Questions tagged [units]

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

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Determining the concentration of chloride in water using ppm vs mg/L

I'm currently testing my drinking water for chloride, but I'm running into issues on units and what is an acceptable level of chloride in drinking water. According to the CDC the acceptable level of ...
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1 vote
1 answer
79 views

How do you rewrite molarity to weight percentage or volume percentage?

Suppose I have a molar concentration $[C]$ in $mol/m^3$ in a continuous reactor, from which the differential equation is as follows: $ \frac{\partial [C]}{\partial t} = k[A][B] + \phi C_{,in} - \phi ...
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-1 votes
1 answer
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Convention for writing concentration units

1st part of my question: Q) The concentration of an NaCl solution is 1M. Express this in percent wt. by vol. unit. Ans: $$\text{In %(w/v) unit}=\frac{58.5\pu g}{1000\pu{mL}}\times \frac{100}{100}=5.85\...
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How to convert from dipole derivative to atomic units?

I need to convert dipole derivative output by Gaussian into atomic units. The dipole derivative is given in Gaussian in the weird units $(\mathrm{km/mol})^{1/2}$. I know that $\pu{1 D^2/(Å^2amu)}$ = $\...
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2 votes
3 answers
650 views

What is the dimension of molar mass?

I was told that the number of moles is ‘dimensionless’. Does this mean that the dimension of molar mass is simply mass and hence the unit g/mol can be expressed as equal or proportional to the dalton?
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How to interpret the “surface power density” of a battery/capacitor

I’ve come across this research and one point of representing a power output confuses me: This enzymatic fuel cell is based on non-immobilized enzymes that exhibit a maximum power output of $\pu{0.8 ...
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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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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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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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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 ...
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2 votes
0 answers
60 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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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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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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1 answer
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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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3 votes
1 answer
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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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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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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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1 vote
1 answer
167 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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6 votes
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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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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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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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7 votes
1 answer
140 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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2 votes
1 answer
281 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-...
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3 votes
1 answer
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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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1 answer
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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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6 votes
2 answers
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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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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1 vote
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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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3 votes
2 answers
404 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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18 votes
1 answer
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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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3 votes
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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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8 votes
1 answer
2k 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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1 answer
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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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4 votes
1 answer
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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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-1 votes
2 answers
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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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0 votes
1 answer
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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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-1 votes
1 answer
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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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1 vote
1 answer
52 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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-2 votes
1 answer
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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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10 votes
2 answers
2k 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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1 answer
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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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-3 votes
3 answers
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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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-3 votes
1 answer
211 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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2 votes
1 answer
201 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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3 votes
3 answers
595 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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4 votes
2 answers
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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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2 votes
3 answers
599 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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0 votes
1 answer
39 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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1 answer
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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