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mmHg
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kPa
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atm
Enter values to see results
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mmHg
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kPa
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atm
The Vapor Pressure Calculator uses the Antoine equation to compute the vapor pressure of common substances at any given temperature. Select from eight substances including water, alcohols, and organic solvents, then enter the temperature to obtain the vapor pressure in mmHg, kPa, and atm. This tool is indispensable for chemical engineers designing distillation columns, environmental scientists assessing volatile organic compound emissions, and students studying phase equilibria.
Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its liquid phase at a given temperature. It is a measure of a liquid's tendency to evaporate and increases exponentially with temperature.
The Antoine equation is a semi-empirical correlation for vapor pressure as a function of temperature:
$$\log_{10}(P) = A - \frac{B}{C + T}$$
where \(P\) is the vapor pressure (typically in mmHg), \(T\) is the temperature in °C, and \(A\), \(B\), \(C\) are substance-specific constants determined experimentally. This equation is a simplified form of the Clausius-Clapeyron equation and provides excellent accuracy over a specified temperature range.
Unit conversions are applied as follows:
$$P_{kPa} = P_{mmHg} \times 0.133322$$
$$P_{atm} = \frac{P_{mmHg}}{760}$$
The Antoine constants for water differ between two temperature ranges (1-100°C and 100-374°C) because a single set of three parameters cannot accurately describe the entire vapor pressure curve. For each substance, the constants are optimized for a specific temperature range, and using them outside this range may introduce significant errors.
A higher vapor pressure at a given temperature indicates a more volatile substance. When the vapor pressure equals the atmospheric pressure, the liquid boils. Substances like acetone and chloroform have high vapor pressures at room temperature, explaining why they evaporate quickly. Water's vapor pressure at 25°C is about 23.8 mmHg (3.17 kPa), which is relatively low, reflecting its strong hydrogen bonding.
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Water's vapor pressure at 25°C is about 23.8 mmHg, which corresponds to about 3.17 kPa. This value is used in humidity calculations — 100% relative humidity means the air holds water vapor at this pressure.
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Acetone has a vapor pressure of about 185 mmHg at 20°C — nearly 8 times that of water. This explains why acetone evaporates rapidly and why you can smell it easily at room temperature.
The Antoine equation is an empirical formula that relates the vapor pressure of a pure substance to temperature: log₁₀(P) = A - B/(C+T). It uses three substance-specific constants (A, B, C) determined from experimental data. It is one of the most widely used correlations in chemical engineering.
Vapor pressure is the pressure exerted by the vapor of a substance in equilibrium with its liquid (or solid) phase in a closed system. It represents the tendency of molecules to escape from the liquid surface. Higher vapor pressure means the substance is more volatile.
As temperature increases, more molecules in the liquid have enough kinetic energy to escape into the vapor phase. This increases the number of gas-phase molecules and thus the pressure they exert. The relationship is exponential — vapor pressure roughly doubles for every 10°C increase for many substances.
The Antoine equation is accurate only within its specified temperature range. It cannot be used near the critical point or at very low temperatures. For wider ranges, more complex equations like the Wagner equation or extended Antoine equation with additional parameters are preferred.
The constants A, B, and C are unique to each substance and determined by fitting experimental vapor pressure data. Constant A mainly affects the magnitude, B relates to the enthalpy of vaporization, and C corrects for the temperature dependence. These constants are tabulated in databases like NIST and the DIPPR project.
A liquid boils when its vapor pressure equals the external (atmospheric) pressure. At sea level (760 mmHg), water boils when its vapor pressure reaches 760 mmHg, which occurs at 100°C. Under vacuum, boiling occurs at lower temperatures because the external pressure is lower.
Vapor pressure is the equilibrium pressure of a pure substance's vapor above its liquid. Partial pressure is the pressure that one component contributes to a gas mixture. In a mixture, the actual vapor pressure of a component is reduced according to Raoult's law: P_i = x_i × P_i°.
Water's vapor pressure curve cannot be accurately described by a single three-parameter Antoine equation over its entire liquid range. The constants for 1-100°C are optimized for lower temperatures, while the 100-374°C set is optimized for higher temperatures. Using the wrong set introduces significant errors.
Relative humidity is defined as the ratio of the actual partial pressure of water vapor in air to the saturation vapor pressure at that temperature, expressed as a percentage. At 100% relative humidity, the air is saturated and the partial pressure of water equals the vapor pressure.
Yes, solids can sublime (transition directly to gas) and have a vapor pressure, though it is usually much lower than that of the liquid phase. Examples include dry ice (solid CO₂), naphthalene (mothballs), and iodine crystals, all of which visibly sublime at room temperature.
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