Van der Waals Constants for Common Gases and Volatile Liquids

In the study of thermodynamics, the ideal gas law provides a useful approximation for how gases behave. However, real gases deviate from this ideal behavior due to the volume occupied by gas molecules and the attractive forces between them. To account for these real-world effects, the Van der Waals equation introduces two specific constants, a and b, to provide a more accurate description of a substance's state.

The constant a represents the attraction between particles (intermolecular forces), while the constant b accounts for the excluded volume—the actual space occupied by the gas molecules themselves.

Key Facts

• Constant a measures the strength of the attractive forces between molecules.
• Constant b represents the volume excluded by the molecules.
• Constants are derived from critical pressure (pc) and critical temperature (Tc).
• The formulas used for calculation are: a = (27/64) * (R²Tc²) / pc and b = (RTc) / (8pc).
• Units for a are typically expressed in L²bar/mol², and b in L/mol.

Calculating and Converting Constants

The Van der Waals constants are generally calculated using the universal gas constant (R) alongside the critical properties of the substance. Because different scientific contexts require different units, conversion factors are often necessary to translate these values into standard SI units.

Unit Conversion Guide

• For Constant a:
  • To convert L²bar/mol² to L²kPa/mol², multiply by 100.
  • To convert L²bar/mol² to m⁶Pa/mol², divide by 10.
• For Constant b:
  • To convert L/mol to m³/mol, divide by 1000.

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Van der Waals Constants Data Table

The following table provides the constants for a wide array of common gases and volatile liquids, ranging from noble gases like Helium to complex organic compounds like Dodecane.

Chemical	a (L²bar/mol²)	b (L/mol)
Acetic acid	17.7098	0.1065
Acetic anhydride	20.158	0.1263
Acetone	16.02	0.1124
Acetonitrile	17.81	0.1168
Acetylene	4.516	0.0522
Ammonia	4.225	0.0371
Aniline	29.14	0.1486
Argon	1.355	0.03201
Benzene	18.24	0.1193
Bromobenzene	28.94	0.1539
Butane	14.66	0.1226
1-Butanol	20.94	0.1326
2-Butanone	19.97	0.1326
Carbon dioxide	3.6	0.04267
Carbon disulfide	11.77	0.07685
Carbon monoxide	1.505	0.0398500
Carbon tetrachloride	19.7483	0.1281
Chlorine	6.579	0.05622
Chlorobenzene	25.77	0.1453
Chloroethane	11.05	0.08651
Chloromethane	7.570	0.06483
Cyanogen	7.769	0.06901
Cyclohexane	23.11	0.1424
Cyclopropane	8.34	0.0747
Decane	52.74	0.3043
1-Decanol	59.51	0.3086
Diethyl ether	17.61	0.1344
Diethyl sulfide	19.00	0.1214
Dimethyl ether	8.180	0.07246
Dimethyl sulfide	13.04	0.09213
Dodecane	69.38	0.3758
1-Dodecanol	75.70	0.3750
Ethane	5.562	0.0638
Ethanethiol	11.39	0.08098
Ethanol	12.18	0.08407
Ethyl acetate	20.72	0.1412
Ethylamine	10.74	0.08409
Ethylene	4.612	0.0582
Fluorine	1.171	0.0290
Fluorobenzene	20.19	0.1286
Fluoromethane	4.692	0.05264
Freon	10.78	0.0998
Furan	12.74	0.0926
Germanium tetrachloride	22.90	0.1485
Helium	0.0346	0.0238
Heptane	31.06	0.2049
1-Heptanol	38.17	0.2150
Hexane	24.71	0.1735
1-Hexanol	31.79	0.1856
Hydrazine	8.46	0.0462
Hydrogen	0.2476	0.02661
Hydrogen bromide	4.510	0.04431
Hydrogen chloride	3.716	0.04081
Hydrogen cyanide	11.29	0.0881
Hydrogen fluoride	9.565	0.0739
Hydrogen iodide	6.309	0.0530
Hydrogen selenide	5.338	0.04637
Hydrogen sulfide	4.490	0.04287
Isobutane	13.32	0.1164
Iodobenzene	33.52	0.1656
Krypton	2.349	0.03978
Mercury	8.200	0.01696
Methane	2.283	0.04278
Methanol	9.649	0.06702
Methylamine	7.106	0.0588
Neon	0.2135	0.01709
Neopentane	17.17	0.1411
Nitric oxide	1.358	0.02789
Nitrogen	1.370	0.0387
Nitrogen dioxide	5.354	0.04424
Nitrogen trifluoride	3.58	0.0545
Nitrous oxide	3.832	0.04415
Octane	37.88	0.2374
1-Octanol	44.71	0.2442
Oxygen	1.382	0.03186
Ozone	3.570	0.0487
Pentane	19.26	0.146
1-Pentanol	25.88	0.1568
Phenol	22.93	0.1177
Phosphine	4.692	0.05156
Propane	8.779	0.08445
1-Propanol	16.26	0.1079
2-Propanol	15.82	0.1109
Propene	8.442	0.0824
Pyridine	19.77	0.1137
Pyrrole	18.82	0.1049
Radon	6.601	0.06239
Silane	4.377	0.05786
Silicon tetrafluoride	4.251	0.05571
Sulfur dioxide	6.803	0.05636
Sulfur hexafluoride	7.857	0.0879
Tetrachloromethane	20.01	0.1281
Tetrachlorosilane	20.96	0.1470
Tetrafluoroethylene	6.954	0.0809
Tetrafluoromethane	4.040	0.0633
Tetrafluorosilane	5.259	0.0724
Tetrahydrofuran	16.39	0.1082
Tin tetrachloride	27.27	0.1642
Thiophene	17.21	0.1058
Toluene	24.38	0.1463
1-1-1-Trichloroethane	20.15	0.1317
Trichloromethane	15.34	0.1019
Trifluoromethane	5.378	0.0640
Trimethylamine	13.37	0.1101
Water	5.536	0.03049
Xenon	4.250	0.05105

Frequently Asked Questions

What do the Van der Waals constants represent?

The constant a represents the attraction between molecules, which reduces the pressure exerted on the walls of a container compared to an ideal gas. The constant b represents the volume occupied by the molecules themselves, which reduces the available space for movement.

How are these constants calculated?

They are typically calculated using the critical temperature (Tc) and critical pressure (pc) of the substance, utilizing the formulas a = (27/64) * (R²Tc²) / pc and b = (RTc) / (8pc).

Why do different gases have different constants?

Different chemicals have different molecular sizes and different strengths of intermolecular attractions. For example, larger molecules like Dodecane have a much higher b value (0.3758 L/mol) than small molecules like Helium (0.0238 L/mol).

How do I convert L²bar/mol² to other units?

To convert L²bar/mol² to L²kPa/mol², you multiply the value by 100. To convert it to m⁶Pa/mol², you divide the value by 10.

What is the difference between an ideal gas and a real gas in this context?

An ideal gas assumes molecules have no volume and no attraction to each other. Real gases, described by the Van der Waals equation, acknowledge that molecules do occupy space and do exert attractive forces on one another.