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Questions tagged [units]

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

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Comparison of oxygen and carbon-12 as standards for mass

While studying about the standards of atomic and molecular mass, I have learnt that while initially hydrogen was used as a standard of mass (its mass taken as unity), according to my book, oxygen was ...
Smarika Singh's user avatar
2 votes
1 answer
113 views

How do I find the amount of moles from the amount of units?

The problem: Find out the amount of moles of a certain substance knowing that the amount of units of said substance is $\pu{1.8E23}$. I have to find how many moles of carbon, hydrogen, and oxygen that ...
Nora's user avatar
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Why isn't the unit of rate constant for any reaction always per second?

It is formally agreed upon that $K_{eq}=\prod_{j}a_j^{\nu_j}$ where $\nu_j$ is stoichiometric coefficient of species $j$ (negative for reactants and positive for products). This equation results in ...
megamonster68's user avatar
-2 votes
1 answer
3k views

Convert daltons (Da) to molar mass (g/mol) [duplicate]

This is probably a stupid question... In converting Da to molar mass (g/mol), Since 1 Da = 1 g/mol, is it logical that 64,776 Da = 64,776 g/mol or am I missing an intermediate calculation? Thank you!
burphound's user avatar
7 votes
1 answer
684 views

Uncertainty when beaker goes by 50s

I am helping my daughter do her chemistry homework and am stumped by this one. I have a beaker that seems to count by 50s. That makes uncertainty calculations awkward. What is the “next unit down”? ...
Vaccano's user avatar
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4 answers
322 views

How to obtain the value of the gas constant R = 8.20574 × 10⁻² L atm K⁻¹ mol⁻¹

While reading a chemistry book, I saw a table of values of the gas constant for different units. For example, $$R=8.20574\times 10^{-2}\ \text{L}\cdot\text{atm}\cdot \text{K}^{-1}\cdot \text{mol}^{-1}\...
xoux's user avatar
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2 votes
2 answers
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What do the units of enthalpy kJ/mol represent?

Enthalpy is making me slightly confused. Here $\Delta H$ refers to the change in enthalpy. Does this refer to the enthalpy change of reaction? Are they the same? It seems that change in enthalpy, ...
Stark's user avatar
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6 votes
1 answer
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What is the correct unit of a RI detector used for HPLC analysis?

I analyze sugar compounds in soft drinks with HPLC using refractive index detection (RID) on Agilent 1260 Infinity Binary LC (System User Guide (PDF)). I am not sure which unit to put next to the $y$-...
Johanna's user avatar
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2 votes
1 answer
1k views

What is the unit of molar density?

I'm reading Unit operations of Chemical Engineering, 5th edition by Warren McCabe. The authors defined the molar density as $\rho_{M} = \frac{n}{V}$. Here, $n$ and $V$ represent the number of moles ...
kamikaze's user avatar
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Masses of atoms and molecules [duplicate]

This is my understanding so far: Average atomic mass is the mass of an element considering isotopes and is measured in atomic mass units. However relative atomic mass is the average mass of an atom ( ...
Quin Gardiner Bax's user avatar
1 vote
1 answer
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Definition of Avogadro's constant

I've read this excerpt from Wikipedia countless times, but I'm still confused: The Avogadro number is the approximate number of nucleons (protons or neutrons) in one gram of ordinary matter. Shouldn'...
Megaptera novaeangliae's user avatar
2 votes
1 answer
318 views

What is the correct method for converting a concentration given in ppm to molarity?

I need to convert a concentration of dissolved boric acid $(\ce{H_3BO_3})$ in water (at high temperature and pressure) from ppm to mol/L. Concentration of boric acid: $c_{ppm}=1500\ \ce{ppm}$ Molar ...
qwebfub3i4u's user avatar
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0 answers
40 views

How to rewrite kinetic mass balance to get to proper SI units?

Suppose from kinetic mass balance I get the following differential equation of the molarity substances $A$,$B$, and $C$ in a reactor, with molar in- or outflow rate $\phi$, and rate constant $k$: $$ \...
user313866's user avatar
1 vote
1 answer
215 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 ...
user313866's user avatar
-1 votes
1 answer
178 views

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\...
tryingtobeastoic's user avatar
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101 views

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 $\pu{1 D^2/(Å^2amu)}$ = $\...
poisonDartFrog's user avatar
3 votes
3 answers
1k 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?
user1007028's user avatar
2 votes
0 answers
227 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 ...
La Cordillera's user avatar
1 vote
0 answers
75 views

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 ...
katara 's user avatar
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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^\...
Apurvium's user avatar
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-4 votes
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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 $$...
Nomas's user avatar
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-1 votes
2 answers
342 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 ...
Vulgar Mechanick's user avatar
2 votes
0 answers
92 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 ...
Gio's user avatar
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1 answer
2k 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 ...
dumb's user avatar
  • 23
1 vote
0 answers
118 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. ...
Mikayla Eckel Cifrese's user avatar
-2 votes
1 answer
153 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 ...
Diego Alves's user avatar
3 votes
1 answer
505 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 ...
olly's user avatar
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23 votes
1 answer
6k views

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 ...
Andyba's user avatar
  • 349
2 votes
2 answers
2k 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?
user103727's user avatar
2 votes
1 answer
483 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 ...
Kurt Hikes's user avatar
  • 1,813
6 votes
5 answers
2k 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' ...
Aerovolo's user avatar
0 votes
0 answers
32 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 ...
andrewH's user avatar
  • 167
3 votes
2 answers
403 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 ...
Rafael Franco's user avatar
7 votes
1 answer
197 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(\...
Frankie S. Palmer's user avatar
2 votes
1 answer
928 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-...
Positron12's user avatar
3 votes
1 answer
142 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 ...
Anton's user avatar
  • 766
0 votes
1 answer
337 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 ...
Andrew Cheong's user avatar
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?
yolo's user avatar
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-4 votes
1 answer
1k 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 ...
Makromat's user avatar
1 vote
0 answers
31 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 ...
g1lb3rt0123's user avatar
3 votes
2 answers
660 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 ...
NayaJoonyeongah's user avatar
19 votes
1 answer
2k 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 ...
Jason C's user avatar
  • 1,172
3 votes
2 answers
2k 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 ...
user avatar
10 votes
1 answer
3k 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?
user95732's user avatar
  • 235
2 votes
1 answer
3k 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 ...
piny's user avatar
  • 39
4 votes
1 answer
187 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 ...
air_nomad's user avatar
  • 123
-1 votes
2 answers
243 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....
The Pointer's user avatar
  • 1,073
0 votes
1 answer
3k 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 ...
Cotton Headed Ninnymuggins's user avatar
-1 votes
1 answer
46 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 ...
Iceheat's user avatar
1 vote
1 answer
56 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})^{-...
Johan's user avatar
  • 197