A little late to this discussion, but I would add a couple of thoughts.

1) How accurate do you need the normal boiling point? There is no question that, when accuracy is important, one needs laboratory measurements to get an accurate vapor pressure vs. temperature curve. If accuracy is less important, then there are some different means of estimating vapor pressure vs. temperature.

2) Estimation techniques that I am aware of are almost all based on the principle of corresponding states. If you are unfamiliar with the principle of corresponding states, put it to your favorite internet search engine or wikipedia or look it up in a good textbook. One equation that I am familiar with is the Riedel equation ln(p)=A+B/T+Cln(T)+DT^6. When temperature and pressure are expressed on a reduced basis, the four parameters of the Riedel equation can be expressed in terms of a single parameter. If you can estimate a critical temperature and pressure, then you can derive the rest of the vapor pressure curve. The accuracy of this technique, of course, depends greatly on the accuracy of the estimation of the critical properties.

3) Do you have access to the textbook *The Properties of Gases and Liquids*? This is a good text used by many engineers as a reference for estimation techniques for many different properties. This text describes Riedel's equation, along with some other techniques for estimating vapor pressures.

**Edited by MrShorty, 11 April 2016 - 10:46 AM.**