# Tag Info

## Hot answers tagged units

39

Yes, you may. It is quite common to convert units into each other. The simplest conversion might be the prefixing of units, e.g. $$\mathrm{\frac{km}{m}}=1000 \Longleftrightarrow \mathrm{1~km = 1000~m}.$$ Another example is the interconversion of units of energy. In some parts of chemistry, it is still quite common to use calories, in others Joule, as a SI ...

39

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 unless you explicitly define the usage of the "parts per something" in the text. This clutters the manuscript with redundant notes and causes overall confusion. ...

38

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 equations that needed an absolute temperature, they used Rankine before Celsius became dominate for scientific calculations. The reason people still sometimes use ...

28

Treating units as if they were algebra is called “dimensional analysis”. One example given in that article is the question of how many seconds are there in two years. $$2\ \mathrm{yr} \times 365\ \mathrm{day}\ \mathrm{yr}^{-1} \times 24\ \mathrm{hr}\ \mathrm{day}^{-1} \times 60\ \mathrm{min}\ \mathrm{hr}^{-1} \times 60\ \mathrm{s}\ \mathrm{min}^{-1} = ... 27 Not only may you, but you absolutely should. As a physics professor, watching students toss away the information contained in units is incredibly frustrating. Typically students drop the units, "do the math," and then put back on whatever final units they think should be there (if they bother with the last step at all). This leads to a shockingly large ... 20 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 Scientific). The values are given in seconds. So, the example OP has given has Herzberg filtration speed of \pu{375 s}. That means the relevant filter paper takes \... 18 The two units torr, and mm of Hg were the same until they were redefined. The torr was named after the Italian Evangelista Torricelli. 1 atmosphere is 101325\ \mathrm{Pa}. The torr is defined as 1/760 of an atmosphere. This is equal to 133.322\overline{368421052631578947}~\mathrm{Pa}, which periodically infinitely repeats. The mm of Hg was defined as ... 18 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°C change is equal to a 1K change. This makes conversion from Celsius to Kelvin pretty easy, involving just the addition or subtration of a certain constant (in ... 18 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 believe the Internet Archive has an English translation of his historically famous book of 19th century. This is where I recall reading this. The original reference ... 16 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\rangle, and \langle0,0,\pm a\rangle. The electrostatic potential of a point charge q is$$V=\frac{kq}R$$where$$k=\frac1{4\pi\epsilon_0}$$is the Coulomb's ... 16 Andselisk correctly identified the law of dilution and the name Ostwald is often connected with it.$$K_\text{dissociation} = \frac{\alpha^2}{1-\alpha}\cdot c$$However, the degree of dissociation is \alpha and has "no" unit, i.e. dimensionless quantity. Therefore the unit for the equilibrium constant is that of a concentration, in SI that would be \pu{... 15 The -1 means "per" unit. So your first example mol/L-1/s-1 is not correct - it would actually be written as mol L-1 s-1, OR mol/(L s). It is also sometimes written as mol/L/s, but the double division is ambiguous and should be avoided unless parentheses are used. If it were mol L-1 s-2, this would mean moles per litre per second per second. This is really ... 15 This is an example of terminology which should be taken with a grain of salt. The term "effective nuclear charge" is often casually symbolized with Z_\mathrm{eff}. This is a universally accepted simplification, but you should keep in mind that the effective nuclear charge is, strictly speaking, Z_\mathrm{eff}e, where e is the elementary ... 14 Authors may be sloppy about notation in this matter. I recommend considering R_\ce{H} \approx \pu{10973 cm-1} and Ry \approx \pu{2.18e-18 J}, noting Ry = hc \cdot R_\ce{H}. Units of wavenumbers (\pu{cm-1}) and energy are often considered interchangeable in practice because they are proportional to each other by the constant value hc. In my notes, ... 13 I currently happen to have the book by Figgis that Max mentioned. Chapter 2 is devoted to a mathematical formulation of crystal field theory, which I did not bother reading in detail because I do not understand any of it. (The corollary is: If you have a better answer, please post it!) It seems that both D and q are collections of constants defined in ... 12 Absolutely! Having taught physics and planetary science for years now, this is how I've seen unit conversion taught most effectively. Example Suppose we wish to convert 423 feet to kilometers, where we know 3280.4 \; \text{ft} = 1 \; \text{km}, or \frac{1 \; \text{km} }{3280.4 \; \text{ft}} =1:$$\require{cancel} 423 \; \text{ft} = \left( 423 \; \...

12

May I manipulate the equation as though $T$ and $B$ were the kinds of symbols that we learned to manipulate in math class? Yes, you can and should (that is, never leave out the units when you plug values into an equation: a quantity is always the product of a numerical value and a unit, and leaving out units is asking for disaster). The set of rules that ...

12

The $^{-1}$ superscript can be thought of as saying "per" or as being the denominator of the fraction. So in your example $\mathrm{mol \cdot L^{-1} sec^{-1}}$ can be thought of as saying moles per liter per second. This is easier than writing $\mathrm{\frac{mol}{(L \cdot sec)}}$ Changing the super script from $1$ to $2$ or $3$ would change the meaning ...

12

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. Tables are available with properties of gases and steam using such units as BTU/R. I can tell you that it is somewhat of a pain to use, because it is often ...

11

Your problem is that you've assumed the density is 1 gram/mL. Remember that a molar is defined as a mole of solvent per liter of solution, not solvent. Usually, in introductory chemistry classes, we skip over the fact that adding solute to a solution increases its density, because it makes life more complicated. As you just found out though, sometimes you ...

11

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 Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013) [1, p. 1516] (reference numbers were updated): Labeling of mass spectra The labeling ...

11

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, the definitions of temperature and entropy are intertwined, with entropy being the more fundamental property. Entropy can be thought of as a potential and ...

10

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 $\alpha$ for the set of spherical harmonics with $L^2Y_l^m(\theta,\phi)=\hbar^2l(l+1)Y_l^m(\theta,\phi)$ is $$\chi_l(R(\alpha))=\sum_{m=-l}^le^{im\alpha}=\frac{\... 10 Rydberg constant R_∞ is usually given in reciprocal length units historically and because it's determined from hydrogen and deuterium transition frequencies [1]. Current value (in \pu{m-1}) is listed at NIST [2] website (accessed 2019-06-05):$$R_∞ = \pu{10973731.568160(21) m-1} Since it's an energy unit, one can convert it to SI rather trivially via ...

9

The turnover frequency is a rate. (It's actually the kinetic rate of the reaction in saturating substrate concentration, normalized to the amount of enzyme.) It might be helpful to think about something like a factory that makes a certain number of widgets in a certain number of time. How would you naturally describe how fast the factory could make widgets? ...

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