# Questions tagged [units]

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

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### Standard litres to kg conversion for Hydrogen [closed]

When looking at hydrogen fuel tanks, they often give the capacity in terms of "standard litres". However, I can't find a definition of what this is. I assumed this may mean a litre of hydrogen gas ...
27 views

### Rate constant converion [closed]

I need to calculate the half life for a first order reaction for which I need the rate constant $k$. From literature I observed that for a very closely simulated experiment I obtained the numerical ...
97 views

### Units for density in nanorod image

I am counting the number of nanorods contained in a sample (see picture below). The length scale is 1 µm. What units does the density have? Is it µm-2?
38 views

### Are the newer measurements for STP commonly used in high school and university textbooks? If not, why?

I read a Wikipedia article that has the following quote: In chemistry, IUPAC changed the definition of standard temperature and pressure (STP) in 1982: Until 1982, STP was defined as a ...
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 ...
3k views

### Why are absolute entropies given the units J mol−1 K−1?

I've just started looking at the concept of entropy and can't understand why absolute (or standard molar entropies) are given the units $\pu{J mol^−1 K^−1}$. Could someone explain the rationale for ...
2k views

### What does atm•L mean? [closed]

In my chemistry book I always see a dot for units. For example, atm•L. Does this mean “1 atm per Liter?” By Atm I mean atmospheric pressure, not atom.
930 views

### Reaction rate constant conversion [closed]

I'm flustered, I'm not into chemistry and need to convert $\mathrm{(nM\cdot s)^{-1}}$ to $\mathrm{(M\cdot s)^{-1}}$. Is it correct to multiply with $10^9$?
69 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 ...
34 views

### Problem with determining the mass value of a soil sample for gravimetric calculations [closed]

I'm having some trouble with how to determine the representative mass value of a prepared soil sample for calculating its $\ce{Cl-}$ concentration in ppm (mg/kg). Recently I carried out a gravimetric ...
55 views

### Convert the lowest IR wavelength to energy in J, frequency in Hz, and wavenumbers in cm-1

Look up the range of wavelengths in nm in the IR region of the electromagnetic spectrum. Convert the lowest IR wavelength to energy in J, frequency in Hz, and wavenumbers in cm-1. I think I’m going ...
5k views

### Why is it written as “joule” and “J”? [closed]

Why is the unit joule is written as "joule" and "J", I mean what does "J" mean and what does "joule" mean.
1k views

### How many mg caffeine is 1mM caffeine?

I am finding some articles about caffeine consumption confusing, because they test in units of mM instead of mg. https://www.ncbi.nlm.nih.gov/m/pubmed/25342885/ Wikipedia says that the molar mass ...