48
votes
Accepted
How to interpret parts-per notations? Is 1 ppb equal to 1 μg/kg?
You are correct suggesting that 1 μg/kg implies 1 ppb, however the reverse is not true. For instance, 1 ppb can also be 1 nmol/mol, and the reader will never have a chance to deduce which one is it ...
- 38.4k
43
votes
Accepted
How much is 1 mμg?
Looks like in 1966 there were still residues of double prefix notation in use.
Here's an entry from Russ Rowlett's compilation of units on ibiblio.org (Rowlett):
millimicro- (mμ-)
an obsolete metric ...
- 1,379
39
votes
Why do we still need to know about the Rankine temperature scale?
Rankine is commonly used in the aerospace industry in the United States.
Rankine is to Fahrenheit what Kelvin is for Celsius. So when people in the United States were creating programs and using ...
- 491
21
votes
Accepted
What is a Herzberg?
Herzberg filtration speed: The time taken to filter $\pu{100 mL}$ water at $\pu{20 ^\circ C}$ through a filter area of $\pu{10 cm2}$ at a constant pressure of $\pu{5 cm}$ water column (Thomas ...
- 41.1k
19
votes
Where does the label ‘Dq’ to denote the field split in coordination compounds come from?
I will expand my previous comment into an answer.
In the $O_h$ case the metal ion is surrounded by $6$ ligand ions, each with charge $q=-Ze$ located at $\langle\pm a,0,0\rangle$, $\langle0, \pm a,0\...
- 1,058
19
votes
Accepted
What is the intuition behind 'mol' as a unit 'symbol'
The symbol mol is due to Ostwald who was a very influential and respected physical chemist more than a century ago. In German, "mole" is "Mol". It is a shortform of Molekül.
I ...
- 42.2k
18
votes
Why do we still need to know about the Rankine temperature scale?
Most of us in the world use the Celsius scale to measure temperature for day-to-day purposes. The Kelvin scale has been designed in such a way, it is not only an absolute temperature scale, but also 1°...
17
votes
Accepted
What is the unit of Z effective?
This is an example of terminology which should be taken with a grain of salt.
The term "effective nuclear charge" is often casually symbolized with $Z_\mathrm{eff}.$ This is a universally ...
- 38.4k
16
votes
Accepted
What is the correct unit for the equilibrium constant?
Andselisk correctly identified the law of dilution and the name Ostwald is often connected with it.
$$K_\text{dissociation} = \frac{\alpha^2}{1-\alpha}\cdot c$$
However, the degree of dissociation is $...
- 44.3k
14
votes
Accepted
Where does the label ‘Dq’ to denote the field split in coordination compounds come from?
I currently happen to have the book by Figgis that Max mentioned. Chapter 2 is devoted to a mathematical formulation of crystal field theory, which I did not bother reading in detail because I do not ...
- 71.7k
14
votes
Accepted
Can Rydberg constant be in joules?
Authors may be sloppy about notation in this matter. I recommend considering $R_\ce{H} \approx \pu{10973 cm-1}$ and $Ry \approx \pu{2.18e-18 J}$, noting $Ry = hc \cdot R_\ce{H}$. Units of wavenumbers $...
- 2,224
13
votes
Where does the label ‘Dq’ to denote the field split in coordination compounds come from?
A more high-level answer this time. In this approach we first find out the form of the term in the potential that will split the $d$ orbitals.
Recall that the character of a proper rotation by angle $\...
- 1,058
12
votes
Why do we still need to know about the Rankine temperature scale?
I am currently studying mechanical engineering in the US, and I have used Rankine.
It is used similarly to Kelvin. For example, in my thermodynamics class we used it to analyze various heat engines. ...
- 221
11
votes
Accepted
Is zero-point vibrational energy an intensive or extensive property?
Mathematically, an extensive property $f$ is one for which $f(\lambda x,\lambda y,z) = \lambda f(x,y,z)$ for extensive variables $x$, $y$, and intensive variable $z$. For example, the Helmholtz free ...
- 5,511
11
votes
Which unit should I use for m/z?
TL;DR: According to current IUPAC recommendations, $m/z$ is an abbreviation for a dimensionless quantity.
Use of thompson unit is indeed currently discouraged.
There is an overview provided in ...
- 38.4k
11
votes
Accepted
Can someone intuitively explain the reason for the units of entropy (J/K )?
The units of of energy over temperature (e.g. J/K) used for entropy in the thermodynamic definition follow from a historical association with heat transfer under temperature gradients, in other words, ...
- 22.7k
11
votes
What is the dimension of molar mass?
According to ISO ISO/IEC 80000 Quantities and units, BIPM The International System of Units (SI), IUPAC Quantities, Units and Symbols in Physical Chemistry (Green Book), NIST Guide for the Use of the ...
- 4,824
10
votes
Can Rydberg constant be in joules?
Rydberg constant $R_∞$ is usually given in reciprocal length units historically and because it's determined from hydrogen and deuterium transition frequencies [1].
Current value (in $\pu{m-1}$) is ...
- 38.4k
9
votes
How to correctly write "7 apples" according to the international system of units (SI)
Mole is just a scale factor
I find this description very intuitive:
A mole is the amount of pure substance containing the same number of
chemical units as there are atoms in exactly 12 grams of ...
- 91
9
votes
Accepted
Is it possible for me to derive Avogadro's number?
To determine Avogadro's number you have to measure the same unit at the atomic and macroscopic scales.
This was first achieved by Millikan who measured the charge of an electron. The charge of one ...
- 4,392
9
votes
Accepted
How to determine omitted units in a publication
These are degrees Celsius.
To prove that, you could search JACS for the publications of the same year/issue for the temperature-related physical constants such as melting point:
Douglas et al. [1] ...
- 38.4k
9
votes
Accepted
Uncertainty when beaker goes by 50s
A real beaker says "approximate volume".
So the volume is approximately between 100 mL and 150 mL. So maybe 125 mL with an uncertainty of 25 mL. Your daughter would know best because she ...
- 41.3k
8
votes
What term should I use when I am quantifying the amount of matter, mass or weight?
Because of the reasoning provided by you, i.e. mass is universal, and weight is the product of mass and magnitude of the local gravitational acceleration, I suggest to stay at mass. (A purist's view, ...
- 30.5k
8
votes
What is the correct unit for the equilibrium constant?
By definition, the equilibrium constant has no units, as we're supposed to be using active masses instead of the molarity/ concentrations of the respective substances.
Active mass is a unit-less ...
- 1,677
8
votes
In biochemistry, when do rate constant units matter in calculating free energy changes?
$K$ should be a unitless value. Otherwise, you'd get a nonsensical unit when you try to take the log of the unit. Each component of the product $K$ is usually an activity, which is frequently similar ...
- 17.5k
8
votes
Why do we still need to know about the Rankine temperature scale?
Mostly I use Rankine as a "way station" when converting between Fahrenheit and Kelvin, which often happens when working with manufacturing facilities in the USA. Because the Rankine and Kelvin scales ...
- 61.1k
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