Questions tagged [units]

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

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93 views

What is the name of the physical quantity that is given in kJ/g?

The chemical potential $\mu$ of a pure phase corresponds to the Gibbs free energy $G$ divided by the amount of substance $n$: $${G\over n} = \mu,$$ so the units of $\mu$ are $\pu{kJ mol^-1}.$ What do ...
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Units of collisional cross-section and reactive cross-section

I haven't been able to find the units of the reactive cross-section (σ* = Pσ) online but when I do a unit analysis from the equation: $$ k = PσN_A √(\frac{8k_BT}{πμ}) e^{-E_a/RT}$$ then when plotting ...
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1answer
60 views

How switching values of $R$, $K_p$ and $K_c$ does not alter the value of change in standard Gibbs energy? [duplicate]

$\Delta_\text r G=RT\ln \frac{Q}{K}$ If we switch $K_p$ from $K_c$, accordingly $Q_p$ will change to $Q_c$. Therefore, value of $\Delta_\text r G$ remains the same. $ΔG^\circ=−RT\ln K=ΔH^\circ−TΔS^\...
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1answer
72 views

Is Avogadro's constant equal to 1? [closed]

The mole is defined to be exactly $\mathrm{mol}=6.02214076\cdot 10^{23}$ entities. Avogadro's constant is defined as $N_A=6.02214076\cdot 10^{23}\mathrm{mol}^{-1}$. Then why is it not usual to write $$...
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2answers
69 views

How does the value of the equilibrium constant give information about the extent of reaction at equilibrium?

Pretty much everywhere I've read, the numerical value of the equilibrium constant gives you information about the relative concentrations of products and reactants in the equilibrium mixture; the ...
2
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0answers
51 views

Units conversion of Henry coefficient in adsorption on solids [closed]

In a database of solid microporous materials I have the Henry coefficient in units of $\pu{mol kg^{-1} Pa^{-1}}.$ How to convert to $\pu{Pa^{-1}}?$ I tried to multiply by the molar mass of the sorbate ...
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1answer
498 views

Will Avogadro's number change if definition of atomic mass unit is changed?

I know that the relation between atomic mass unit and Avogadro's number is a reciprocal relation. My question is if the definition of atomic mass unit is changed, suppose instead of 1/12th mass of ...
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44 views

Why isn't entropy dimensionless?

The usual units are J/K, however the only reason temperature has different units than energy is because we didn't used to know temperature was just the average kinetic energy of atoms/molecules. ...
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1answer
76 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 ...
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1answer
194 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 ...
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1answer
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How much is 1 mμg?

So I stumbled on a research paper by Okuda and Takedatsu [1] 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 ...
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2answers
83 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?
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1answer
124 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 ...
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5answers
962 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' ...
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29 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 ...
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2answers
106 views

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

I'm reading Agre's paper on aquaporins [1] 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 ...
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99 views

Help with Forster distance conversion for FRET

For my project on Fluorescence, I'm trying to simplify the equation for the Forster distance (I obtained this from the Lakowicz book). To keep track, the units are in square brackets. $$R_0 = \left(\...
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1answer
172 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-...
3
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1answer
73 views

Are the units of mole of oxygen molecules the same with the units of mole of nitrogen molecules?

Suppose we have in a box $3 \,{\rm mol}$ $\ce{O}$ and $3 \,{\rm mol}$ $\ce{N}$. Is the addition of these two quantities meaningfull? We do it all the time when we want to find the molar fraction. I ...
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1answer
170 views

Is it accurate to describe these radiation measurement units in terms of where-it's-measured and relationship-to-time?

I'm reading about measuring radiation and trying to come up with a more intuitive retelling for an audience familiar with math but not necessarily chemistry or any physical science. Is any or all of ...
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2answers
2k views

What is the intuition behind 'mol' as a unit 'symbol'

Why are moles written with 'mol' as opposed to something which is easier to write with one character, or even two?
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1answer
63 views

Calculate concentration of 10 µM to ng/ml Cannabidol [closed]

I would like to calculate the concentration of 10 µM unit in vitro to blood plasma unit ng/ml? What formula do I need to use? Do I need to use molecular weight? In my case, it is 314,47 g/mol for ...
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21 views

Thermochemistry: On Units of Enthalpy Change and Implicit Given of Initial Temperature of Calorimeter

I would like to ask two separate questions. But first, so that we are on the same page, the experiment used a constant-pressure calorimeter, and it is a neutralization reaction of 1M of HCl and 1M of ...
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2answers
214 views

Why is the units of rate constant dependent on the order of the reaction?

In many textbooks, it is written that For order (m + n), the rate constant, k has units of mol1−(m+n)·L(m+n)−1·s−1 However, by the Arrhenius equation, k can be expressed as k = PZexp(-Ea/RT) and ...
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1answer
1k views

What is a Herzberg?

