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It means that if the molecule contains 3 protons less than the query molecule (compare Ionic 

Form: H4L and the left part of the equation in the brackets), for the current dissociation step, the 

pK

a

 value is 17.05.  The number of the atom dissociating is defined in the brackets after the 

semicolon (4 in this case) in accordance with the numbering in the corresponding MOL file.  The 

subscript after "pKa" simply denotes the running number of the pK

a

 value.  

pKa2(H2L/HL+H; 3) = 14.19   0.20 

...  

pKa3(H3L/H2L+H; 5) = 9.51   0.20 

...  

pKa4(H4L/H3L+H; 9) = 5.60   0.30  

This line means that if the molecule contains as many protons as the specified molecule, for the 

current dissociation step the pK

a

 value is equal to 5.6.  The dissociating atom stands under 

number 9.  

pKa5(H5L/H4L+H; 4) =  1.82   0.20  

If the molecule contains 1 proton more than the drawn structure, the pK

a

 value for this step is 

equal to  1.82.  Atom 4 is dissociating, but this time it turns from the positively charged into a 

neutral one.  As compared to the first pK

a

 value (pK

a

 = 17.05), this atom became negatively 

charged. 

4.20 

H(vap) 

This module calculates enthalpy of vaporization values. 

Prior to calculation, you should select the units in which the enthalpy of vaporization is to be 

calculated. 

The resultant dialog box contains structure, enthalpy of vaporization value, and pressure at which 

it was calculated.  All of this data can be printed at once by clicking Print or you may wish to edit 

the default report page in ChemSketch and then print it (click Insert to start editing). 

4.20.1 Limitations 

Restrictions are: 

A maximum of 255 atoms excluding hydrogen can be calculated; and 

Only C, H, O, S, P, N, F, Cl, Br, I, Se, Si, Ge, Pb, Sn, As, and B are permitted elements. 

4.20.2  What is Enthalpy of Vaporization? 

The following equations are used to calculate the enthalpy of vaporization at different 

temperatures: 

4 1

. 9

(n

1)

0 2343

.

0 0021576

.

(BP

in C

)

(1) 

760

60

where 

n = pre assigned number related to the class of compound, which can be estimated according to 

the dependence of functional groups present in the compound.  For example, for ethanol and 

amyl alcohol (non aromatic OH containing) n=7, for acetic acid n=4, for different amines n=3, etc. 

and 

BP

760 

 the boiling point at 760 mm Hg (normal boiling point) 

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