Raoult's Law Vapor Pressure Calculator

Calculate solution vapor pressure instantly using our Raoult's Law calculator. Enter mole fraction and pure solvent vapor pressure to get accurate results for chemistry, distillation, and solution analysis.

What is Raoult's Law?

Raoult's Law is a fundamental principle in physical chemistry that describes how the vapor pressure of a solution relates to the mole fraction of its components. This vapor pressure calculator applies Raoult's Law to determine solution vapor pressure quickly and accurately.

According to Raoult's Law, the partial vapor pressure of each component in an ideal solution equals the vapor pressure of the pure component multiplied by its mole fraction. This principle is essential for understanding solution behavior, distillation processes, and colligative properties in chemistry and chemical engineering.

When a solvent contains a non-volatile solute, the vapor pressure decreases compared to the pure solvent. Our Raoult's Law calculator provides the mathematical relationship to calculate this reduction, making it indispensable for solution chemistry applications.

Raoult's Law Formula and Calculation

Raoult's Law is expressed by the following equation:

$P_{solution} = X_{solvent} \times P^{\circ}_{solvent}$

Where:

• $P_{solution}$ is the vapor pressure of the solution (typically measured in kPa, mmHg, or atm)
• $X_{solvent}$ is the mole fraction of the solvent in the solution (dimensionless, ranging from 0 to 1)
• $P^{\circ}_{solvent}$ is the vapor pressure of the pure solvent at the same temperature (in the same pressure units)

The mole fraction ($X_{solvent}$) is calculated as:

$X_{solvent} = \frac{n_{solvent}}{n_{solvent} + n_{solute}}$

Where:

• $n_{solvent}$ is the number of moles of solvent
• $n_{solute}$ is the number of moles of solute

Understanding the Variables

1. Mole Fraction of Solvent ($X_{solvent}$):

   • This is a dimensionless quantity that represents the proportion of solvent molecules in the solution.
   • It ranges from 0 (pure solute) to 1 (pure solvent).
   • The sum of all mole fractions in a solution equals 1.

2. Pure Solvent Vapor Pressure ($P^{\circ}_{solvent}$):

   • This is the vapor pressure of the pure solvent at a specific temperature.
   • It's an intrinsic property of the solvent that depends strongly on temperature.
   • Common units include kilopascals (kPa), millimeters of mercury (mmHg), atmospheres (atm), or torr.

3. Solution Vapor Pressure ($P_{solution}$):

   • This is the resulting vapor pressure of the solution.
   • It's always less than or equal to the vapor pressure of the pure solvent.
   • It's expressed in the same units as the pure solvent vapor pressure.

Edge Cases and Limitations

Raoult's Law has several important edge cases and limitations to consider:

1. When $X_{solvent} = 1$ (Pure Solvent):

   • The solution vapor pressure equals the pure solvent vapor pressure: $P_{solution} = P^{\circ}_{solvent}$
   • This represents the upper limit of the solution's vapor pressure.

2. When $X_{solvent} = 0$ (No Solvent):

   • The solution vapor pressure becomes zero: $P_{solution} = 0$
   • This is a theoretical limit, as a solution must contain some solvent.

3. Ideal vs. Non-ideal Solutions:

   • Raoult's Law applies strictly to ideal solutions.
   • Real solutions often deviate from Raoult's Law due to molecular interactions.
   • Positive deviations occur when the solution vapor pressure is higher than predicted (indicating weaker solute-solvent interactions).
   • Negative deviations occur when the solution vapor pressure is lower than predicted (indicating stronger solute-solvent interactions).

4. Temperature Dependence:

   • The vapor pressure of the pure solvent varies significantly with temperature.
   • Raoult's Law calculations are valid at a specific temperature.
   • The Clausius-Clapeyron equation can be used to adjust vapor pressures for different temperatures.

5. Assumption of Non-volatile Solute:

   • The basic form of Raoult's Law assumes the solute is non-volatile.
   • For solutions with multiple volatile components, a modified form of Raoult's Law must be used.