Whatman lists the flow rates for their filters in a unit called a 'Herzberg': What is this? I've been searching around and I did find information about Herzberg flow rate testers, for example this ...
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2answers
979 views

What is the dimension of Avogadro's constant?

What is the dimension of Avogadro's constant ($N_\mathrm{A}$). On Wikipedia it says it is dimensionless, but in Nigel Wheatley's article (pdf) On the dimensionality of the Avogadro constant and the ...
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1answer
1k views

What is the unit of Z effective?

Z effective is the net positive charge experienced by an electron. But what is the unit of this net charge: coulomb (C) or microcoulomb (μC), or something else?
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1answer
1k views

Are atomic mass unit (amu), unified mass unit (u) and Dalton (Da) the same unit but with different names? [duplicate]

I recently read up on some chemistry and found these units of mass: atomic mass unit (amu) unified atomic mass unit (u) Dalton (Da) Are they all the same unit but just with different names or are ...
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1answer
156 views

Heat Transfer Coefficient Problem

The Problem An unusual reference gives the formula below for the heat transfer coefficient of an emulsion. $$H = 0.0061\left(X^{0.6} + 0.038T\right)\cdot V^{0.35}$$ In the formula $T$ is the ...
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2answers
55 views

Calculating the molar volume of a perfect gas under standard ambient temperature and pressure [duplicate]

I am trying to calculate the molar volume of a perfect gas under standard ambient temperature and pressure, but I'm having problems with the units. $$\begin{align} V_m = \dfrac{RT}{p} &= \dfrac{(8....
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1answer
884 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 ...
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1answer
33 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 ...
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1answer
46 views

Question on notation

Here, when they write $v/(\mathrm{mol\ s^{-1}\ (kg\ \text{protein})^{-1}})$ is $0.30$ for the first row first column, do they mean that the value is $0.30\ \mathrm{mol\ s^{-1}\ (kg\ \text{protein})^{-...
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1answer
35 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 ...
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2answers
1k views

Can someone intuitively explain the reason for the units of entropy (J/K )? [duplicate]

If entropy is just a measure of the "randomness" or "chaos" of a system, then why does it have specific units. What is the reason behind the units of entropy being J/K?
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1answer
35 views

Difference between "physical quantities" and "units"?

My textbook, Atkins' Physical Chemistry, 11th edition, by Atkins, de Paula, and Keeler, says the following: Although it is good practice to use only SI units, there will be occasions where accepted ...
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3answers
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 ...
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1answer
127 views

What is the equivalence of 40 ppt to pg/mL? [closed]

Please help me converting 40 ppt to pg/mL. I have tried an online calculator and done it manually and there are differences. I`m aware that ppt can be used as parts per trillion or per thousand but I ...
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1answer
142 views

Up to which dilution can one assume that 1 mg/l = 1 ppm?

For aqueous solutions it is often assumed that 1 milligram per liter equals 1 ppm. Are there any guidelines up to which concentration this shortcut is admissible? Are there any formulas which ...
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3answers
527 views

Can I express the rate of a reaction as percent per second (%/s)?

Rate is calculated as change in concentration over change in time, so usually $\pu{M/s}$ $(\pu{mol L-1 s-1}).$ Say, I'm calculating it for $20\%$ solution of ammonium hydroxide, so my final rate is ...
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2answers
329 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 ...
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3answers
422 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, ...
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1answer
33 views

What is the use of both mol and mass representing the same thing i.e. amount of something? [closed]

A mol is a unit but also has a magnitude. How can a unit have a magnitude? And if mole is used to measure an amount of substance than what does mass measure?
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1answer
2k views

Meaning of mass concentration "units" % m/v [closed]

In a saline solution that is 5.750% (m/v) potassium iodide $(\ce{KI}),$ there are: a) 5.750 mL $\ce{KI}$ per 100.0 mL $\ce{H2O}$ b) 575.0 mg $\ce{KI}$ per 10.00 mL of $\ce{KI}$ solution c) 5.750 g $\...
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Interlayer binding energy: how to convert to meV/Ų?

I've noticed that in most papers the interlayer binding energy is given in units of meV/Ų. But how do I get these units? Do I divide the interlayer binding energy I get by the area of my unit cell?
3
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1answer
856 views

How is the ln k term in the linearised Arrhenius equation acceptable?

In my syllabus we only study about rate laws under the topic "Rates of reactions", but as I was interested in it, I started to search about that topic. And then I found the Arrhenius equation $$\ln k ...
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1answer
626 views

Units of the rate constant for reactions whose order is greater than 1

In many textbooks, it is written that: The value of rate constant depends on the nature of the reactants, temperature and catalyst. It is independent of the concentration of the reactants. However,...
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1answer
230 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.
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2answers
745 views

Why is the equilibrium constant unitless? [duplicate]

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....
4
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0answers
243 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 $...