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Previous articles on Henry's Law generated several additional questions, which are answered here.
Henry's Law is a commonly used method for calculating phase equilibria, particularly, but not exclusively, the solubility of a gas in a liquid. An earlier article (I) presented various forms of Henry's Law and concentrated on the effect of pressure and solute concentration. A subsequent article (2) covered the application of Henry's Law to multicomponent mixtures. A third article (3) placed Henry's Law in a historical context.
Those publications proved to be quite popular and generated a lot of discussion, which has become the basis of this article. Here, a few more Henry's Law concepts are presented in the hope that it will answer some of the questions posed since the original article.
HENRY'S LAW vs. RAOULT'S LAW
Perhaps the most common question I have been asked over the years is "When should I use Henry's Law and when should I use Raoult's Law?" Although they appear to be significantly different, the basis for the two approaches is similar.
Equilibrium calculations
In the hypothetical example illustrated in Figure 1, we can assume ideal Raoultian behavior for component 1 for compositions between 100% and about 92%. Such an approximation results in an error of less than 1%. However, assuming ideality in a Henrian sense would result in a very large error.
Assuming that ideality for the Henry's Law standard state applies for component 1 for the composition range of about 0% to 15% would result in errors less than 10%. On the other hand, there are significant deviations from Raoult's Law - such an assumption would result in an error of more than 60%, in the worst case. At 15% component 1, the actual fugacity is 23.2 kPa; the estimate from the simple Henry's Law is 25.8 kPa, whereas the strict Raoult's Law yields 15.2 kPa.
There are only very rare cases where a system is ideal in both the Raoult sense and the Henry sense. These are systems that obey the strict Raoult's Law over the entire range of composition.
Henry or Raoult which to use?
The choice of using Henry's Law or Raoult's Law is merely one of convenience. At first glance, Raoult's Law appears to be simple to apply....