How to Use the Vapor Pressure Calculator

Our Raoult's Law vapor pressure calculator is designed for quick and accurate calculations. Follow these steps to calculate solution vapor pressure:

1. Enter the Mole Fraction of the Solvent:

   • Input a value between 0 and 1 in the "Mole Fraction of Solvent (X)" field.
   • This represents the proportion of solvent molecules in your solution.
   • For example, a value of 0.8 means that 80% of the molecules in the solution are solvent molecules.

2. Enter the Pure Solvent Vapor Pressure:

   • Input the vapor pressure of the pure solvent in the "Pure Solvent Vapor Pressure (P°)" field.
   • Make sure to note the units (the calculator uses kPa by default).
   • This value is temperature-dependent, so ensure you're using the vapor pressure at your desired temperature.

3. View the Result:

   • The calculator will automatically compute the solution vapor pressure using Raoult's Law.
   • The result is displayed in the "Solution Vapor Pressure (P)" field in the same units as your input.
   • You can copy this result to your clipboard by clicking the copy icon.

4. Visualize the Relationship:

   • The calculator includes a graph showing the linear relationship between mole fraction and vapor pressure.
   • Your specific calculation is highlighted on the graph for better understanding.
   • This visualization helps to illustrate how the vapor pressure changes with different mole fractions.

Input Validation

The calculator performs the following validation checks on your inputs:

• Mole Fraction Validation:

  • Must be a valid number.
  • Must be between 0 and 1 (inclusive).
  • Values outside this range will trigger an error message.

• Vapor Pressure Validation:

  • Must be a valid positive number.
  • Negative values will trigger an error message.
  • Zero is allowed but may not be physically meaningful in most contexts.

If any validation errors occur, the calculator will display appropriate error messages and will not proceed with the calculation until valid inputs are provided.

Practical Examples

Let's walk through some practical examples to demonstrate how to use the Raoult's Law Calculator:

Example 1: Aqueous Solution of Sugar

Suppose you have a solution of sugar (sucrose) in water at 25°C. The mole fraction of water is 0.9, and the vapor pressure of pure water at 25°C is 3.17 kPa.

Inputs:

• Mole fraction of solvent (water): 0.9
• Pure solvent vapor pressure: 3.17 kPa

Calculation: $P_{solution} = X_{solvent} \times P^{\circ}_{solvent} = 0.9 \times 3.17 \text{ kPa} = 2.853 \text{ kPa}$

Result: The vapor pressure of the sugar solution is 2.853 kPa.

Example 2: Ethanol-Water Mixture

Consider a mixture of ethanol and water where the mole fraction of ethanol is 0.6. The vapor pressure of pure ethanol at 20°C is 5.95 kPa.

Inputs:

• Mole fraction of solvent (ethanol): 0.6
• Pure solvent vapor pressure: 5.95 kPa

Calculation: $P_{solution} = X_{solvent} \times P^{\circ}_{solvent} = 0.6 \times 5.95 \text{ kPa} = 3.57 \text{ kPa}$

Result: The vapor pressure of ethanol in the mixture is 3.57 kPa.

Example 3: Very Dilute Solution

For a very dilute solution where the mole fraction of the solvent is 0.99, and the pure solvent vapor pressure is 100 kPa:

Inputs:

• Mole fraction of solvent: 0.99
• Pure solvent vapor pressure: 100 kPa

Calculation: $P_{solution} = X_{solvent} \times P^{\circ}_{solvent} = 0.99 \times 100 \text{ kPa} = 99 \text{ kPa}$

Result: The vapor pressure of the solution is 99 kPa, which is very close to the pure solvent vapor pressure as expected for a dilute solution.

Raoult's Law Applications and Use Cases

Raoult's Law vapor pressure calculations have numerous applications across chemistry, chemical engineering, and industrial processes:

1. Distillation Processes

Distillation is one of the most common applications of Raoult's Law. By understanding how vapor pressure changes with composition, engineers can design efficient distillation columns for:

• Petroleum refining to separate crude oil into various fractions
• Production of alcoholic beverages
• Purification of chemicals and solvents
• Desalination of seawater

2. Pharmaceutical Formulations

In pharmaceutical sciences, Raoult's Law helps in:

