# Which units is this conversion factor for (molar volume)? 0.023901488

• I
bumblebee77
TL;DR Summary
This number (0.023901488) is in an old code that I'm trying to use. It's most likely a conversion factor for molar volume or maybe pressure. I cannot figure out what the units are that are being converted. Can anyone please help?
I'm getting the wrong results when using an old, undocumented code and just realized there's a number lurking in it that I can't account for. It's:

0.023901488

and it is multiplied with molar volume and pressure. I have searched for a couple of hours but just can't figure out what the units are. One of them is probably cm3/mol. Has anyone ever encountered it? I would be very grateful for any help.

bumblebee77
I just realized that when molar volume (units of length^3/mol) and pressure (units of mass/(length*time^2)) are multiplied, the result is energy/mol ((mass * length^2) / (s^2 mol)).

I found a table online that shows 1 erg = 2.3901E-11 kilocalories.

I still don't know what my conversion factor (0.023901488) converts (should be able to figure it out but my brain is not working!). I'm pretty sure the code is treating my volumes as cm3/mol. If anyone has any bright ideas, they would be very welcome, thanks!

2022 Award
Well, if you think everything is cgs then ##pV## would be in ergs. Multiplying by ##2.39×10^{-2}## is the same as multiplying your conversion factor by ##10^9## - so instead of converting to ##\mathrm{kCal}## it's converting to ##\mathrm{\mu Cal}##.

vanhees71 and Ibix
bumblebee77
0.023901488 is the molar volume, in metres cubed per mole.
At a temperature of approximately 10 or 15 deg C, and at a pressure of 1 Atm or maybe 1 bar.
https://en.wikipedia.org/wiki/Molar_volume#Ideal_gases
Thank you, Baluncore. I'm not dealing with an ideal gas, but with fluid.

bumblebee77
Well, if you think everything is cgs then ##pV## would be in ergs. Multiplying by ##2.39×10^{-2}## is the same as multiplying your conversion factor by ##10^9## - so instead of converting to ##\mathrm{kCal}## it's converting to ##\mathrm{\mu Cal}##.
Thank you, Ibix. I will think this over!

Staff Emeritus
This is why one should never put comments in code. We'd miss out on all this fun!

anorlunda, malawi_glenn and Bystander
We'd miss out on all this fun!
I really like the way the code is withheld, so we can't cheat by reconstructing the expression. That's what makes an entertaining storyteller.

Gold Member
I have seen passages in textbooks that seem to be keen on 15 °C. Defining the specific heat of water to be some specific amount of heat energy associated with a change in temperature from 14.5 °C to 15.5 °C. If memory serves it's 4.184 J/g. The mixing of cgs units and MKS units is, I think, one of the things the CGPM tried to eliminate with the invention of the SI in 1960.

The goal was to have a coherent system of units where conversion factors other than 1, for example 0.023901488, would not be necessary.

bumblebee77
I really like the way the code is withheld, so we can't cheat by reconstructing the expression. That's what makes an entertaining storyteller.
Not allowed to post it! It's pretty hairy anyway. Figuring out as much as I did for the question took three days. What I'm wondering is whether anyone in my field knows what the units should be.

Anyway, I figured it out. The pressure is in bar and the volume is in cm3/mol. The conversion factor gives me cal/mol. Thank you very much for your interest and suggestions, everyone!

bumblebee77
I have seen passages in textbooks that seem to be keen on 15 °C. Defining the specific heat of water to be some specific amount of heat energy associated with a change in temperature from 14.5 °C to 15.5 °C. If memory serves it's 4.184 J/g. The mixing of cgs units and MKS units is, I think, one of the things the CGPM tried to eliminate with the invention of the SI in 1960.

The goal was to have a coherent system of units where conversion factors other than 1, for example 0.023901488, would not be necessary.
In my field, it's still very common to see non-SI units and because people think of them as traditional, they're often not even identified! So much of my life is about figuring out units. Thanks for the interesting comment on the history of SI.

Gold Member
The pressure is in bar and the volume is in cm3/mol.
If it's measured in cm3/mol, it's not a volume, perhaps the chemists call it the molar volume? A volume would be measured in cm3.

Gold Member
In my field, it's still very common to see non-SI units and because people think of them as traditional, they're often not even identified!
It's very common in almost every field to use non-SI units. The CGPM has gone so far as to approve some non-SI units for use with the SI.

Leaving the units unidentified is a huge no-no in physics. Anyone who has taken a physics course has hopefully had that practice drummed out of them.

I remember the confusion I had learning how to calculate the body mass index (BMI). People told me to just use the web, just enter your height and weight and it will tell you your BMI. I finally figured out how to do it myself, but I would have been saved a lot of effort if someone had just told me that the units of BMI are kg/m2. All you do is take your mass in kilograms and divide it by the square of your height in meters.

bumblebee77
It's very common in almost every field to use non-SI units. The CGPM has gone so far as to approve some non-SI units for use with the SI.

Leaving the units unidentified is a huge no-no in physics. Anyone who has taken a physics course has hopefully had that practice drummed out of them.

I remember the confusion I had learning how to calculate the body mass index (BMI). People told me to just use the web, just enter your height and weight and it will tell you your BMI. I finally figured out how to do it myself, but I would have been saved a lot of effort if someone had just told me that the units of BMI are kg/m2. All you do is take your mass in kilograms and divide it by the square of your height in meters.
That is interesting! I'm embarrassed that I never even thought about the units as far as BMI. Now I'm afraid to check.

Yes, it was certainly drummed into me to be clear about units. My colleagues think I'm an amateur when I do it. There are a couple of very old codes that people use and I have found that parts of them are not consistent with other parts. It's a bit scary to think what results might be out there that are based on the codes.

bumblebee77
If it's measured in cm3/mol, it's not a volume, perhaps the chemists call it the molar volume? A volume would be measured in cm3.
Yes, you're right.