# Vapor Pressure Calculator

Free vapor pressure calculator. Calculate vapor pressure using the Clausius-Clapeyron equation or Antoine equation parameters.

## What this calculates

Calculate the vapor pressure of a substance at any temperature using either the Clausius-Clapeyron equation (with two known data points) or the Antoine equation (with empirical parameters).

## Inputs

- **Method** — options: Clausius-Clapeyron (two temperatures), Antoine Equation — Select the calculation method.
- **Known Vapor Pressure (P₁)** (mmHg) — min 0 — Known vapor pressure at temperature T₁ (e.g., water at 25°C = 23.8 mmHg).
- **Temperature T₁** (K) — min 0 — Temperature at which P₁ is known.
- **Temperature T₂** (K) — min 0 — Temperature at which to calculate vapor pressure.
- **ΔH_vap (Heat of Vaporization)** (kJ/mol) — min 0 — Enthalpy of vaporization in kJ/mol (water = 40.67).
- **Antoine A** — Antoine parameter A (water: 8.07131 for mmHg, °C).
- **Antoine B** — Antoine parameter B (water: 1730.63).
- **Antoine C** — Antoine parameter C (water: 233.426).
- **Temperature** (°C) — Temperature in Celsius for Antoine equation.

## Outputs

- **Vapor Pressure** (mmHg) — Calculated vapor pressure.
- **Vapor Pressure** (atm) — Vapor pressure in atmospheres.
- **Calculation** — formatted as text — Step-by-step calculation.

## Frequently Asked Questions

**Q: What is vapor pressure?**

A: Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid (or solid) at a given temperature. When vapor pressure equals atmospheric pressure, the liquid boils.

**Q: What is the Clausius-Clapeyron equation?**

A: The Clausius-Clapeyron equation relates vapor pressure to temperature: ln(P₂/P₁) = -(ΔH_vap/R)(1/T₂ - 1/T₁). It uses a known vapor pressure at one temperature to calculate at another.

**Q: What is the Antoine equation?**

A: The Antoine equation is an empirical formula: log₁₀(P) = A - B/(C+T). A, B, and C are substance-specific constants determined experimentally. It is more accurate than the Clausius-Clapeyron equation over wider temperature ranges.

**Q: Why does boiling point change with altitude?**

A: At higher altitudes, atmospheric pressure is lower. Since a liquid boils when its vapor pressure equals atmospheric pressure, it boils at a lower temperature at higher altitudes. This is why water boils below 100°C on mountains.

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Source: https://vastcalc.com/calculators/chemistry/vapor-pressure
Category: Chemistry
Last updated: 2026-04-21