• Predicting drug solubility in different solvents
• Understanding the stability of liquid formulations
• Developing controlled-release mechanisms
• Optimizing extraction processes for active ingredients

3. Environmental Science

Environmental scientists use Raoult's Law to:

• Model the evaporation of pollutants from water bodies
• Predict the fate and transport of volatile organic compounds (VOCs)
• Understand the partitioning of chemicals between air and water
• Develop remediation strategies for contaminated sites

4. Chemical Manufacturing

In chemical manufacturing, Raoult's Law is essential for:

• Designing reaction systems involving liquid mixtures
• Optimizing solvent recovery processes
• Predicting product purity in crystallization operations
• Developing extraction and leaching processes

5. Academic Research

Researchers use Raoult's Law in:

• Studying thermodynamic properties of solutions
• Investigating molecular interactions in liquid mixtures
• Developing new separation techniques
• Teaching fundamental concepts of physical chemistry

Alternatives to Raoult's Law

While Raoult's Law is a fundamental principle for ideal solutions, several alternatives and modifications exist for non-ideal systems:

1. Henry's Law

For very dilute solutions, Henry's Law is often more applicable:

$P_i = k_H \times X_i$

Where:

• $P_i$ is the partial pressure of the solute
• $k_H$ is Henry's constant (specific to the solute-solvent pair)
• $X_i$ is the mole fraction of the solute

Henry's Law is particularly useful for gases dissolved in liquids and for very dilute solutions where solute-solute interactions are negligible.

2. Activity Coefficient Models

For non-ideal solutions, activity coefficients ($\gamma$) are introduced to account for deviations:

$P_i = \gamma_i \times X_i \times P^{\circ}_i$

Common activity coefficient models include:

• Margules equations (for binary mixtures)
• Van Laar equation
• Wilson equation
• NRTL (Non-Random Two-Liquid) model
• UNIQUAC (Universal Quasi-Chemical) model

3. Equation of State Models

For complex mixtures, especially at high pressures, equation of state models are used:

• Peng-Robinson equation
• Soave-Redlich-Kwong equation
• SAFT (Statistical Associating Fluid Theory) models

These models provide a more comprehensive description of fluid behavior but require more parameters and computational resources.

History of Raoult's Law

Raoult's Law is named after French chemist François-Marie Raoult (1830-1901), who first published his findings on vapor pressure depression in 1887. Raoult was a professor of chemistry at the University of Grenoble, where he conducted extensive research on the physical properties of solutions.

François-Marie Raoult's Contributions

Raoult's experimental work involved measuring the vapor pressure of solutions containing non-volatile solutes. Through meticulous experimentation, he observed that the relative lowering of vapor pressure was proportional to the mole fraction of the solute. This observation led to the formulation of what we now know as Raoult's Law.

His research was published in several papers, with the most significant being "Loi générale des tensions de vapeur des dissolvants" (General Law of Vapor Pressures of Solvents) in Comptes Rendus de l'Académie des Sciences in 1887.

Evolution and Significance

Raoult's Law became one of the foundational principles in the study of colligative properties—properties that depend on the concentration of particles rather than their identity. Along with other colligative properties like boiling point elevation, freezing point depression, and osmotic pressure, Raoult's Law helped establish the molecular nature of matter at a time when atomic theory was still developing.

The law gained further significance with the development of thermodynamics in the late 19th and early 20th centuries. J. Willard Gibbs and others incorporated Raoult's Law into a more comprehensive thermodynamic framework, establishing its relationship with chemical potential and partial molar quantities.

In the 20th century, as understanding of molecular interactions improved, scientists began to recognize the limitations of Raoult's Law for non-ideal solutions. This led to the development of more sophisticated models that account for deviations from ideality, expanding our understanding of solution behavior.

Today, Raoult's Law remains a cornerstone of physical chemistry education and a practical tool in many industrial applications. Its simplicity makes it an excellent starting point for understanding solution behavior, even as more complex models are used for non-ideal systems.

