Non-ideal gas - Van der Waal's Equation and Constants

The ideal gas law treats the molecules of a gas as point particles with perfectly elastic collisions. This works well for dilute gases in many experimental circumstances.

However,  gas molecules are not point masses, and there are many cases gases need to be treated as non-ideal.  Johannes D. van der Waals suggested a modification to take into account molecular size and molecular interaction forces. It is usually referred to as the van der Waals equation of state:

[P + a(n/V)²] (V/n - b) = RT

The constants a and b are called  van der Waals constants. They have positive values and are characteristic of the individual gas. If a gas behaves ideally, both a and b are zero, and van der Waals equations approaches the ideal gas law PV=nRT. 

The constant a provides a correction for the intermolecular forces.
Constant b adjusts for the volume occupied by the gas particles. It is a correction for finite molecular size and its value is the volume of one mole of the atoms or molecules.

See also the Gas Compressibility Factor, z-factor for air, dependent on pressure and temperature and used to account for deviation from the ideal situation.
How to calculate total pressure and partial pressures from Ideal gas law

* To convert a into atm L²/mol² multiply by 0.986 atm/bar.    To convert a into kPa L²/mol² multiply by 100.0 kPa/bar

Name	Molecular Formula	a* bar L2/mol2	b L/mol
Aluminum trichloride	AlCl3	42.63	0.245
Ammonia	NH3	4.225	0.03713
Ammonium chloride	NH4Cl	2.38	0.00734
Argon	Ar	1.355	0.03201
Boron trichloride	BCl3	15.6	0.1222
Boron trifluoride	BF3	3.98	0.05443
Diborane	B2H6	6.048	0.07437
Bromine	Br2	9.75	0.0591
Perchloryl fluoride	ClFO2	7.371	0.0713
Chlorine pentafluoride	ClF5	9.58	0.08204
Phosphonium chloride	ClH4P	4.111	0.04545
Chlorine	Cl2	6.343	0.05422
Trichlorofluorosilane	Cl3FSi	15.67	0.1273
Fluorine	F2	1.171	0.02896
Germanium tetrachloride	GeCl4	23.12	0.1489
Nitrogen trifluoride	F3N	3.58	0.05453
Phosphorus trifluoride	F3P	4.954	0.0651
Tetrafluorohydrazine	F4N2	7.426	0.08564
Germane	GeH4	5.743	0.06555
Helium	He	0.0346	0.0238
Hydrogen bromide	HBr	4.5	0.04415
Hydrogen chloride	HCl	3.7	0.04061
Hydrogen cyanide	HCN	11.29	0.08806
Hydrogen fluoride	HF	9.565	0.0739
Hydrogen iodide	HI	6.309	0.05303
Hydrogen	H2	0.2453	0.02651
Water	H2O	5.537	0.03049
Hydrogen sulphide	H2S	4.544	0.04339
Hydrogen selenide	H2Se	5.523	0.0479
Krypton	Kr	2.325	0.0396
Silane	SiH4	4.38	0.0579
Silicon tetrachloride	SiCl4	20.96	0.147
Silicon tetrafluoride	SiF4	5.259	0.07236
Titanium(IV) chloride	TiCl4	25.47	0.1423
Mercury	Hg	5.193	0.01057
Nitric oxide	NO	1.46	0.0289
Nitrogen dioxide	NO2	5.36	0.0443
Nitrogen	N2	1.37	0.0387
Nitrous oxide	N2O	3.852	0.04435
Hydrazine	N2H4	8.46	0.0462
Neon	Ne	0.208	0.01672
Oxygen	O2	1.382	0.03186
Ozone	O3	3.57	0.0487
Phosphorus	P	53.6	0.157
Phosphine	PH3	4.696	0.05157
Radon	Rn	6.601	0.06239
Stannic chloride	SnCl4	27.25	0.1641
Sulphur	S	24.3	0.066
Sulphur dioxide	SO2	6.865	0.05679
