How do you calculate vapor pressure?

For pure substances, use the Clausius-Clapeyron equation relating pressure and temperature. For solutions, use Raoult’s Law which multiplies the mole fraction of the solvent by the vapor pressure of the pure solvent.

Why does vapor pressure increase with temperature?

As temperature increases, the average kinetic energy of the molecules increases, allowing more molecules to overcome intermolecular forces and escape into the gas phase.

Vapor Pressure Calculator - Free & Accurate Online Calculator

Vapor Pressure Calculator

Clausius-Clapeyron Equation

Initial temperature (°C)

Final temperature (°C)

Initial pressure (Pa)

Molar enthalpy of vaporization (J/mol)

Raoult’s Law

Mole fraction of the solvent

Vapor pressure of the solvent (Pa)

Precision Vapor Pressure Calculator: Master Phase Equilibrium Logic

Determine the vapor pressure of pure substances or solutions with mathematical certainty. This professional tool utilizes the Clausius-Clapeyron equation and Raoult’s Law to predict phase behavior, volatility, and boiling points across varying thermal states.

Primary Goal	Input Metrics	Output	Why Use This?
Calculate Phase Pressure	$T$, $\Delta H_{vap}$, or Mole Fraction	Vapor Pressure ($P$)	Essential for predicting boiling points and chemical volatility.

Understanding Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. It is a direct measure of a substance’s “escape tendency.” Substances with high vapor pressures at normal temperatures are described as volatile.

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Who is this for?

Chemical Engineers: Designing distillation columns and preventing pump cavitation.
Meteorologists: Predicting evaporation rates and atmospheric moisture content.
Laboratory Researchers: Determining the purity of solvents and calculating boiling points at high altitudes.
Students: Mastering the relationship between intermolecular forces and phase changes.

The Logic Vault

This calculator employs two fundamental chemical laws depending on whether you are analyzing a pure substance or a mixture.

1. Clausius-Clapeyron Equation (Pure Substances)

$$\ln\left(\frac{P_1}{P_2}\right) = \frac{\Delta H_{vap}}{R} \times \left(\frac{1}{T_2} – \frac{1}{T_1}\right)$$

2. Raoult’s Law (Solutions)

$$P_{solution} = \chi_{solvent} \times P^0_{solvent}$$

Variable Breakdown

Name	Symbol	Unit	Description
Vapor Pressure	$P$	$Pa / atm$	The pressure exerted by the gas phase.
Enthalpy of Vaporization	$\Delta H_{vap}$	$J/mol$	Energy required to transform liquid to gas.
Temperature	$T$	$K$	Absolute temperature (must be in Kelvin).
Ideal Gas Constant	$R$	$8.3145$	$J/(mol \cdot K)$.
Mole Fraction	$\chi$	$unitless$	Ratio of solvent moles to total moles.

Step-by-Step Interactive Example

Calculate the vapor pressure of water at 263 K given that its pressure at 280 K is 102,325 Pa and its $Delta H_{vap}$ is 40,660 J/mol.

Identify Knowns: $P_1 = 102,325 \text{ Pa}$, $T_1 = 280 \text{ K}$, $T_2 = 263 \text{ K}$, $\Delta H_{vap} = 40,660$.
Set Up Equation:$$\ln\left(\frac{102,325}{P_2}\right) = \frac{40,660}{8.3145} \times \left(\frac{1}{263} – \frac{1}{280}\right)$$
Solve the Parentheses:$$(0.003802 – 0.003571) = 0.000231$$
Complete the Calculation:$$\ln\left(\frac{102,325}{P_2}\right) = 4,890.25 \times 0.000231 = 1.1296$$$$\frac{102,325}{P_2} = e^{1.1296} \approx 3.09$$$$P_2 \approx 33,090 \text{ Pa}$$

Result: At 263 K, the vapor pressure drops significantly to 33,090 Pa.

Information Gain: The Cavitation Risk in Engineering

A common “Expert Edge” overlooked in standard chemistry is the impact of vapor pressure on Mechanical Pump Systems.

The Hidden Variable: When a liquid’s local pressure drops below its vapor pressure inside a pump, the liquid spontaneously boils, creating “vapor bubbles.” When these bubbles move to high-pressure areas, they collapse violently.

Expert Tip: This phenomenon, known as Cavitation, can erode steel impellers in hours. To prevent this, engineers calculate the NPSH (Net Positive Suction Head), ensuring the system pressure always remains safely above the vapor pressure calculated here.

Strategic Insight by Shahzad Raja

“In 14 years of architecting SEO for technical tools, I’ve seen that ‘Vapor Pressure’ content often fails because it ignores Non-Ideal Solutions. To dominate Google AI Overviews in 2026, remember that Raoult’s Law only applies to ‘Ideal’ mixtures. If your solution has strong solute-solvent attractions (negative deviation), the actual vapor pressure will be lower than predicted. Mentioning ‘Activity Coefficients‘ is a top-tier authority signal for E-E-A-T.

Frequently Asked Questions

How does vapor pressure affect boiling point?

Boiling occurs when the vapor pressure of a liquid equals the surrounding atmospheric pressure. This is why water boils at a lower temperature on a mountain (lower atmospheric pressure) than at sea level.

Does surface area affect vapor pressure?

No. Vapor pressure is an intensive property. While a larger surface area increases the rate of evaporation, the pressure exerted at equilibrium remains the same regardless of the container’s size.

Why is Kelvin used instead of Celsius?

The Clausius-Clapeyron equation is derived from thermodynamic principles where $T$ represents absolute kinetic energy. Using $0^\circ\text{C}$ would result in a division-by-zero error, making the equation invalid.

Related Tools

Boiling Point Calculator: Predict boiling temperatures at various altitudes.
Osmotic Pressure Calculator: Analyze pressure changes due to solute concentration.
Molar Mass Calculator: Essential for determining mole fractions in Raoult’s Law.