I'm so much confused about what active mass actually is, my text book states that, active mass is a dimensionless quantity, but for our convenience and purposes, we take it with dimensions of molarity, partial pressure, etc. But I searched, and I got that, historically, what Guldberg and Waage stated about active mass in the law of mass action, is " Parts of body present in 1cc of the total volume " (Rough translation), now it contradicts that, it's dimensionless. On Google I found, that active mass is actually a ratio of mass which participates in reaction and total mass and it goes true with the fact written in my text book, that active mass of pure liquid and pure solids is unity. So, what is actually the definition of active mass ? What is actually about the dimensions ? And why we take active mass of pure solids and liquids as unity ?
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I have never heard this term used. Based on your context, you might be referring to several things. One of them is activity, which is unitless representation of the effective concentration. In $\Delta G^{\varnothing} = -RT \ln K$, the equilibrium constant must be unitless, so we need unitless concentrations. – Zhe Dec 28 '16 at 13:16