Programming Examples for Vapor Pressure Calculations

Implement Raoult's Law calculations in various programming languages for automated vapor pressure analysis:

1' Excel formula for Raoult's Law calculation
2' In cell A1: Mole fraction of solvent
3' In cell A2: Pure solvent vapor pressure (kPa)
4' In cell A3: =A1*A2 (Solution vapor pressure)
5
6' Excel VBA Function
7Function RaoultsLaw(moleFraction As Double, pureVaporPressure As Double) As Double
8    ' Input validation
9    If moleFraction < 0 Or moleFraction > 1 Then
10        RaoultsLaw = CVErr(xlErrValue)
11        Exit Function
12    End If
13    
14    If pureVaporPressure < 0 Then
15        RaoultsLaw = CVErr(xlErrValue)
16        Exit Function
17    End If
18    
19    ' Calculate solution vapor pressure
20    RaoultsLaw = moleFraction * pureVaporPressure
21End Function
22

1def calculate_vapor_pressure(mole_fraction, pure_vapor_pressure):
2    """
3    Calculate the vapor pressure of a solution using Raoult's Law.
4    
5    Parameters:
6    mole_fraction (float): Mole fraction of the solvent (between 0 and 1)
7    pure_vapor_pressure (float): Vapor pressure of the pure solvent (kPa)
8    
9    Returns:
10    float: Vapor pressure of the solution (kPa)
11    """
12    # Input validation
13    if not 0 <= mole_fraction <= 1:
14        raise ValueError("Mole fraction must be between 0 and 1")
15    
16    if pure_vapor_pressure < 0:
17        raise ValueError("Vapor pressure cannot be negative")
18    
19    # Calculate solution vapor pressure
20    solution_vapor_pressure = mole_fraction * pure_vapor_pressure
21    
22    return solution_vapor_pressure
23
24# Example usage
25try:
26    mole_fraction = 0.75
27    pure_vapor_pressure = 3.17  # kPa (water at 25°C)
28    
29    solution_pressure = calculate_vapor_pressure(mole_fraction, pure_vapor_pressure)
30    print(f"Solution vapor pressure: {solution_pressure:.4f} kPa")
31except ValueError as e:
32    print(f"Error: {e}")
33

1/**
2 * Calculate the vapor pressure of a solution using Raoult's Law.
3 * 
4 * @param {number} moleFraction - Mole fraction of the solvent (between 0 and 1)
5 * @param {number} pureVaporPressure - Vapor pressure of the pure solvent (kPa)
6 * @returns {number} - Vapor pressure of the solution (kPa)
7 * @throws {Error} - If inputs are invalid
8 */
9function calculateVaporPressure(moleFraction, pureVaporPressure) {
10    // Input validation
11    if (isNaN(moleFraction) || moleFraction < 0 || moleFraction > 1) {
12        throw new Error("Mole fraction must be a number between 0 and 1");
13    }
14    
15    if (isNaN(pureVaporPressure) || pureVaporPressure < 0) {
16        throw new Error("Pure vapor pressure must be a positive number");
17    }
18    
19    // Calculate solution vapor pressure
20    const solutionVaporPressure = moleFraction * pureVaporPressure;
21    
22    return solutionVaporPressure;
23}
24
25// Example usage
26try {
27    const moleFraction = 0.85;
28    const pureVaporPressure = 5.95; // kPa (ethanol at 20°C)
29    
30    const result = calculateVaporPressure(moleFraction, pureVaporPressure);
31    console.log(`Solution vapor pressure: ${result.toFixed(4)} kPa`);
32} catch (error) {
33    console.error(`Error: ${error.message}`);
34}
35

1public class RaoultsLawCalculator {
2    /**
3     * Calculate the vapor pressure of a solution using Raoult's Law.
4     * 
5     * @param moleFraction Mole fraction of the solvent (between 0 and 1)
6     * @param pureVaporPressure Vapor pressure of the pure solvent (kPa)
7     * @return Vapor pressure of the solution (kPa)
8     * @throws IllegalArgumentException If inputs are invalid
9     */
10    public static double calculateVaporPressure(double moleFraction, double pureVaporPressure) {
11        // Input validation
12        if (moleFraction < 0 || moleFraction > 1) {
13            throw new IllegalArgumentException("Mole fraction must be between 0 and 1");
14        }
15        
16        if (pureVaporPressure < 0) {
17            throw new IllegalArgumentException("Pure vapor pressure cannot be negative");
18        }
19        
20        // Calculate solution vapor pressure
21        return moleFraction * pureVaporPressure;
22    }
23    
24    public static void main(String[] args) {
25        try {
26            double moleFraction = 0.65;
27            double pureVaporPressure = 7.38; // kPa (water at 40°C)
28            
29            double solutionPressure = calculateVaporPressure(moleFraction, pureVaporPressure);
30            System.out.printf("Solution vapor pressure: %.4f kPa%n", solutionPressure);
31        } catch (IllegalArgumentException e) {
32            System.err.println("Error: " + e.getMessage());
33        }
34    }
35}
36

