Calculating Vapor Pressure From Boiling Point

Vapor Pressure Equation:

\[ P = P_0 \exp\left[ -\frac{\Delta H_{vap}}{R} \left( \frac{1}{T} - \frac{1}{T_b} \right) \right] \]

Vapor Pressure (P):

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1. What is the Vapor Pressure Equation?

The vapor pressure equation calculates the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature. It's derived from the Clausius-Clapeyron relation and is particularly useful for estimating vapor pressure from boiling point data.

2. How Does the Calculator Work?

The calculator uses the vapor pressure equation:

\[ P = P_0 \exp\left[ -\frac{\Delta H_{vap}}{R} \left( \frac{1}{T} - \frac{1}{T_b} \right) \right] \]

Where:

\( P \) — Vapor pressure (Pa)
\( P_0 \) — Standard pressure (101325 Pa)
\( \Delta H_{vap} \) — Enthalpy of vaporization (J/mol)
\( R \) — Gas constant (8.314 J/mol·K)
\( T \) — Temperature (K)
\( T_b \) — Boiling point (K)

Explanation: The equation relates vapor pressure to temperature using the enthalpy of vaporization and the known boiling point of the substance.

3. Importance of Vapor Pressure Calculation

Details: Vapor pressure calculations are essential in chemical engineering, atmospheric science, and industrial processes for predicting evaporation rates, designing distillation systems, and understanding phase behavior of substances.

4. Using the Calculator

Tips: Enter enthalpy of vaporization in J/mol, temperature in Kelvin, and boiling point in Kelvin. All values must be positive and valid.

5. Frequently Asked Questions (FAQ)

Q1: What is the standard pressure value used?
A: The calculator uses 101325 Pa as the standard atmospheric pressure at sea level.

Q2: Why is temperature required in Kelvin?
A: The equation uses absolute temperature scale, making Kelvin the appropriate unit for thermodynamic calculations.

Q3: How accurate is this calculation?
A: The accuracy depends on the precision of input values and assumes ideal behavior. It's most accurate for substances with constant enthalpy of vaporization.

Q4: Can this be used for mixtures?
A: This equation is primarily for pure substances. Mixtures require more complex calculations accounting for composition.

Q5: What are typical vapor pressure values?
A: Vapor pressure ranges from near 0 Pa for non-volatile substances to 101325 Pa at boiling point for many liquids.