Accurately calculate the weight of benzene codistilled with each gram of water during steam distillation.
Codistillation Ratio Calculator
Enter the vapor pressures and molecular weights to determine the mass ratio.
Vapor pressure of benzene at the distillation temperature (approx 69°C for mixture).
Please enter a valid positive number.
Vapor pressure of water at the distillation temperature.
Please enter a valid positive number.
Standard molecular weight of Benzene (C₆H₆).
Please enter a valid positive number.
Standard molecular weight of Water (H₂O).
Please enter a valid positive number.
Benzene Codistilled per Gram of Water
0.000 g
Mass Ratio (A:B)
0:1
Benzene Mass %
0%
Total Pressure (System)
0 mmHg
Calculation Logic: Mass Ratio = (PBenzene × MWBenzene) / (PWater × MWWater)
Composition Breakdown Table
Table 1: Detailed breakdown of the distillate composition based on current inputs.
Component
Vapor Pressure (mmHg)
Molecular Weight (g/mol)
Weight Fraction
Distillate Mass Composition
Benzene
Water
Figure 1: Visual representation of the mass proportion of Benzene vs Water in the distillate.
What is Benzene Codistillation?
Benzene codistillation, often referred to in the context of steam distillation, is a separation process used to purify or isolate temperature-sensitive organic compounds like benzene. When we calculate the weight of benzene codistilled with each gram of water (steam), we are determining the efficiency and yield of this purification process.
This technique relies on the principle that for immiscible liquids (like benzene and water), the total vapor pressure is the sum of the individual vapor pressures of the pure components. This allows the mixture to boil at a temperature lower than the boiling point of either component, preventing the thermal decomposition of sensitive organic compounds.
Chemists, chemical engineers, and industrial process managers use this calculation to estimate solvent recovery rates, design distillation columns, and ensure environmental compliance in waste stream management. Understanding the mass relationship is crucial for operational planning and cost estimation.
Benzene Codistillation Formula and Mathematical Explanation
The theoretical basis for steam distillation is derived from Dalton's Law of Partial Pressures. Since the liquids are immiscible, they exert their own vapor pressures independently. The number of moles of each substance in the vapor phase is proportional to its partial pressure.
The core formula to calculate the weight ratio is:
Assume a mixture of benzene and water boils at 69.4°C at standard atmospheric pressure (760 mmHg). At this temperature, the vapor pressure of water is approximately 225 mmHg. Since total pressure is 760 mmHg, the vapor pressure of benzene is 760 – 225 = 535 mmHg.
PBenzene: 535 mmHg
PWater: 225 mmHg
Ratio Calculation: (535 × 78.11) / (225 × 18.015)
Result: Approximately 10.31 grams of benzene per gram of water.
Interpretation: For every 1 gram of steam used, you recover over 10 grams of benzene, making this a highly efficient process.
Example 2: Reduced Pressure Distillation
In an industrial setting operating under a partial vacuum where the total system pressure is 400 mmHg. Suppose the mixture boils at a temperature where water vapor pressure is 100 mmHg.
PWater: 100 mmHg
PBenzene: 300 mmHg (400 – 100)
Ratio Calculation: (300 × 78.11) / (100 × 18.015)
Result: Approximately 13.01 grams of benzene per gram of water.
Interpretation: Lowering the pressure changed the vapor pressure ratios, potentially increasing the efficiency of benzene recovery per unit of steam.
How to Use This Benzene Codistillation Calculator
Identify Vapor Pressures: Enter the vapor pressure of benzene and water at the boiling temperature of the mixture. If you only know the total pressure and temperature, use steam tables to find the water pressure, then subtract it from the total to find the benzene pressure.
Verify Molecular Weights: The defaults are set for Benzene (78.11) and Water (18.015). If you are using isotopes or different purities, adjust these slightly.
Review the Ratio: The calculator immediately displays the weight of benzene recovered for every single gram of water distilled.
Analyze Mass Percentage: Check the "Benzene Mass %" to see the purity of the distillate stream before separation.
Key Factors That Affect Codistillation Results
Several physical and economic factors influence the results when you calculate the weight of benzene codistilled with each gram:
1. Temperature of the Mixture
Vapor pressure is exponentially related to temperature. A slight change in boiling temperature can significantly alter the ratio of $P_A$ to $P_B$, changing the distillate composition.
2. Atmospheric Pressure
The system boils when the sum of vapor pressures equals the external pressure. High-altitude labs or vacuum systems will see different boiling points and thus different mass ratios.
3. Miscibility
This calculation assumes benzene and water are immiscible. If any mutual solubility occurs (which increases with temperature), the theoretical yield may deviate slightly from the ideal calculation.
4. Molecular Weight Differences
Benzene is much heavier than water (78 vs 18 g/mol). This high molecular weight leverages the vapor pressure, allowing significant mass transfer even if benzene's partial pressure drops.
5. Equipment Efficiency
In real-world distillation columns, "holdup" and plate efficiency can affect the actual recovery rates compared to the theoretical maximum calculated here.
6. Purity of Components
Impurities in the benzene or water can lower their respective vapor pressures (Raoult's Law effects for soluble impurities), altering the final codistillation ratio.
Frequently Asked Questions (FAQ)
Why is the weight of benzene so much higher than water?
Because Benzene has a much higher molecular weight (78.11 g/mol) compared to water (18.02 g/mol) and a relatively high vapor pressure. The formula multiplies these factors, amplifying the benzene mass.
Can I use this for other solvents like Toluene?
Yes, but you must change the Molecular Weight input to match Toluene (92.14 g/mol) and input the correct vapor pressure for Toluene at the boiling temperature.
Does this calculator assume ideal gas behavior?
Yes, the derivation relies on the Ideal Gas Law and Dalton's Law. For most industrial steam distillation pressures, this approximation is sufficient.
How do I find the vapor pressure at a specific temperature?
You typically use the Antoine Equation or consult standard chemical engineering steam tables and vapor pressure charts for Benzene.
What is the boiling point of a benzene-water mixture?
At 1 atm (760 mmHg), the heterogeneous azeotrope of benzene and water boils at approximately 69.25°C.
Is the codistillate pure benzene?
No, the distillate is a mixture of water and benzene. They will separate into two layers upon condensation due to immiscibility, allowing for easy physical separation.
Does the ratio change during the distillation?
As long as both liquid phases (water and benzene) are present in the boiling flask, the temperature and vapor composition remain constant. Once one phase is exhausted, the temperature changes.
Why is steam distillation used for benzene?
It allows benzene to be distilled below its normal boiling point (80.1°C), which is safer and requires less energy input for the heating source.
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