Active mass represents the effective concentration of a substance participating in a chemical reaction
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Retrieved chemistry discussions verify that active mass is a historical term denoting molar concentration or amount per unit volume participating in a chemical reaction.
# Why is active mass of a pure solid or liquid always taken as unity?
Tags: physical-chemistry, concentration, phase
- Score: 17
- Views: 33720
- Answers: 4
- Answered: yes
- Asked by: user5764
- Asked: 2014-10-31
- Edited: 2015-10-23
- Site: chemistry
## Question
Active mass is defined as the molar concentration ie. number of Gram-moles per litre.
My book then wrote
Active mass of pure solid/liquid is always 1 .
The book reasoned that
Molar concentration is directly proportional to density. Since density of solid or liquid always remains constant, the active mass is taken as 1 .
Really? Doesn't the density of the solid reactant decrease when the forward reaction is about to reach the equilibrium??
In a word, I am greatly confused. What is the reason for taking the active mass 1 ? Please help.
## Answers
### Answer by Jason Patterson (score: 24 [ACCEPTED])
Does the activity of a solid or liquid change over the course of a reaction?
The density of a solid or liquid reactant doesn't change over the course of a reaction. The mass and volume do as it is consumed, but the ratio of the two is constant. If the reaction causes a temperature change then there are small cha
# What is the meaning of 'mass action'?
- Tags: equilibrium
- Score: 5
- Views: 880
- Answers: 1
- Asked by: Yusuke Terada (123 rep)
- Asked on: Mar 11, 2018
- Last active: Nov 15, 2018
- License: CC BY-SA 3.0
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## Question
I understand the statement of 'Law of mass action', but I don't understand clearly what 'mass action' means.
I investigated it and found the following descriptions.
- The law was originally suggested as a law about 'active mass'. \[from [Wikipedia](https://en.wikipedia.org/wiki/Law_of_mass_action)\]
- The term 'active mass' is a historical term that professional chemists do not use anymore. 'Active mass' meant 'amount per unit volume'. \[from [Chemistry Stack Exchange #58141](https://chemistry.stackexchange.com/questions/58141/what-is-active-mass/59577#59577)\]
From these explanations, I understood that 'active mass' corresponds to 'concentration' in modern words, which is usually expressed in the unit 'mole per liter' today.
However, a question still remains for me. What does the term 'mass' intend to originally? I am not a native English speaker, so I cannot understand the nuance well.
The word 'mass' has some different meanings.
- a physical quant
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