Raoults Law Calculator - Ideal Solution Partial Pressures

A Raoult's law calculator is a chemistry tool that turns a solvent mole fraction and the pure-component vapor pressures of a binary mixture into the partial pressure of each component, the total vapor pressure over the solution, and the vapor pressure lowering relative to the pure solvent. According to LibreTexts Chemistry, the partial vapor pressure of each component in an ideal solution equals the mole fraction of that component multiplied by its pure-component vapor pressure.

• • General chemistry homework: Verify a textbook partial pressure or vapor pressure lowering problem for benzene-toluene, ethanol-water, or another ideal binary pair.
• • Distillation planning: Predict how a liquid mixture vaporizes by reading the partial pressure ratio and vapor-phase enrichment.
• • Colligative property context: Connect this law to boiling-point elevation, which shares the same solvent-versus-solute framing.
• • Lab solvent selection: Compare how the same mole fraction of two solutes changes the vapor pressure of a working solvent.

The law describes the vapor pressure of an ideal solution, where unlike-molecule forces match like-molecule forces. The vapor pressure lowering delta-P = x_B * P_A* feeds the boiling-point elevation formula, which is why both calculators live in the same category.

The law consumes a solvent mole fraction, so the Mole Fraction Calculator is the right place to compute x_A when the inputs are grams and molar masses instead of mole counts.

The calculator reads the solvent and solute presets, lets you override the pure-component vapor pressures, and applies the textbook relationships P_A = x_A * P_A*, P_B = x_B * P_B*, and P_total = P_A + P_B. It also returns the vapor pressure lowering, the relative lowering as a percentage, and y_A = P_A / P_total.

• x_A: Mole fraction of the solvent in the liquid solution, 0 to 1. The solute mole fraction is x_B = 1 - x_A.
• P_A*: Pure-component vapor pressure of the solvent at the system temperature, in mmHg.
• P_B*: Pure-component vapor pressure of the solute. Set to 0 for non-volatile solutes such as salts or sugars.
• delta-P: Vapor pressure lowering, equal to P_A* - P_A = x_B * P_A*. The relative lowering equals x_B.
• y_A: Vapor-phase mole fraction of the solvent, equal to P_A / P_total. Highlights vapor-phase enrichment of the more volatile component.

If your system is not at 25 C, replace the preset vapor pressures with values from the NIST WebBook for your working temperature.

According to LibreTexts Chemistry - Raoult's Law, Raoult's law states that the vapor pressure of a solvent above a solution is equal to the vapor pressure of the pure solvent at the same temperature scaled by the mole fraction of the solvent present.

According to Wikipedia - Raoult's law, the total vapor pressure of an ideal solution is the sum of the partial pressures of the components and the vapor pressure lowering equals the solute mole fraction times the pure solvent vapor pressure.

Vapor pressure lowering and boiling-point elevation share the same x_B * P_A* physics, so the Boiling Point Elevation Calculator extends the same colligative-property input to a new boiling point.

Four ideas come up every time you read about this law. Skim them once so the result panel reads like a derivation rather than a black box.

These four concepts work together. The ideal-solution assumption lets you use the simple x_i * P_i* form, the mole fraction sets the dose of each component, and the comparison to Henry's law tells you where the assumption starts to break down.

Both laws describe an ideal-behaviour baseline, and the Ideal Gas Calculator handles the gas-phase side of the same kind of problem.

Run the calculator in under a minute. The defaults reproduce a textbook benzene-toluene worked example at x_A = 0.40, so you only need to change fields if your problem is different.

1. 1 Pick the solvent preset: Choose water, ethanol, methanol, benzene, toluene, n-hexane, acetone, diethyl ether, chloroform, or carbon tetrachloride to load the matching pure-component vapor pressure at 25 C.
2. 2 Pick the solute preset: Choose the second component or pick Non-volatile solute for salts, sugars, or other solids with P_B* = 0.
3. 3 Type the solvent mole fraction: Enter x_A between 0 and 1. The solute mole fraction x_B is computed as 1 - x_A.
4. 4 Override P_A* and P_B* if needed: Type custom pure-component vapor pressures if your system is at a different temperature or you have measured data.
5. 5 Read the result panel: The primary panel shows the partial pressure of the solvent. The secondary panel shows the partial pressure of the solute, total pressure, lowering, relative lowering, and y_A.

A laboratory wants to predict whether benzene will preferentially evaporate from a 60/40 toluene/benzene mixture. Set the solvent preset to toluene and the solute preset to benzene, type x_A = 0.60 for toluene, and read y_A in the result panel. A y_A for benzene greater than 0.40 confirms the vapor is enriched in benzene, which is the same physical effect that drives a benzene-toluene distillation column.

When the available data is mass rather than mole counts, the Grams to Moles Calculator converts grams of solute and solvent into moles so the mole fraction input above can be filled in directly.

Knowing the partial pressure and the vapor pressure lowering for a given mole fraction turns a textbook formula into a number you can act on. These are the most common practical wins.

• • Built-in vapor pressure presets: Loads CRC Handbook and NIST WebBook vapor pressures for ten common solvents at 25 C, so no printed reference is needed for textbook problems.
• • Handles non-volatile solutes: Returns the textbook colligative form P_soln = x_A * P_A* and the relative lowering equal to x_B for salts, sugars, and other non-volatile solutes.
• • Computes vapor-phase enrichment: Reports y_A = P_A / P_total so you can see at a glance whether the more volatile component concentrates in the vapor, the core physical effect behind fractional distillation.
• • Three pressures from one panel: Shows partial pressures P_A and P_B alongside the total pressure P_total from a single x_A input, faster than computing each piece separately by hand.
• • Pairs with related colligative tools: Sits next to the boiling-point elevation and mole fraction calculators, so a colligative-property problem can be cross-checked in the same session.
• • Transparent override path: Lets you type custom P_A* and P_B* values for non-standard temperatures or measured vapor pressures without losing the preset defaults.

For coursework the calculator is the fastest way to verify a homework answer; for lab work it is a sanity check before a distillation is set up; for chemical engineering it provides the partial-pressure basis used in VLE tables.

Recipes and lab stock solutions are usually quoted as mass percent or molarity, and the Concentration Calculator turns those numbers into the mole counts that feed the law.

Three inputs and two physical limits drive every result. The notes below explain what each factor does and where the underlying equation stops being trustworthy.

• • The law assumes an ideal solution. Polar or hydrogen-bonding mixtures deviate and need an activity-coefficient correction.
• • The preset vapor pressures are tabulated at 25 C. Override P_A* and P_B* for other temperatures.
• • The law models the liquid phase. The reported y_A value assumes the vapor behaves as an ideal gas.

If the calculator returns a value that surprises you, check the mole fraction first (typo between x_A and x_B is the most common mistake) and then check whether the chosen solvent and solute are really expected to behave ideally. For dilute non-volatile solutes the relative lowering equals x_B exactly, the same number that feeds the boiling-point elevation formula.

According to NIST Chemistry WebBook, the pure-component vapor pressures at 25 degrees Celsius used as calculator defaults come from peer-reviewed thermochemistry data and should be replaced by values at the actual system temperature for high-precision work.

Replacing the preset vapor pressures with values at a different temperature often requires the molar mass and density of each component, both of which the Mole / Molar Mass Calculator provides.