Find Vapor Pressure Calculator

Easily calculate the vapor pressure of a substance at a specific temperature using the Antoine equation with our Vapor Pressure Calculator.

Calculate Vapor Pressure

Antoine Constants for Common Substances (P in mmHg, T in °C)

Substance	A	B	C	T range (°C)
Water	8.07131	1730.63	233.426	1 – 100
Ethanol	8.20417	1642.89	230.300	-57 – 80
Methanol	8.08097	1582.271	239.726	-44 – 65
Benzene	6.90565	1211.033	220.790	6 – 80
Acetone	7.11714	1210.595	229.664	-32 – 56

Source: NIST WebBook and various handbooks. Constants vary slightly with source and temperature range.

What is Vapor Pressure?

Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid’s evaporation rate. It relates to the tendency of particles to escape from the liquid (or a solid). A substance with a high vapor pressure at normal temperatures is often referred to as volatile.

Essentially, it’s a measure of how readily a substance will turn into a gas at a certain temperature. The higher the vapor pressure, the more volatile the substance and the more easily it evaporates.

Who should use a Vapor Pressure Calculator?

A Vapor Pressure Calculator is useful for:

• Chemists and Chemical Engineers: For designing distillation processes, understanding reaction conditions, and safety assessments involving volatile liquids.
• Meteorologists: To understand and predict weather patterns, especially humidity and evaporation rates.
• Environmental Scientists: To model the transport and fate of pollutants in the atmosphere and water bodies.
• Students and Educators: For learning and teaching concepts in physical chemistry and thermodynamics.
• Pharmacists and Material Scientists: When dealing with the formulation and stability of products containing volatile components.

Common Misconceptions

• Vapor pressure is the same as boiling point: While related, they are different. The boiling point is the temperature at which the vapor pressure equals the surrounding atmospheric pressure. Vapor pressure exists at all temperatures below boiling.
• Vapor pressure only applies to liquids: Solids also have a vapor pressure (sublimation), though it’s often much lower than that of liquids.
• Vapor pressure increases with external pressure: Vapor pressure is an intrinsic property of the substance at a given temperature and is largely independent of external pressure, although the boiling point is affected by external pressure.

Vapor Pressure Formula and Mathematical Explanation

One of the most common empirical equations used to estimate vapor pressure is the Antoine equation:

log₁₀(P) = A – (B / (T + C))

Where:

• P is the vapor pressure (often in mmHg, bar, or kPa).
• T is the temperature (often in °C or K, depending on the constants).
• A, B, and C are the Antoine constants, which are specific to each substance and the units used for P and T, as well as the temperature range.

From this equation, we can solve for P:

P = 10^{(A – (B / (T + C)))}

Variables Table

Variable	Meaning	Unit	Typical Range
P	Vapor Pressure	mmHg, bar, Pa, kPa	Varies widely with substance and temperature
T	Temperature	°C, K	Depends on the validity range of A, B, C
A	Antoine Constant A	Dimensionless (for log10)	Typically 6-9 (for P in mmHg, T in °C)
B	Antoine Constant B	°C or K	Typically 1000-2000 (for P in mmHg, T in °C)
C	Antoine Constant C	°C or K	Typically 200-250 (for P in mmHg, T in °C)

Practical Examples (Real-World Use Cases)

Example 1: Vapor Pressure of Water at 25°C

Let’s find the vapor pressure of water at 25°C using the Antoine constants from the table (A=8.07131, B=1730.63, C=233.426 for T in °C, P in mmHg).

• T = 25 °C
• A = 8.07131
• B = 1730.63
• C = 233.426

log₁₀(P) = 8.07131 – (1730.63 / (25 + 233.426))

log₁₀(P) = 8.07131 – (1730.63 / 258.426)

log₁₀(P) = 8.07131 – 6.6970

log₁₀(P) = 1.37431

P = 10^1.37431 ≈ 23.67 mmHg

So, the vapor pressure of water at 25°C is approximately 23.67 mmHg.

Example 2: Vapor Pressure of Ethanol at 78.37°C (Boiling Point at 1 atm)

Let’s find the vapor pressure of ethanol at its normal boiling point, 78.37°C (A=8.20417, B=1642.89, C=230.300 for T in °C, P in mmHg).

• T = 78.37 °C
• A = 8.20417
• B = 1642.89
• C = 230.300

log₁₀(P) = 8.20417 – (1642.89 / (78.37 + 230.300))

log₁₀(P) = 8.20417 – (1642.89 / 308.67)

log₁₀(P) = 8.20417 – 5.3223

log₁₀(P) = 2.88187

P = 10^2.88187 ≈ 761.9 mmHg

This is very close to 760 mmHg (1 atm), which is expected at the normal boiling point.

