Talk:Van der Waals constants (data page)

My copy of HPC (45th edition, 1960-1961, CRC Press) lists the critical values for ethyl acetate as 20.45 and 0.1412. I've graphed the values using Van Der Waals equation, and looking at the critical temperature that would get rid of the loop. In case of the values above, Tc is 523.26K or thereof (523 is listed in my edition of HPC). Using values from this entry (20.82 and 0.1412) critical temperature would have to be higher. --stany

• Your comment raises a number of points, which I shall try to deal with in non-random order.

1. The values in this table are for the coefficients of the van der Waals equation, not for the critical temperature and critical pressure.
2. Values for the critical tempertaure and pressure can be calculated algebraically from the van der Waals constants (there is no need to draw a graph), although I must admit that I can't remember the equations and derivations off hand (it's been three years since I taught this course at college).
3. The van der Waals constants are usually determined by linear regression from a number of data points. Critical temperatures and critical pressures are usually determined by extrapolation. There is no requirement that there be exact agreement (although obviously large discrepancies between the two would indicate problems in either the measurements or in the understanding of the system).
4. The experimental values for the critical temperature and critical pressure of ethyl acetate are highly uncertain: NIST gives T_c = 530±20 K, p_c = 38.3–42.8 bar. This in itself is enough to explain the slight discrepancy which you have noticed.

I hope this helps to clear things up: it short, it's down to experimental uncertainty! Physchim62 10:42, 18 September 2005 (UTC)

• I apologize for using "critical values" in my original comment (English is not my first language). I've meant coefficients of the van der Waals equation. Method I were using was, in fact, linear regression. Thank you very much for your clarification. --stany

style, units

I changed from decimal commas to decimal points because

• Quote from Wikipedia:Manual of Style#Scientific style: "Wikipedia Style for numbers is 12,345,678.901"
• Quoted Source (CRC) was written using decimal points

Also, according to the manual, SI units are preferred, so eventually we should convert. PAR 22:18, 6 November 2005 (UTC)

The style for numbers was a simple error: I uploaded the figures into (my French version of) Excel to covert from atm to bar and to calculate gas densities, but I forgot to reconvert the commas to points for this table!

The units I use are SI compatible (1 bar = 100 kPa; 1 L = 0.001 m³) and in accordance with IUPAC practice: I have corrected your conversions, which were incorrect for a.

Thanks for your vigilant interest in this table! Physchim62 (talk) 11:59, 12 November 2005 (UTC)

Right, that was a mistake. I looked up the bar (unit) article (which I should have done before) and it says "A bar (symbol bar) is a unit of pressure. It is not an SI unit. It is accepted (although discouraged) for use with the SI." Seems like that article is suggesting a conversion. PAR 14:27, 12 November 2005 (UTC)

IUPAC classes the bar as a unit used with the SI, although I seem to remember that there is a footnote to that table in the printed version which states that the bar and the angström "should not be used in circumstances where they are not used already," or words to that effect. The bar is commonly used for pressures above atmospheric, and personally I don't see what is gained by converting everyting to MPa! Still, it is good to have the conversion at hand on the table, thanks for thinking of that. Physchim62 (talk) 09:29, 13 November 2005 (UTC)

bar or atm?

Does the table really give a in L²atm/mol², as stated in its header? For water, other sources available in the Web give a = 5.46 L²atm/mol² [1] or 5.537 L²bar/mol² [2]. My printed copy of Handbook of Physical Quantities edited by I.S.Grigoriev and E.Z.Meilikhov (1996) gives 0.5524 N m⁴ / mol². All this makes me think that the value given in the article (5.536) is actually in L²bar/mol² (bar, not atm!). Could the authors of the article please check/correct this? Thanking you, Potekhin (talk) 08:11, 14 February 2014 (UTC)

a value for benzene seams to be wrong in comparison with other literatur — Preceding unsigned comment added by 178.142.9.69 (talk) 12:38, 2 October 2020 (UTC)

Comparison of values of 'a' for H2 & He

Why value for He is so small comparison to H2. The van der Waals forces increases with increase in molecular mass. But in this case it is not, or there's any other factor? apurva_thatsit 12:45, 29 July 2021 (UTC)