Enter Solution Data
Use grams, grams per mole, degrees Celsius, and kilopascals. The automatic reference uses water temperature.
Example Data Table
This example uses sucrose as a nonvolatile solute at 25 °C.
| Input | Example value | Purpose |
|---|---|---|
| Water mass | 100 g | Converts to water moles. |
| Sucrose mass | 10 g | Converts to solute moles. |
| Sucrose molar mass | 342.2965 g/mol | Relates solute mass to amount. |
| van’t Hoff factor | 1 | Represents one particle per molecule. |
| Temperature | 25 °C | Estimates pure-water reference pressure. |
Formula Used
This calculator uses Raoult’s law for a nonvolatile solute.
Psolution = Xwater × P°water
Xwater = nwater ÷ (nwater + i × nsolute)
nwater = mwater ÷ 18.01528
nsolute = msolute ÷ Msolute
P°water is the pure-water vapor pressure. Xwater is the water mole fraction. The factor i accounts for dissolved particles from electrolyte dissociation.
How to Use This Calculator
- Enter the mixture temperature and water mass.
- Enter solute mass and its molar mass.
- Set i to one for sucrose, glucose, or similar nonelectrolytes.
- Choose automatic pressure estimation or enter a measured pure-water pressure.
- Select Calculate Vapor Pressure to view results above the form.
- Download the data as CSV or use the print option for a PDF copy.
Understanding Solution Vapor Pressure
Why Solution Vapor Pressure Changes
A liquid surface contains molecules with different energies. Some molecules escape into the gas phase. Their escaping tendency produces vapor pressure. Pure water has a pressure set mainly by temperature. Warmer water has faster molecules. More molecules can leave the surface. Therefore vapor pressure rises quickly as temperature increases.
A dissolved nonvolatile solute changes this balance. Solute particles occupy part of the liquid mixture. They reduce the mole fraction of water. Fewer water molecules are available at the surface, on average. The solution then produces less vapor than pure water at the same temperature. This effect is called vapor-pressure lowering.
The size of the change depends on particle amount. It does not depend only on solute mass. Molecular mass matters because it converts mass into moles. A small mass of a low-molar-mass solute can create many particles. A larger mass of a heavy solute can create fewer particles. Mole fractions make this comparison fair.
Role of Water and Solute Ratio
The water-to-solute ratio is useful for preparation work. It provides a quick mass-based description of a mixture. However, Raoult’s law uses mole fraction instead of mass ratio. The calculator therefore converts both materials to moles. It uses water’s molar mass of 18.01528 grams per mole.
For a nonelectrolyte, each solute molecule normally counts as one dissolved particle. Examples include sucrose and many organic compounds. Electrolytes can separate into ions. Sodium chloride can behave as roughly two particles per formula unit in dilute water. The van’t Hoff factor estimates this extra particle count. Real solutions may show smaller values because ions interact.
Choosing the Reference Pressure
A reliable reference pressure is essential. This page can estimate pure-water pressure from temperature using a standard Antoine relation. That estimate is suited to water between 1 and 100 degrees Celsius. A measured reference pressure can be entered instead. This is helpful when using a laboratory table, a controlled system, or a different valid temperature range.
The calculation assumes the solute is nonvolatile. A volatile solute contributes its own vapor pressure. That situation needs a two-component vapor calculation. Concentrated solutions may also depart from ideal behavior. Activity coefficients are then more appropriate than simple mole fractions. Use this page as a clear first estimate, not a replacement for measured process data.
Practical Uses and Checks
This method supports classroom experiments, food formulation studies, and basic solution design. It can also help explain freezing-point and boiling-point effects. Those properties are related to the number of dissolved particles. Always record units before entering values. Use grams for masses and grams per mole for molar mass.
Check whether the solute can evaporate. Check whether it dissociates in water. Keep the temperature within the stated automatic range. Compare the calculated pure-water pressure with a trusted reference when accuracy matters. Finally, interpret a low solution pressure correctly. It indicates reduced water escaping tendency, not necessarily a slow chemical reaction.
Repeat calculations after changing one input, so trends remain easy to identify and explain.
Frequently Asked Questions
1. Which law does this calculator apply?
It applies Raoult’s law. The method multiplies pure-water vapor pressure by the mole fraction of water in the solution.
2. Which solutes are suitable?
Use nonvolatile solutes, such as sucrose or glucose. A volatile solute needs a multi-component vapor-pressure model.
3. Why is solute molar mass required?
Molar mass converts solute mass into moles. Mole count determines the water mole fraction used by the calculation.
4. What does the van’t Hoff factor do?
It estimates the number of dissolved particles produced per solute unit. Use one for nonelectrolytes. Electrolytes often need a larger value.
5. What temperature range supports automatic pressure?
Automatic water-pressure estimation supports temperatures from 1 to 100 °C. Enter a custom measured pressure outside that range.
6. When should custom pure-water pressure be used?
Use custom pressure when a laboratory measurement or trusted table is available. It can better match your specific temperature and conditions.
7. Can this model handle salt solutions?
Yes, as an estimate. Enter an appropriate van’t Hoff factor. Real salts may depart from ideal behavior, especially at higher concentration.
8. Does lower vapor pressure mean water boils sooner?
No. At the same external pressure, a lower solution vapor pressure usually means a slightly higher boiling temperature is needed.
9. Can I enter kilograms instead of grams?
Convert kilograms to grams first. Both masses must use grams because the calculator pairs them with molar masses in grams per mole.
10. Is the calculation exact for concentrated mixtures?
Not always. Concentrated or strongly interacting solutions can require activity coefficients and measured property data for improved accuracy.
11. What does pressure lowering show?
It shows the difference between pure-water vapor pressure and solution vapor pressure. A larger difference indicates a stronger particle effect.