Sulphur hexafluoride	SF6	7.857	0.08786
Selenium	Se	33.4	0.0675
Uranium(VI) fluoride	UF6	16.01	0.1128
Tungsten(VI) fluoride	WF6	13.25	0.1063
Xenon	Xe	4.192	0.05156
Xenon difluoride	XeF2	12.46	0.07037
Xenon tetrafluoride	XeF4	15.52	0.09035
Chlorotrifluoromethane	CClF3	6.873	0.0811
Trichlorofluoromethane	CCl3F	14.68	0.1111
Tetrachloromethane	CCl4	20.01	0.1281
Tetrafluoromethane	CF4	4.04	0.06325
Carbon monoxide	CO	1.472	0.03948
Carbon oxysulphide	COS	6.975	0.06628
Carbon dioxide	CO2	3.658	0.04286
Carbon disulphide	CS2	11.25	0.07262
Trichloromethane	CHCl3	15.34	0.1019
Trifluoromethane	CHF3	5.378	0.06403
Dichloromethane	CH2Cl2	12.44	0.08689
Difluoromethane	CH2F2	6.184	0.06268
Chloromethane	CH3Cl	7.566	0.06477
Fluoromethane	CH3F	5.009	0.05617
Nitromethane	NH3NO2	17.18	0.1041
Methane	CH4	2.3	0.04301
Methanol	CH3OH	9.472	0.06584
Methanethiol	CH3SH	8.911	0.06756
Methylamine	CH3NH2	7.106	0.05879
1,1,2-Trichlorotrifluoroethane	C2Cl2F3	20.25	0.1481
Tetrafluoroethylene	C2F4	6.954	0.08085
Cyanogen	C2N2	7.803	0.6952
Acetylene	C2H2	4.516	0.0522
1,1-Difluoroethylene	C2H2F2	6	0.07058
1,1,1-Trichloroethane	C2H3Cl3	20.15	0.1317
Fluroethylene	C2H3F	5.984	0.06504
1,1,1-Trifluoroethane	C2H3F3	9.302	0.09572
Acetonitrile	C2H3N	17.89	0.1169
Ethylene	C2H4	4.612	0.05821
1,1-Dichloroethane	C2H4Cl2	15.73	0.1072
1,2-Dichloroethane	C2H4Cl2	17	0.108
Ethylene oxide	C2H4O	8.922	0.06779
Acetic acid	CH3COOH	17.71	0.1065
Methyl formate	C2H4O2	11.54	0.08442
Bromoethane	C2H5Br	11.89	0.08406
Chloroethane	C2H5Cl	11.7	0.09
Fluoroethane	C2H5F	8.17	0.07758
Ethane	C2H6	5.57	0.06499
Dimethyl ether	C2H6O	8.69	0.07742
Ethanol	C2H5OH	12.56	0.0871
Dimethyl sulphide	C2H5SH	13.34	0.09453
Ethanethiol	C2H5SH	13.23	0.09447
Dimethylamine	C2H7N	10.44	0.0851
Ethylamine	C2H7N	10.79	0.08433
Perfluoropropane	C3F8	12.96	0.1338
Propanenitrile	C3H5N	21.57	0.1369
Propene	C3H6	8.438	0.08242
Cyclopropane	C3H6	8.293	0.0742
Acetone	C3H6O	16.02	0.1124
Propanal	C3H6O	14.08	0.09947
Ethyl formate	C3H6O2	15.91	0.1115
Methyl acetate	C3H6O2	15.75	0.1108
Propanoic acid	C3H6O2	23.49	0.1386
1-Chloropropane	C3H7Cl	16.11	0.1141`
Propane	C3H8	9.385	0.09044
1-Propanol	C3H8O	16.26	0.108
2-Propanol	C3H8O	15.82	0.1109
Ethyl methyl ether	C3H8O	12.7	0.1034
Ethyl methyl sulphide	C3H8S	19.45	0.13
Propylamine	C3H9N	15.26	0.1094
Trimethylamine	C3H9N	13.37	0.1101
Furan	C4H4O	12.74	0.0926
Thiophene	C4H4S	17.21	0.1058
Pyrrole	C4H4N	18.82	0.1049
1,3-Butadiene	C4H6	12.17	0.102
Acetic anhydride	C4H6O3	26.8	0.157
Butanenitrile	C4H7N	25.76	0.1568
1-Butene	C4H8	12.76	0.1084
Cyclobutane	C4H8	12.39	0.096
2-Butanone	C4H8O	19.97	0.1326
Tetrahydrofuran	C4H8O	16.39	0.1082
1,4-Dioxane	C4H8O2	19.29	0.1171
Ethyl acetate	C4H8O2	20.57	0.1401
Methyl propanoate	C4H8O2	20.51	0.1377
Propyl formate	C4H8O2	20.79	0.1377
Butanoic acid	C4H8O2	28.18	0.1609
Pyrrolidine	C4H9N	16.84	0.1056
Butane	C4H10	13.93	0.1168
Isobutane	C4H10	13.36	0.1168
1-Butanol	C4H10O	20.9	0.1323
2-Methyl-2-propanol	C4H10O	18.81	0.1324
2-Methyl-1-propanol	C4H10O	20.35	0.1324
