Questions tagged [units]

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

Filter by
Sorted by
Tagged with
33
votes
6answers
3k views

May I treat units (e.g. joules, grams, etc.) in equations as variables?

Suppose I have two units, Tonks, $T$, and Borks, $B$, as well as the equation $\frac{50T}{20B}=x$. May I manipulate the equation as though $T$ and $B$ were the kinds of symbols that we learned to ...
24
votes
5answers
6k views

Why do we still need to know about the Rankine temperature scale?

I am learning process principles in chemical engineering and I was taught various temperature units like Fahrenheit, Celsius, Kelvin, and Rankine. I see the first three being used quite commonly, but ...
15
votes
1answer
2k views

Is 1 ppb equal to 1 μg/kg?

In an article I recently submitted, a reviewer asked that I provide a concentration in μg/kg instead of ppb (parts per billion), and mentions that the later is not correct. I am not a chemist, and I ...
14
votes
1answer
26k views

What is the difference between the units 'torr' and 'mm of Hg'?

According to present definition torr and mm of Hg differ though slightly. I am still confused over it. Can someone write an explicit answer differentiating between them and the reason for ...
14
votes
2answers
682 views

Is zero-point vibrational energy an intensive or extensive property?

The zero-point vibrational energy (ZPVE) is the non-zero amount of non-electronic energy that molecules have even at zero kelvin and is purely quantum mechanical in nature. For a molecule with $c$ ...
13
votes
3answers
2k views

Where does the label ‘Dq’ to denote the field split in coordination compounds come from?

As established in a previous question, coordination compounds typically have a field split between the $\mathrm{t_{2g}}$ and the $\mathrm{e_g}$ d-orbitals.[1] This energy difference can be explained ...
10
votes
2answers
5k views

What is the correct unit for the equilibrium constant?

I have the following equation $$K_a = \dfrac{c\alpha^2}{1-\alpha},$$ with units $c= \pu{mol/cm^3}$ and $\alpha$ is the same. I'm not sure how to get the correct units for $K_a$. What I have come up ...
9
votes
3answers
994 views

Can Rydberg constant be in joules?

In my textbook (Chemistry Part - I for Class XI published by NCERT), there is an equation for the energy of an electron in an energy state: $$E_n = -R_\mathrm H\left(\frac{1}{n^2}\right)$$ and there ...
9
votes
2answers
528 views

How to determine omitted units in a publication

I have found a 1954 paper in J. Am. Chem. Soc. which gives various temperatures in degrees but does not specify units. A snippet of the introduction is: Titanium reacts appreciably with fluorine ...
9
votes
3answers
531 views

Units of rate constant (cycles per second vs radians per second) in Eyring equation

In the Eyring equation (EE), $$k = \frac{k_\mathrm B T}{h} \exp\left(\frac{-\Delta G_{\mathrm f}}{RT}\right),$$ the units of $k$ are $\mathrm{s^{-1}}$. However, in general rate constants are usually ...
8
votes
6answers
3k views

How to correctly write “7 apples” according to the international system of units (SI)

According to international system of units (SI), we can write "7 kg of apples" to refer to the mass of these apples. However, if we want refer to the amount of apples, that is, the number of ...
8
votes
1answer
533 views

Effects of Changing Avogadro's Constant

The Avogadro project suggests that we redefine the Avogadro constant to be equal to our best known estimate, $N_\mathrm{A} = 6.02214179 \times 10^{23}$, and redefine the kilogram based on the Avogadro ...
7
votes
1answer
1k views

Units in modified Arrhenius equation?

The modified Arrhenius equation is used to express the rate constant in a chemical mechanism model I'm working with. The equations is as follows: $$k_\mathrm{f} = A\times T^b\times\exp\left(-\frac{E_\...
7
votes
2answers
2k views

When to use Da or u for mass units

Following up on the question Units of mass on the atomic scale, Daltons (Da) and unified mass units (u) are equivalent, but which is used seems to depend on context. For example, when discussing ...
6
votes
1answer
574 views

Does adding prefixes to SI units make them no longer SI units?

Our chemistry professor said that "mg for mass, g/m3 for density, ms for time are all SI units". I want to make sure now, even if we added a prefix to the unit, will it still be an SI unit?
6
votes
2answers
254 views

In biochemistry, when do rate constant units matter in calculating free energy changes?

