Take the 2-minute tour ×
Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers and students. It's 100% free, no registration required.

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.

share|improve this question

4 Answers 4

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.

share|improve this answer
    
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. –  Curt F. Mar 10 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.

share|improve this answer

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.

share|improve this answer
    
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. –  Nathaniel Oct 28 '12 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.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.