The author replies to Miller

In his letter in reference to our article, Dr. Miller made a valid point. However, I do not quite agree with his statement "the term [partial volume] is confusing" even though his approach may be pedagogically more sound. While his derivation is simple and avoids bringing up the concept of partial volume at the beginning, his term "effective volume" of air that appears at the end of his derivation is identical to the partial volume of air.

The concept of partial volume and the law of partial volume for a mixture of gases, to my knowledge, have never been formally and openly discussed in any type of textbook. Nonetheless, their validity is obvious. I would like to formally present the Law of Partial Volume as follows:

The law of partial volume states that the total volume of a gas mixture is the sum of the partial volumes of the individual gases. That is, the volume occupied by the gas mixture is a composite of the volumes occupied by each component. Suppose we have a mixture of gases A and B. Then

V_total = V_A + V_B

where V_A and V_B are the partial volumes of gas A and B, respectively. Equations 4 and 5 in the article can then be readily derived from the law of partial volume and Avogadro's law:

V_air/V_total= V_air/(V_air + V_H2O)

= (n_airRT/P)/(n_airRT/P +n_H2ORT/P)

= n_air/(n_air + n_H2O) = P_air/(P_air + P_H2O)

= P_air/P_total

Therefore,

V_air = V_total (P_air/P_total)

or

V_H2O = V_total (P_H2O/P_total)

The concept of volume is simpler and more obvious than the concept of pressure; hence one might assert that the law of partial volume is more obvious than Dalton's law of partial pressure. Perhaps this is why chemists and textbook authors have not addressed such a law. If one can understand the concept of partial pressure, the concept of partial volume should not be confusing, which is contrary to Miller's statement.

Miller termed the left side of his third equation as "effective volume" of air, which is the volume contributed from air in the gas mixture, namely, the "partial volume" of air. It is my hope that the term partial volume and the law of partial volume will be included in beginning chemistry textbooks. Or is it too obvious to discuss?