Clausius-Clapeyron Vapor Pressure Calculator

Estimate vapor pressure at a new temperature with Clausius-Clapeyron

The Clausius-Clapeyron equation is one of the fastest ways to estimate how a liquid's equilibrium vapor pressure shifts when temperature changes. Instead of hunting through a full property table for every condition, you start from one trusted vapor-pressure point, combine it with an enthalpy of vaporization, and project the pressure to a nearby temperature. This calculator performs that projection directly. It is especially useful when you are sketching out laboratory work, comparing evaporation behavior, estimating headspace pressure, or solving phase-equilibrium homework where the substance stays the same but the temperature does not.

What the tool does is narrowly defined and very physical. It is not trying to reproduce an entire phase diagram or a multicomponent process model. It answers one focused question: if a pure substance has vapor pressure P₁ at temperature T₁, what vapor pressure P₂ should you expect at a new temperature T₂, assuming the enthalpy of vaporization is reasonably constant across that interval? That framing matters because it tells you exactly what belongs in the inputs. You are not entering unrelated pressures or arbitrary temperatures; you are entering a matched equilibrium pair for the same substance and asking how that equilibrium changes as the temperature moves.

A helpful way to read the result is to think of it as temperature sensitivity turned into pressure sensitivity. Vapor pressure does not climb in a simple straight line. For many substances it increases rapidly as temperature rises, which is why even a modest warming step can noticeably change evaporation, boiling tendency, or container pressure. The Clausius-Clapeyron relation captures that nonlinear behavior in a compact form, so a small change in temperature can be translated into a much larger change in pressure without needing to interpolate an entire table by hand.

What each input means in plain language

For the Clausius-Clapeyron vapor-pressure calculation, each input has a specific job. Reference Pressure P₁ is the known equilibrium vapor pressure at the reference condition. In practice, that number usually comes from a handbook, a data sheet, an experiment, or a previously trusted estimate. If you have a vapor-pressure point for water, ethanol, acetone, or another volatile liquid at one temperature, that value belongs in the first pressure field. The key requirement is consistency: the pressure must describe the same substance and the same liquid-vapor equilibrium point as the temperature beside it.

Reference Temperature T₁ is the temperature associated with that known pressure. New Temperature T₂ is the temperature where you want the estimate. Both must be entered in Kelvin because the formula uses reciprocal temperature, 1/T. Using Celsius directly would shift the scale by 273.15 and break the calculation. If you only have Celsius data, convert it first by adding 273.15. A quick built-in self-check is simple: if you enter the same number for T₁ and T₂, the answer should come back essentially equal to the reference pressure you started with.

Enthalpy of Vaporization ΔH_vap measures how much energy is required to vaporize one mole of the substance. Larger values usually make vapor pressure change more sharply with temperature because the exponential term becomes more sensitive. The form on this page accepts kJ/mol, while the gas constant in the script is stored in J/(mol·K), so the calculator converts ΔH_vap to joules internally by multiplying by 1000. That unit conversion is small, but it is crucial; skipping it would make the exponent wildly wrong even if the rest of the inputs were reasonable.

If you want a quick checklist before calculating, use this one:

• Use a real vapor-pressure reference pair for the same substance: P₁ at T₁.
• Enter both temperatures in Kelvin, not Celsius.
• Keep the pressure units consistent with the label shown here, which is kPa.
• Use an enthalpy of vaporization value that applies to the same phase change and is reasonable over the temperature range you are studying.

Those four checks catch most bad results before they happen. In other words, this calculator is usually less likely to fail than the inputs are. When an answer looks odd, the first thing to suspect is mixed units or a reference pressure that does not actually match the reference temperature.

Clausius-Clapeyron formula used for vapor pressure estimates

The integrated Clausius-Clapeyron equation used here can be written in several algebraically equivalent forms. A common version is shown below, because it directly relates two vapor-pressure points for the same substance:

The script rearranges that expression into a direct exponential calculation for P₂:

P₂ = P₁ × exp[(ΔH_vap × 1000 / R) × (1/T₁ − 1/T₂)]

That form makes the direction of the change easy to interpret. If the new temperature T₂ is higher than T₁, then 1/T₂ is smaller than 1/T₁. The bracketed temperature term becomes positive, the exponential factor becomes greater than one, and the estimated vapor pressure rises. If T₂ is lower than T₁, the exponent becomes negative and the estimated vapor pressure drops. So the model behaves in the direction your physical intuition expects.

That algebraic rearrangement is useful because it shows where the sensitivity comes from. The temperature difference appears inside an inverse term and then inside an exponential, so the response is nonlinear rather than additive. A 5 K shift at one starting point may barely move the result, while the same shift elsewhere can have a much stronger effect. The calculator is therefore best understood as a ratio-based estimator: it is tracking how much the new equilibrium pressure multiplies relative to the reference point, not how many kilopascals temperature simply adds.