1#' Calculate the vapor pressure of a solution using Raoult's Law
2#'
3#' @param mole_fraction Mole fraction of the solvent (between 0 and 1)
4#' @param pure_vapor_pressure Vapor pressure of the pure solvent (kPa)
5#' @return Vapor pressure of the solution (kPa)
6#' @examples
7#' calculate_vapor_pressure(0.8, 3.17)
8calculate_vapor_pressure <- function(mole_fraction, pure_vapor_pressure) {
9  # Input validation
10  if (!is.numeric(mole_fraction) || mole_fraction < 0 || mole_fraction > 1) {
11    stop("Mole fraction must be a number between 0 and 1")
12  }
13  
14  if (!is.numeric(pure_vapor_pressure) || pure_vapor_pressure < 0) {
15    stop("Pure vapor pressure must be a positive number")
16  }
17  
18  # Calculate solution vapor pressure
19  solution_vapor_pressure <- mole_fraction * pure_vapor_pressure
20  
21  return(solution_vapor_pressure)
22}
23
24# Example usage
25tryCatch({
26  mole_fraction <- 0.9
27  pure_vapor_pressure <- 2.34  # kPa (water at 20°C)
28  
29  result <- calculate_vapor_pressure(mole_fraction, pure_vapor_pressure)
30  cat(sprintf("Solution vapor pressure: %.4f kPa\n", result))
31}, error = function(e) {
32  cat("Error:", e$message, "\n")
33})
34

1function solution_vapor_pressure = raoultsLaw(mole_fraction, pure_vapor_pressure)
2    % RAOULTS_LAW Calculate the vapor pressure of a solution using Raoult's Law
3    %
4    % Inputs:
5    %   mole_fraction - Mole fraction of the solvent (between 0 and 1)
6    %   pure_vapor_pressure - Vapor pressure of the pure solvent (kPa)
7    %
8    % Output:
9    %   solution_vapor_pressure - Vapor pressure of the solution (kPa)
10    
11    % Input validation
12    if ~isnumeric(mole_fraction) || mole_fraction < 0 || mole_fraction > 1
13        error('Mole fraction must be a number between 0 and 1');
14    end
15    
16    if ~isnumeric(pure_vapor_pressure) || pure_vapor_pressure < 0
17        error('Pure vapor pressure must be a positive number');
18    end
19    
20    % Calculate solution vapor pressure
21    solution_vapor_pressure = mole_fraction * pure_vapor_pressure;
22end
23
24% Example usage
25try
26    mole_fraction = 0.7;
27    pure_vapor_pressure = 4.58;  % kPa (water at 30°C)
28    
29    result = raoultsLaw(mole_fraction, pure_vapor_pressure);
30    fprintf('Solution vapor pressure: %.4f kPa\n', result);
31catch ME
32    fprintf('Error: %s\n', ME.message);
33end
34

Frequently Asked Questions About Vapor Pressure and Raoult's Law

What is Raoult's Law and how does it calculate vapor pressure?

Raoult's Law states that the vapor pressure of a solution equals the pure solvent vapor pressure multiplied by the solvent's mole fraction. The Raoult's Law formula is P = X × P°, where P is solution vapor pressure, X is mole fraction, and P° is pure solvent vapor pressure.

How do you calculate vapor pressure using Raoult's Law?

To calculate vapor pressure with Raoult's Law: (1) Determine the mole fraction of solvent, (2) Find the pure solvent vapor pressure at your temperature, (3) Multiply these values together. Our vapor pressure calculator automates this process for accurate results.

