# Is there a standard symbol for molar density?

Is there a standard symbol for molar density, i.e. $n/V$, where $n$ is the total number of moles of every species in a system?

I realise that there is standard term "molar volume", which is the reciprocal of what I'm asking for, but my equations are going to look pretty silly with $1/V_m$ written everywhere. As a simple example, I want to say that if my system consists of a mixture of $\ce{A}$ and $\ce{B}$ with the reaction $\ce{A <-> B}$, the total concentration of $\ce{A}$ and $\ce{B}$ will be constant and we can write $$\ce{[B]} = c - \ce{[A]},$$ where $c$ is the thing I'm looking for the symbol for.

I didn't find "molar density" in the IUPAC green book, but I don't know my way around it very well, and if it has a different name it might be hiding somewhere I didn't think to look.

To my knowledge, n/V is known as molar concentration and uses symbol $C$ and it seems okay for me to use $C_t$ i.e. total concentration to mark sum of concentrations of all particles in system.

• To my mind this is the best answer. If the OP agrees that "concentration" or at least "molar concentration" is the correct term, I'd recommend accepting this one. If not, I wonder if they would mind explaining a bit more why "concentration" is not the word they are looking for. Commented Mar 10, 2015 at 14:13

I don't know of a standard symbol for molar density, but rho ($\rho$) is a standard symbol for density in general, so if there's no better symbol, I'd go for something like $\rho_m$. As long as you explicitly define it somewhere, it should be okay.

I agree with Aesin, but I have another comment to make. The term "molar density" has a confused meaning. That is because the density of one mole of a substance is identical to the density of two moles of the substance, or any number of moles of the substance.

The term "molar" is most often used to make an extensive quantity into an intensive one.

An extensive quantity is one that doubles if you take two identical copies of a system and put them together. Volume is extensive, temperature is not, it is intensive. Most importantly, density is already intensive, so that "molar density" is essentially devoid of any meaning other than that the question states, the inverse of the molar volume.

• That's a fair point. I intended "molar density" to be read as "particle density expressed in (pseudo)units of moles", but I can see that it could just as easily be read as "density per mole", which would make no sense as you say. Commented Oct 28, 2012 at 0:17

In enzyme kinetics, there is a similar situation, where the symbol $E_t$ is used to indicate the "total enzyme concentration".

In other words, there can be free enzyme with concentration [E], substrate bound enzyme with concentration [ES], and inhibitor bound enzyme [EI], but for the total concentration of all forms of the enzyme the symbol $E_t$ with no brackets is often used.

See for example page 354 of Essentials of Chemical Reaction Engineering by H. Scott Fogler where he has a note about using that symbol.