Clausius-Clapeyron Vapor Pressure Calculator

Estimate how vapor pressure changes with temperature

The Clausius-Clapeyron equation is one of the standard shortcuts chemists and engineers use when they know a vapor pressure at one temperature and need a reasonable estimate at another. Instead of searching through a full data table for every possible temperature, you start from a known equilibrium point, combine it with an enthalpy of vaporization value, and project the vapor pressure to a new temperature. This calculator performs that step directly. It is especially useful for quick lab planning, distillation and evaporation discussions, storage checks, and homework problems where the temperature changes but the substance stays the same.

What the tool is doing is narrow and specific. It is not trying to model every detail of a phase diagram. It is answering one focused question: if a pure substance has vapor pressure P₁ at temperature T₁, what vapor pressure P₂ should we expect at a new temperature T₂, assuming the enthalpy of vaporization stays reasonably constant over that interval? That framing matters because it tells you what kind of input belongs here. You are not entering any random pressure or any random temperature. You are entering one known vapor-pressure reference point for the same substance, then asking how that equilibrium value shifts.

A helpful way to think about the result is that the calculator translates temperature sensitivity into pressure sensitivity. Vapor pressure does not rise in a simple linear way with temperature. For many substances it rises rapidly as temperature increases, which is why a modest warming step can dramatically increase evaporation tendency, headspace pressure, or the ease of boiling under fixed external conditions. The Clausius-Clapeyron relation captures that nonlinearity in a compact form.

What each input means in plain language

Reference Pressure P₁ is the known vapor pressure at the reference temperature. In practice, that number usually comes from a handbook, a specification sheet, an experiment, or a previously trusted calculation. If you have a value 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 reference 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 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 first by adding 273.15. A quick built-in self-check is this: if you enter the same number for T₁ and T₂, the answer should return essentially the same 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 mean vapor pressure changes more sharply with temperature because the exponential term becomes more sensitive. The form 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 important; skipping it would make the answer off by a factor of a thousand in the exponent.

If you want a compact 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 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 prevent most bad results. In other words, the calculator is usually less likely to fail than the inputs are. When answers look strange, the first suspect is almost always mixed units or a reference pressure that does not actually match the reference temperature.

Formula used by the calculator

The integrated Clausius-Clapeyron equation used here can be written in several algebraically equivalent forms. A common version is shown below. 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.

The two MathML blocks below are retained as general calculator templates. They are not the substance-specific law used by this page, but they are still a fair reminder that every calculator turns a set of inputs into a defined output and that weighted terms can matter more than others. For this tool, the actual Clausius-Clapeyron expression above is the formula to focus on.

There is also a practical interpretation hidden inside the equation. Temperature appears inside an inverse term and then inside an exponential, so the response is nonlinear. Equal 5 K steps do not always feel equally important. Depending on the starting temperature and the value of ΔH_vap, a small warm-up near room temperature can change vapor pressure far more than many people expect from casual intuition.

Worked example

Suppose you know an approximate vapor pressure of 3.17 kPa at 298.15 K and you 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 pattern rather than serve as a replacement for 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 the result and when to be cautious

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

People commonly use Clausius-Clapeyron estimates when they need a quick 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 phenomenon the equation is designed to reveal.