In my experience, it is always been the ratio of products to reactants that mattered, so as long as your units are consistent; what they are doesn't matter. However, an in-class problem today required ...
6
votes
1answer
1k views

What units do I use for energy in the Boltzmann distribution?

I am a mathematics student doing a course in thermodynamics and am working on the basics now. The question I am asked is: Suppose two states differ in energy by one electronvolt. What can be said ...
6
votes
2answers
895 views

Why a specific temperature for value of 1 calorie?

Everything in chemistry often comes with a strong reason and facts behind it, but I stuck in definition of $1$ calorie. My reference book define one calorie as amount of heat required to raise the ...
6
votes
1answer
110 views

What advantage is there in reporting the Lennard-Jones well depth in wavenumber units?

The Lennard-Jones well depth $\epsilon$ is typically given in energy units ($\mathrm{kJ}$ or $\mathrm{kcal}$; sometimes per mole). Jasper and Miller, however, reported $\epsilon$ values in reciprocal ...
5
votes
3answers
5k views

How to retrieve Dalton value from m/z value?

As I understand, Dalton (Da) is the standard way for representing mass unit. As for $m/z$, which is a specific notation for mass spectrometry, quoting for wikipedia: This notation eases data ...
5
votes
3answers
476 views

What term should I use when I am quantifying the amount of matter, mass or weight?

I am writing an academic article but I got confused as I am writing about the weight a specific metal got by using balance scale. Should I use the term mass, which has a unit of kg, or weight, which ...
5
votes
2answers
315 views

What are the units of P and V when R is expressed in terms of J/(mol K)?

I have learnt the ideal gas equation $PV=nRT$. However, I have some doubts regarding the units of the quantities in this equation. When we take the gas constant $R=8.314\ \mathrm{J\ mol^{-1}\ K^{-1}}...
5
votes
1answer
167 views

Why the term 'unified' in “unified mass unit”? [closed]

Why different unit symbols - amu, u and dalton are used for representing atomic mass? And what does the term 'unified' mean in "unified mass unit"? Thank you.
5
votes
2answers
142 views

Force Fields and units

I am a math student trying to implement a simple force field in python for determining the energy of a small molecule and trying to minimize it in order to find a stable configuration. I was wondering ...
5
votes
1answer
117 views

Amount of substance expressed in yoctomole units

I stumbled upon a couple of papers where amounts were expressed in yoctomoles ([ymol], $\pu{10^{-24} mol}$) and I find it somewhat bothersome as 1 ymol would correspond to about 60% of atom/molecule/...
4
votes
4answers
32k views

What does the ‘−1’ superscript mean in units?

For example, the rate of a chemical reaction can be expressed in $\mathrm{mol}/\mathrm{L}^{-1}/\mathrm{sec}^{-1}$. Why is it ‘−1’ and not, say, ‘−2’? Does it change the meaning if the minus is removed ...
4
votes
4answers
1k views

Is the ideal gas constant not a constant if Celsius is used instead of Kelvin? [closed]

For example, Charles' law does not work with Celsius. Why is this? I can only think that the gas constant is not a constant when Celsius is used rather than Kelvin.
4
votes
2answers
480 views

Relation of pressure and temperature for an ideal gas at constant volume

I have a tank of ideal gas with volume $3.1\ \mathrm L$; I have $R = 8.314$, and 11 moles. The tank is filled at $23\ \mathrm{^\circ C}$ (not sure why that's relevant) and the tank will rupture at $P =...
4
votes
3answers
490 views

What is the physical quantity/magnitude of pH?

I have a database to classify variables/parameters in water resources as follows: Variable: Precipitation (P); Quantity: Length; Units of measurement: mm (millimeters). Following the example, what ...
4
votes
1answer
910 views

Units of mass on the atomic scale

what are the systems of recording atomic masses and their units? I know that the nucleon number is the number of protons and neutrons I also know that the mass number on the periodic table is the ...
4
votes
1answer
64 views

Is the kilogram still consistent with the old definition?

My old understanding was that a gram is the amount of mass in a cubic centimeter of water. But then I recently learned that the second was redefined to be some 9 billion cycles of a Caesium atom. ...
4
votes
1answer
1k views

Do ALL conversion factors have an infinite number of sigfigs?

