# Questions tagged [units]

For questions about the definition and usage of units of measurement.

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### Is the negative excess volume of ethanol/water the cause of my math error?

I have a binary mixture, say ethanol at $90\%wt$ ethanol, $10\%wt$ water. According to Perry's Chemical Engineers' Handbook 9th Ed., that mixture has a density of $0.81797g/cc$ (at $20^{\circ}C$). ...
2answers
263 views

### Regarding units of water potential and chemical potential

If water potential is nothing but the chemical potential of a given solution, then why do their units differ? If the unit for the latter is taken as 'Energy per molecule' (which makes sense to me ...
1answer
68 views

### Would 10.0 mL of water at 10 °C have the same mass as 10.0 mL of water at 25 °C? [closed]

I am reading a book and it gives the following answer: No, the density of water changes with temperature. 10.0 mL of water at 10 °C would have a higher mass than 10.0 mL of water at 25 °C because the ...
2answers
5k views

### How to convert ppm to g/m3?

I'm not very comfortable with chemistry and I need to convert $\ce{CH4}$ concentration values in $\pu{ppm}$ to $\pu{g/m3}$. Is that possible? I've already researched a bit and realised that for water ...
1answer
170 views

### Why are these two methods of converting from ppm to g/m3 equivalent?

I've found two methods to convert from $ppm$ to $g/m^3$. They both appear to give equivalent results, but I don't understand why. Can anyone join the dots for me and explain why they are equivalent ...
1answer
3k views

### How much is 1 mμg?

So I stumbled on a research paper by Okuda and Takedatsu  that uses mμg as a symbol for the mass unit. I couldn't find anything about this measure unit anywhere except a recommendation not to use ...
2answers
54 views

### Is 1% m/m the same as 1% w/w?

Hydrocortisone cream seems to be sold as 1% w/w, but my cream is 1% m/m, which is confusing. Are they the same units or the same concentrations?
1answer
65 views

### What is the 'electronic energy', in units of 'negative' hartrees, of a molecule?

On a NIST (National Institute for Standards and Technology) page on the molecular details of water and it's various isotopologues, I came across this: Electronic energy: -76.4089547246 hartree There ...
5answers
714 views

### Why do the units of rate constants change, and what does that physically mean?

Just to be clear, I do understand that the units of the rate constant k is selected to make the equation dimensionally consistent. That is not what I am asking. It should be the case that k 'means' ...
1answer
79 views

### Why does osmolarity take into account both penetrating and non-penetrating solutes?

According to Wikipedia: … osmolarity takes into account the total concentration of penetrating solutes and non-penetrating solutes, whereas tonicity takes into account the total concentration of non-...
2answers
93 views

### What is the difference in mosM and mOsm units, if there is any at all?

I'm reading Agre's paper on aquaporins  and he uses a unit called "mosM" throughout the paper, for example: … ocytes were transferred from 200 mosM to 70 mosM modified Barth's buffer. I ...
0answers
27 views

### When multiple sources give the content of an ingredient my an unqualified percentage, can I safely assume this is by weight? Or; it's not a wart

I am trying to make a simple scented shampoo and body soap for my own use and for Christmas gifts. I statr with a base of 32 oz – (a bit under a kg) of a mild commercial castille soap (Dr. Bronner’s ...
0answers
95 views

2answers
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### How to convert g/L to mol/L

I know I should divide $\pu{g L-1}$ by the molar mass of the substance, but I don't seem to find the specific answer on Google. So just to be sure: If I have $10^{-5}~\pu{g L-1}~\ce{Cu^2+}$ solution, ...
2answers
3k views

### Atomic Mass of ¹²C before 1961

Question: Prior to 1961, the atomic mass unit was defined as 1/16 the mass of the atomic mass of oxygen; that is, the atomic mass of oxygen was defined as exactly $\pu{16 amu}$. What was the ...
1answer
31 views

### Should I relate the number of moles between substances when converting weight units?

Good day! I graduated from an undergrad degree of chemistry almost 3 years ago but I'm frustrated that I fail to reconcile these things: In gravimetric analysis, to get the correct amount for the ...
1answer
393 views

### Why is gas pressure measured in mmHg?

Pressure can be measured in $\pu{mmHg}$ which is equal to the pressure exerted by a “column of mercury” of a height expressed in $\pu{mm},$ at $\pu{0 °C}$ under Earth’s normal gravity. Does the ...
3answers
292 views

### About the use of pascal or atmosphere in chemistry

I was wondering if there are specific formulae in chemistry for which it is "compulsory" or convenient to use atmosphere (atm) rather than pascal (or even mmHg) as a unit of measurement. Of course, ...
1answer
40 views