Vapor Pressure and Heat of Vaporization Lab Calculator

Calculator Inputs

Sample Name

Temperature Unit

Pressure Unit

Trial 1 Temperature

Trial 1 Vapor Pressure

Trial 1 Pressure Correction

Trial 2 Temperature

Trial 2 Vapor Pressure

Trial 2 Pressure Correction

Prediction Temperature

Target Boiling Pressure

Accepted Heat Value

Extra Lab Rows

Use the selected units. Example: 70,31.16

Example Data Table

Trial	Temperature °C	Vapor Pressure kPa	Use
1	60	19.95	Lower temperature point
2	70	31.16	Extra regression point
3	80	47.37	Higher temperature point

Formula Used

Clausius Clapeyron linear form:
ln(P) = (-ΔHvap / R)(1 / T) + C

Slope form:
slope = (ln(P2) - ln(P1)) / ((1 / T2) - (1 / T1))

Heat of vaporization:
ΔHvap = -slope × R

Two point form:
ln(P2 / P1) = (-ΔHvap / R) × ((1 / T2) - (1 / T1))

Pressure prediction:
P3 = exp(slope × (1 / T3) + intercept)

Boiling temperature estimate:
T = 1 / ((ln(Ptarget) - intercept) / slope)

Percent error:
Percent error = |calculated - accepted| / accepted × 100

Temperature is converted to kelvin. Pressure is converted to kPa for logarithmic work.

How to Use This Calculator

Enter at least two measured vapor pressures and their temperatures.

Select the units used in your lab notebook.

Add pressure corrections when your experiment needs them.

Paste extra temperature and pressure rows for regression analysis.

Enter a prediction temperature to estimate vapor pressure there.

Enter a target pressure to estimate a boiling temperature.

Use the CSV or PDF buttons to save your report data.

Vapor Pressure Lab Calculation Guide

Purpose of the Lab

A vapor pressure lab links pressure, temperature, and molecular escape. As liquid warms, more molecules gain enough energy to enter the vapor phase. The measured pressure rises because vapor molecules strike the container walls more often. This calculator organizes those readings and turns them into usable lab values.

What the Calculator Solves

The main result is the molar heat of vaporization. It is found from the slope of a Clausius Clapeyron plot. The plot uses natural log pressure on the vertical axis. It uses reciprocal absolute temperature on the horizontal axis. A steeper negative slope means more energy is needed to vaporize the liquid.

Why Temperature Units Matter

All temperatures must be converted to kelvin before calculations. Celsius and Fahrenheit are common in lab notebooks, but they cannot be used directly in reciprocal temperature formulas. A small unit mistake can change the slope and give a poor heat value. The tool converts entered data before solving.

Pressure Corrections

Many experiments need pressure corrections. A manometer height difference, trapped air correction, or calibration offset may change the measured pressure. Use positive corrections when pressure should increase. Use negative corrections when pressure should decrease. The corrected pressure is the value used in the logarithm.

Using Several Trials

Two points can estimate heat of vaporization. More trials are better. Extra rows allow a linear regression fit. The regression gives slope, intercept, and R squared. A value near one suggests the data follows the expected straight line. A lower value may show leaks, unstable temperature, or reading errors.

Prediction and Boiling Point

After the slope and intercept are known, the calculator predicts vapor pressure at another temperature. It can also estimate the boiling temperature for a chosen pressure. Normal boiling point uses 101.325 kPa. Lower target pressure gives a lower boiling temperature. Higher target pressure gives a higher boiling temperature.

Reporting Results

Use the table, formula notes, and downloads for a clear lab report. Always include units, corrected pressures, kelvin temperatures, and assumptions. Compare your heat value with an accepted value only when the same substance is used. Discuss likely error sources, especially temperature lag, pressure leaks, and calibration limits during each careful lab run.

FAQs

What is vapor pressure?

Vapor pressure is the pressure made by vapor above a liquid. It rises as temperature rises. It shows how easily molecules escape from the liquid phase.

What is heat of vaporization?

Heat of vaporization is the energy needed to convert one mole of liquid into vapor. In this calculator, it is reported in kJ/mol.

Why must temperature be in kelvin?

The Clausius Clapeyron equation uses reciprocal absolute temperature. Kelvin starts at absolute zero, so it works correctly in this relationship.

Can I use more than two data points?

Yes. Add extra rows in temperature, pressure format. The calculator uses linear regression when more than two valid data points are available.

What does R squared mean here?

R squared shows how closely the lab data fits a straight line. Values closer to one usually mean a better Clausius Clapeyron trend.

What pressure should I use for normal boiling point?

Use 101.325 kPa for normal boiling point. Leave the target pressure blank, and the calculator uses that value by default.

How do pressure corrections work?

Corrections are added to measured pressure before calculation. Enter a negative value when the measured pressure should be reduced.

Why is my heat value negative?

A negative heat value usually means the data trend is reversed or inconsistent. Check pressure units, temperature units, and trial order.