Conversion factors have an infinite number of sigfigs, right? They should be exact values; but not all of them are, are they? I looked at http://www.webassign.net/question_assets/buelemphys1/...
4
votes
2answers
4k views

SI Units for Isothermal Incompressibility and Specific heat capacity at constant volume

I am looking for the SI units of isothermal incompressibility and specific heat capacity at constant volume. What I found was that isothermal compression was measured in " $\mathrm{m^2}\ \mathrm{N^{-...
4
votes
1answer
746 views

How to compute the residence time for given flow rate in standard cubic meters?

I'm given a flow rate of a gas mixture of $3\ \mathrm{Nm^3/h}$ in standard cubic meters through a cylinder of length $50\ \mathrm{cm}$ and cross section $70\ \mathrm{cm^2}$, can I compute how long a ...
4
votes
1answer
275 views

What's an “amagat”? How is it different from the Loschmidt Number?

At class, we were (briefly) taught about the Loschmidt Number/Constant ($n_0$), which isn't really different from the Wikipedia definition, The Loschmidt constant or Loschmidt's number (symbol: $...
4
votes
1answer
839 views

How to find the units of the rate constant?

Consider the reaction $$\ce{2A + B -> Products}$$ When the concentration of $\ce{B}$ alone was doubled, the half-life did not change. When the concentration of $\ce{A}$ alone was doubled, the rate ...
4
votes
0answers
35 views

What does “at%” mean and how it differs from “mol%”?

Sorry if my question is a bit obvious, but for some reason I'm getting confused with the percentages in this question: Concentration of $\ce{A}$ is $\pu{80 at\%}$ and concentration of $\ce{B}$ is $...
4
votes
0answers
195 views

Difference between change in energy level and hyperfine energy level?

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. --> This ...
4
votes
0answers
932 views

What do we have both r.a.m. and r.m.m.?

Read Orthocresol's answer here first: What is the difference between "molecular mass", "average atomic mass" and "molar mass"? Orthocresol, in his answer, says that ...
4
votes
0answers
488 views

Calculating volume ethanol in an fermented aqueous solution of sucrose, water, and ethanol

I've been working on a biology experiment where I calculate the fermentation yield of yeast under various light induced stress responses inhibiting the translation of various fermentation enzymes. My ...
3
votes
3answers
967 views

Is 44.5 mg/L a measurement of weight or volume?

I have a test result that I am trying to fully grasp. I need to determine how much of a contaminant I need to reach that amount in 4,800 gallons. Making the conversion to liters is no problem, nor is ...
3
votes
4answers
10k views

Why is the units of kcat 1/s?

I understand that $k_\text{cat}$ measures the turnover number of an enzyme. This measure is therefore a quantity of molecule conversions per unit of time. I suspect that my problem is more that of a ...
3
votes
2answers
181 views

Why is the equilibrium constant unitless?

I read that the equilibrium constant is unitless because the molar activities of each of the species are used in the equilibrium expression, not the actual concentrations themselves. I understand that....
3
votes
1answer
5k views

How is the unit of measure “equivalent” abbreviated?

I usually use "eq" but I have been told to use "equiv." in order to avoid confusion with "equation". Is there a reference I can check?
3
votes
2answers
2k 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 16 amu. What was the mass of ...
3
votes
1answer
107 views

Is it possible for me to derive Avogadro's number?

To my understanding the mole is the unit used to translate between mass on the atomic level and mass in the macro level, defined as the number of atoms in $12$ grams of Carbon-$12$, which apparently ...
3
votes
1answer
126 views

Order of the reaction between hemoglobin and oxygen

I have the reaction: $$\ce{Hb + O2 <=>[$k$][$k'$] HbO2}$$ where $\ce{Hb}$ is hemoglobin, $k$ is the rate constant of the forward reaction, $k'$ is the rate constant of the inverse reaction. On ...
3
votes
1answer
766 views

What does 10 Vols mean?

I am currently working on a project and the protocol says the following, "Wash resin 3 times 10 vols of the extract buffer." This is very confusing to me and I am not exactly sure what the 10 vols ...
3
votes
1answer
159 views

What's with the weird definition for the Calorie? [duplicate]

I came across this definition for the calorie, which I found... strange: "The (15°) calorie is the heat required to raise the temperature of one gram of water from 14.5°C to 15.5°C." Subsequent ...
3
votes
1answer
14k views

Convert from Pa to Pa to J/dm^3 [closed]

Given the value 380 PaK^-1, how do I convert Pa to Jdm-3? I know that 1 joule = 1 Nm, and that 1 pascal = 1 N/m^2. But where does the dm^-3 (or L) come into this? I don't know where to start.