How to Use This Vapor Pressure Calculator

1. Enter Temperature (T): Input the temperature of the substance in degrees Celsius (°C) into the “Temperature (T in °C)” field.
2. Enter Antoine Constants (A, B, C): Input the substance-specific Antoine constants A, B, and C into their respective fields. Ensure these constants are for the correct units (P in mmHg and T in °C for the defaults provided in the table). You can find these constants in chemical handbooks or online databases like the NIST WebBook. Our table provides some examples.
3. Calculate: The calculator automatically updates the results as you type. You can also click the “Calculate” button.
4. View Results: The “Primary Result” shows the calculated vapor pressure (P) in mmHg (assuming the constants are for mmHg). Intermediate values like log₁₀(P), T+C, and B/(T+C) are also displayed.
5. Reset: Click “Reset” to return to the default values (water at 25°C).
6. Copy Results: Click “Copy Results” to copy the main result and intermediate values to your clipboard.
7. View Chart: The chart below the calculator visualizes how the vapor pressure changes with temperature around the value you entered, using the provided A, B, and C constants.

The Vapor Pressure Calculator provides a quick way to estimate vapor pressure based on the Antoine equation, which is very useful within the specified temperature range for the given constants.

Key Factors That Affect Vapor Pressure

1. Temperature: This is the most significant factor. As temperature increases, the kinetic energy of the molecules increases, and more molecules have enough energy to escape from the liquid phase into the gas phase, thus increasing the vapor pressure. Our Vapor Pressure Calculator directly uses temperature.
2. Intermolecular Forces (IMFs): The strength of the forces holding molecules together in the liquid phase (like hydrogen bonds, dipole-dipole interactions, London dispersion forces) significantly affects vapor pressure. Stronger IMFs mean molecules are held more tightly, resulting in lower vapor pressure at a given temperature. The Antoine constants (A, B, C) empirically account for the nature of the substance and its IMFs.
3. Substance Identity: Different substances have different molecular structures and IMFs, leading to different vapor pressures even at the same temperature. This is reflected in their unique Antoine constants used by the Vapor Pressure Calculator.
4. Molar Mass (to some extent): Lighter molecules generally tend to be more volatile (higher vapor pressure) than heavier ones *if* the intermolecular forces are similar, but IMFs are usually more dominant.
5. Purity of the Substance: The presence of impurities, especially non-volatile solutes, can lower the vapor pressure of a liquid (Raoult’s Law). The Antoine equation and our Vapor Pressure Calculator assume a pure substance.
6. Surface Area (for rate, not equilibrium): While surface area affects the *rate* of evaporation, it does not affect the equilibrium vapor pressure in a closed system.

Frequently Asked Questions (FAQ)

Q: What units does the Vapor Pressure Calculator use?
A: The calculator assumes temperature (T) is in Celsius (°C). The vapor pressure (P) unit depends on the Antoine constants A, B, and C used. The default constants and the table provided are generally for P in mmHg. If your constants are for other units (like bar or Pa), the result will be in those units.

Q: How accurate is the Antoine equation and this Vapor Pressure Calculator?
A: The Antoine equation is an empirical correlation and provides good accuracy within the temperature range for which the constants were derived. Outside this range, the accuracy can decrease. For very high precision or wider temperature ranges, more complex equations like the Wagner equation might be needed.

Q: Where can I find Antoine constants for different substances?
A: Antoine constants are available in many chemical engineering handbooks (like Perry’s Chemical Engineers’ Handbook), scientific literature, and online databases such as the NIST Chemistry WebBook or the Dortmund Data Bank.

Q: Can I use this calculator for mixtures?
A: No, this Vapor Pressure Calculator using the simple Antoine equation is designed for pure substances. Calculating the vapor pressure of mixtures is more complex and requires considering Raoult’s Law (for ideal solutions) or activity coefficient models (for non-ideal solutions).

Q: What if my temperature is outside the valid range for the constants?
A: The results may be less accurate. It’s best to find Antoine constants specific to your temperature range of interest if possible.

Q: Does external pressure affect vapor pressure?
A: The vapor pressure itself is an intrinsic property at a given temperature and is not significantly affected by the total external pressure. However, the boiling point (where vapor pressure equals external pressure) is affected.

Q: What is the difference between log10 and ln in vapor pressure equations?
A: Some forms of the Antoine or similar equations use the natural logarithm (ln) instead of the base-10 logarithm (log10). The constants A, B, and C will be different depending on which logarithm is used. Our Vapor Pressure Calculator uses log10.

Q: Can I calculate the boiling point using this information?
A: Yes, the boiling point at a given external pressure (e.g., 760 mmHg for normal boiling point) is the temperature at which the vapor pressure equals that external pressure. You would need to rearrange the Antoine equation to solve for T given P.

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