calculate vapor pressure using the Clausius-Clapeyron equation if we know temperature and enthalpy of vaporization. For solutions, we use Raoult’s law by calculating the mole fraction of the solvent.

What is the formula for vapor pressure?

The most used formula is ln(P2/P1) = ΔH/R × (1/T1 − 1/T2). For solutions, another formula is P_solution = P_solvent × X_solvent.

What is the vapor pressure of water at 70°F?

70°F is 21.1°C. The vapor pressure of water at 21.1°C is about 18.65 mmHg or 2.49 kPa.

What is the vapor pressure of water at 25 degrees?

At 25°C, the vapor pressure of water is about 23.8 mmHg or 3.17 kPa (0.0313 atm).

Can I calculate in Fahrenheit?

Yes, you can enter temperature in Fahrenheit and our vapor pressure calculator will convert to Kelvin automatically for accurate calculations.