The size of ΔH_vap matters directly in the exponent as well. Larger enthalpies make the pressure estimate more temperature-sensitive, while smaller enthalpies soften the change. That is why two substances that start at similar vapor pressures can diverge quickly once you move to a warmer or cooler condition. The equation is compact, but it is still carrying real thermodynamic meaning: the more energy it takes to escape the liquid phase, the more sharply the equilibrium pressure responds to temperature.

There is also a practical reason the calculator keeps the input unit in kJ/mol while the gas constant is handled in J/(mol·K). The exponent must be dimensionless, so the energy term and the gas constant need to be in compatible units before they are combined. Converting ΔH_vap inside the calculation keeps the displayed input friendly while preserving the math that the equation requires.

At the same time, the formula has limits. It assumes the enthalpy of vaporization does not change much over the temperature interval, which is a good approximation only over moderate ranges. When the temperature gap gets large, when the substance is near a critical region, or when exact design work matters, the estimate should be treated as a quick approximation rather than the final property value.

Worked example: estimating vapor pressure at a warmer temperature

Suppose you know an approximate vapor pressure of 3.17 kPa at 298.15 K and want an estimate at 308.15 K. Use an enthalpy of vaporization of 43.9 kJ/mol as an illustrative value. Plugging those numbers into the formula gives an exponent of about 0.575, so the new pressure estimate is:

P₂ ≈ 3.17 × e^0.575 ≈ 5.63 kPa

That result tells the story more clearly than the algebra alone. A 10 K increase does not merely add a fixed amount of pressure; it multiplies the original vapor pressure by about 1.78 in this example. If you are checking solvent loss, vent loading, or whether a liquid becomes easier to boil at the warmer condition, that multiplier is often more useful than the raw exponent itself.

The short table below keeps the same reference point and the same ΔH_vap while changing only the new temperature. The numbers are approximate and meant to show the trend rather than replace a full property table:

A useful sanity test comes from the special case where T₂ = T₁. Then the bracketed temperature term becomes zero, the exponential term becomes one, and the calculator should return P₂ = P₁. If it does not, a unit error or typing error is likely. That is the fastest reasonableness check available on this page.

How to read a Clausius-Clapeyron vapor-pressure result

Once you click the button, the result area reports the estimated vapor pressure at the new temperature, the pressure change relative to the reference point, and the constants used in the exponent. In everyday use, the most important part is usually the first line: the new vapor pressure in kPa. The second line helps you see whether the change is modest or dramatic. The third line reminds you what assumptions were fed into the exponent so you can document or audit the estimate later.

If the result is much larger than expected, do not assume the calculator is broken. A large change can be physically reasonable because vapor pressure is highly temperature-sensitive. Instead, walk through three checks. First, verify that both temperatures are truly in Kelvin. Second, confirm that ΔH_vap is in kJ/mol before you enter it. Third, make sure P₁ and T₁ are a matched pair for the same substance. Those three checks catch almost every outlier.

You should also know where the approximation becomes less trustworthy. The integrated Clausius-Clapeyron form is most reliable over moderate temperature ranges where ΔH_vap does not vary too much. Over a very wide range, real-property data can curve away from the simple constant-enthalpy model. Near critical conditions, over very large temperature jumps, or when precise design work is required, property tables or more detailed vapor-pressure correlations are better choices.

For teaching and quick estimation, though, this calculator does exactly what it should. It shows how one clean reference point can be projected to a nearby temperature, it highlights the nonlinear character of vapor pressure, and it gives you a consistent starting estimate that is easy to compare across scenarios.

Practical uses and assumptions for vapor-pressure estimates

People commonly use Clausius-Clapeyron estimates when they need a fast answer about volatility. Examples include checking whether a solvent bottle stored in a warmer room will build more headspace pressure, estimating how a vacuum distillation condition changes when the liquid warms slightly, comparing evaporation tendency between two nearby temperatures, or solving chemistry exercises about phase equilibrium. In all of those cases, the same core assumptions apply: one substance, one known vapor-pressure point, Kelvin temperatures, and a roughly constant enthalpy of vaporization over the temperature interval.

• Same substance throughout: the reference point and the new condition must describe the same material.
• Equilibrium vapor pressure: P₁ should be an equilibrium value, not an arbitrary operating pressure from an unrelated process line.
• Moderate range preferred: the farther T₂ moves from T₁, the more you should treat the result as an estimate rather than a property-table replacement.
• Rounded display: tiny differences caused by rounding are normal; what matters most is the order of magnitude and trend.

If you need to compare several temperatures, a useful habit is to keep P₁, T₁, and ΔH_vap fixed while changing only T₂. That isolates the temperature effect and makes the trend easier to interpret. If the output climbs very quickly as T₂ rises, that is not a flaw in the calculator. It is the behavior the equation is designed to reveal.