When does Raoult's Law apply to vapor pressure calculations?

Raoult's Law applies best to ideal solutions with chemically similar components at moderate temperatures and pressures. It's most accurate for dilute solutions and when solvent-solute interactions are similar to solvent-solvent interactions.

What causes deviations from Raoult's Law in vapor pressure?

Positive deviations occur when actual vapor pressure exceeds Raoult's Law predictions due to weaker solvent-solute interactions. Negative deviations happen when molecular attractions reduce vapor pressure below predicted values.

How does temperature affect Raoult's Law vapor pressure calculations?

Temperature directly affects pure solvent vapor pressure (P°) but not the Raoult's Law relationship itself. Higher temperatures increase pure solvent vapor pressure exponentially, proportionally increasing solution vapor pressure.

Can Raoult's Law calculate vapor pressure for multiple volatile components?

Yes, for mixtures with multiple volatile components, each contributes to total vapor pressure according to Raoult's Law. Total pressure equals the sum of partial pressures: P_total = Σ(X_i × P°_i) for each volatile component.

How is Raoult's Law used in distillation vapor pressure analysis?

In distillation, Raoult's Law predicts vapor composition above liquid mixtures. Components with higher vapor pressures concentrate in vapor phase, enabling separation through repeated vaporization-condensation cycles.

What pressure units work with the Raoult's Law calculator?

Our vapor pressure calculator accepts any pressure units (kPa, mmHg, atm, torr) as long as both input and output use the same units. Common conversions: 1 atm = 101.325 kPa = 760 mmHg = 760 torr.

How accurate is Raoult's Law for vapor pressure prediction?

Raoult's Law provides excellent accuracy for ideal solutions and good approximations for many real solutions. Accuracy decreases with increasing solution non-ideality, requiring activity coefficient corrections for precise vapor pressure calculations.

References

1. Atkins, P. W., & de Paula, J. (2014). Atkins' Physical Chemistry (10th ed.). Oxford University Press.

2. Levine, I. N. (2009). Physical Chemistry (6th ed.). McGraw-Hill Education.

3. Smith, J. M., Van Ness, H. C., & Abbott, M. M. (2017). Introduction to Chemical Engineering Thermodynamics (8th ed.). McGraw-Hill Education.

4. Prausnitz, J. M., Lichtenthaler, R. N., & de Azevedo, E. G. (1998). Molecular Thermodynamics of Fluid-Phase Equilibria (3rd ed.). Prentice Hall.

5. Raoult, F. M. (1887). "Loi générale des tensions de vapeur des dissolvants" [General law of vapor pressures of solvents]. Comptes Rendus de l'Académie des Sciences, 104, 1430–1433.

6. Sandler, S. I. (2017). Chemical, Biochemical, and Engineering Thermodynamics (5th ed.). John Wiley & Sons.

7. Denbigh, K. G. (1981). The Principles of Chemical Equilibrium (4th ed.). Cambridge University Press.

8. "Raoult's Law." Wikipedia, Wikimedia Foundation, https://en.wikipedia.org/wiki/Raoult%27s_law. Accessed 25 July 2025.

9. "Vapor Pressure." Chemistry LibreTexts, https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/States_of_Matter/Phase_Transitions/Vapor_Pressure. Accessed 25 July 2025.

10. "Colligative Properties." Khan Academy, https://www.khanacademy.org/science/chemistry/states-of-matter-and-intermolecular-forces/mixtures-and-solutions/v/colligative-properties. Accessed 25 July 2025.

Start Calculating Vapor Pressure Now

Use our Raoult's Law vapor pressure calculator for instant, accurate solution analysis. Perfect for chemistry students, researchers, and engineers working with distillation, solution chemistry, and colligative properties.

Calculate your solution's vapor pressure in seconds - enter your values above and get precise results based on proven Raoult's Law principles.

---

Meta Title: Raoult's Law Vapor Pressure Calculator | Free Chemistry Tool Meta Description: Calculate solution vapor pressure with our free Raoult's Law calculator. Enter mole fraction and pure solvent vapor pressure for instant, accurate chemistry results.