Diethyl ether	C4H10O	17.46	0.1333
Diethyl sulphide	C4H10S	22.85	0.1462
Butylamine	C4H11N	19.41	0.1301
Diethylamine	C4H11N	19.4	0.1383
Tetramethylsilane	C4H12Si	20.81	0.1653
Furfural	C5H4O2	22.23	0.1182
Pyridine	C5H5N	19.77	0.1137
Cyclopentene	C5H8	15.61	0.1097
1-Pentene	C5H10	17.86	0.137
2-Methyl-1-butene	C5H10	16.9	0.129
2-Methyl-2-butene	C5H10	17.26	0.1279
Cyclopentane	C5H10	16.94	0.118
Tetrahydropyran	C5H10O	20.02	0.1247
Isobutyl formate	C5H10O2	22.82	0.1476
Propyl acetate	C5H10O2	26.23	0.17
Ethyl propanoate	C5H10O2	25.86	0.1688
Methyl butanoate	C5H10O2	25.83	0.1661
Methyl isobutanoate	C5H10O2	24.87	0.1639
Piperidine	C5H11N	20.84	0.125
Pentane	C5H12	19.13	0.1451
Isopentane	C5H12	18.29	0.1415
Neopentane	C5H12	17.17	0.141
1-Pentanol	C5H12O	25.81	0.1564
Bromobenzene	C6H5Br	28.96	0.1541
Chlorobenzene	C6H5Cl	25.8	0.1454
Fluorobenzene	C6H5F	20.1	0.1279
Iodobenzene	C6H5I	33.54	0.1658
Benzene	C6H6	18.82	0.1193
Phenol	C6H5OH	22.93	0.1177
Aniline	C6H5NH2	29.14	0.1486
Cyclohexanone	C6H10O	31.1	0.17
Hexanenitrile	C6H11N	35.5	0.1996
Cyclohexane	C6H12	21.95	0.1413
Cyclohexanol	C6H11OH	28.93	0.1586
Pentyl formate	C6H12O2	27.97	0.173
Isobutyl acetate	C6H12O2	29.05	0.1845
Ethyl butanoate	C6H12O2	30.53	0.1922
Ethyl 2-methylpropanoate	C6H12O2	29.05	0.1872
Methyl pentanoate	C6H12O2	29.39	0.1847
Hexane	C6H14	24.97	0.1753
2,3-Dimethylbutane	C6H14	23.29	0.166
1-Hexanol	C6H14O	31.35	0.1829
Triethylamine	C6H15N	27.59	0.1836
Dipropylamine	C6H15N	24.82	0.1591
Benzonitrile	C7H5N	33.89	0.1727
Benzaldehyde	C7H6O	30.3	0.1553
Toluene	C7H8	24.89	0.1499
o-Cresol	C7H8O	28.33	0.1447
m-Cresol	C7H8O	31.86	0.1609
p-Cresol	C7H8O	28.11	0.1422
Benzyl alcohol	C7H8O	34.7	0.173
Anisole	C7H8O	28.6	0.1579
Heptane	C7H16	30.89	0.2038
Heptanol	C7H15OH	37.22	0.2097
Ethylbenzene	C8H10	30.86	0.1782
o-Xylene	C8H10	31.06	0.1756
m-Xylene	C8H10	31.41	0.1814
p-Xylene	C8H10	31.54	0.1824
Phenetole	C8H10O	35.7	0.1966
N,N-Dimethylaniline	C8H11N	37.92	0.1967
Octane	C8H18	37.86	0.2372
2,5-Dimethylhexane	C8H18	35.49	0.2299
1-Octanol	C8H18O	43.42	0.2371
Quinoline	C9H7N	36.7	0.1672
Cumene	C9H12	36.2	0.2044
Propylbenzene	C9H12	37.14	0.2073
1,2,4-Trimethylbenzene	C9H12	38.03	0.2088
Mesitylene	C9H12	37.87	0.2118
Nonane	C9H20	45.11	0.2702
1-Nonanol	C9H20O	50	0.2654
Naphthalene	C10H8	40.32	0.192
Butylbenzene	C10H14	44.07	0.2378
Isobutylbenzene	C10H14	40.4	0.2215
o-Cymene	C10H14	42.7	0.234
p-Cymene	C10H14	45.27	0.2478
p-Diethylbenzene	C10H14	45.03	0.2439
1,2,4,5-Tetramethylbenzene	C10H14	45.8	0.24
Decane	C10H22	52.88	0.3051
1-Decanol	C10H22O	57.45	0.2971
Undecane	C11H24	60.88	0.3396
Biphenyl	C12H10	47.16	0.213
Dodecane	C12H26	69.14	0.3741
1-Dodecanol	C12H26O	72.69	0.3598
Diphenylmethane	C13H12	60.46	0.2798
Tridecane	C13H28	77.94	0.4109
1-Tridecanol	C13H28O	81.2	0.3942
1-Tetradecanol	C14H30O	89.91	0.4289
Pentadecane	C15H32	96.5	0.4857

* To convert a into atm L2/mol2 multiply by 0.986 atm/bar

 * To convert a into kPa L2/mol2 multiply by 100.0 kPa/bar

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