Questions tagged [units]
For questions about the definition and usage of units of measurement.
147
questions
0
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1answer
24 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 ...
0
votes
1answer
34 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})^{-...
-2
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1answer
27 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 ...
10
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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?
0
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1answer
22 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 ...
-3
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3answers
298 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 ...
-3
votes
1answer
25 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 ...
1
vote
1answer
41 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 ...
3
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3answers
484 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 ...
4
votes
2answers
115 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 ...
1
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3answers
96 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 ...
0
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1answer
31 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?
0
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1answer
81 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....
1
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0answers
21 views
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
votes
1answer
74 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
$$\ln k ...
1
vote
1answer
163 views
Units of the rate constant for reactions whose order is greater than 1
In many textbooks, it is written that:
The value of rate constant depends on the nature of the reactants, temperature and catalyst. It is independent of the concentration of the reactants.
However,...
5
votes
1answer
220 views
Why the term 'unified' in “unified mass unit”? [closed]
Why different unit symbols - amu, u and dalton are used for representing atomic mass? And what does the term 'unified' mean in "unified mass unit"?
Thank you.
4
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2answers
306 views
Why is the equilibrium constant unitless?
I read that the equilibrium constant is unitless because the molar activities of each of the species are used in the equilibrium expression, not the actual concentrations themselves. I understand that....
4
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0answers
98 views
What does “at%” mean and how it differs from “mol%”?
Sorry if my question is a bit obvious, but for some reason I'm getting confused with the percentages in this question:
Concentration of $\ce{A}$ is $\pu{80 at\%}$ and concentration of $\ce{B}$ is $...
1
vote
3answers
78 views
Soil conductivity: How do I get a near equivalent between meq/100 g to mS/cm?
I have two soil reports, one that reports CEC as 45 meq/100 g and another that reports 6 mS/cm using 1:1 water soil.
I can see that the two are, if not apples and oranges, they are at best tangerines ...
9
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3answers
3k views
Can Rydberg constant be in joules?
In my textbook (Chemistry Part - I for Class XI published by NCERT), there is an equation for the energy of an electron in an energy state: $$E_n = -R_\mathrm H\left(\frac{1}{n^2}\right)$$ and there ...
1
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2answers
341 views
How do I convert salinity measured in mg/L Cl into parts per thousand as NaCl?
I have collected water data using a refractometer that records salinity in ‰ (parts per thousand) as $\ce{NaCl}$. I want to compare my data to other data but every journal article I read has the ...
2
votes
2answers
136 views
How will the periodic table change due to redefinition of the kilogram (May 2019)?
The definition of a kilogram has recently been changed. What effect, if any, will this change have on data on the periodic table?
2
votes
1answer
377 views
Barron states that 4.18×10⁸ joules equal 1 kcal, is this correct?
Here is a paragraph in Barron's SAT chemistry prep book:
Work itself is measured in Joules, and so is energy. In some problems, however, energy may be expressed in kilocalories. The relationship ...
0
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2answers
463 views
How are lowercase k and uppercase K used in general chemistry courses? [closed]
Where do lowercase k and uppercase K make appearances in general chemistry, and what do they signify?
0
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2answers
2k views
How are lowercase m and uppercase M used in general chemistry courses? [closed]
Where do lowercase m and uppercase M make appearances in general chemistry, and what do they signify?
3
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1answer
174 views
IUPAC Recommendations for Concentration Units (ppb, ppm)
In a recent interesting post, (Is 1 ppb equal to 1 μg/kg?)
it was pointed out that IUPAC advises to abandon parts per million, parts per billion quantities and instead it suggests to employ micromole/ ...
15
votes
1answer
3k 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 ...
9
votes
2answers
533 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 ...
1
vote
2answers
77 views
Does the prefix of an inverted unit also get inverted?
What is the energy of radiation that has a frequency of $\pu{2.51 \times 10^11 ms-1}$?
(a) $\pu{1.66 \times 10^-19 J}$ (supposedly correct)
(b) $\pu{1.66 \times 10^-22 J}$
(c) $\pu{7.92 \...
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1answer
562 views
Standard litres to kg conversion for Hydrogen [closed]
When looking at hydrogen fuel tanks, they often give the capacity in terms of "standard litres". However, I can't find a definition of what this is.
I assumed this may mean a litre of hydrogen gas ...
0
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2answers
52 views
Unit analysis for neutralization reaction
What volume of $\pu{0.500 mol L-1}$ $\ce{HCl}$ (in $\pu{mL}$) is required to neutralize $\pu{2.02 g}$ of solid $\ce{NaOH}$ pellets?
So far I have tried regular unit analysis:
2.02 g NaOH * 1mol ...
0
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1answer
193 views
Types of concentration units
I would like to ask about a concentration unit.
What does it exactly mean when stock concentration of something is,
for example, 5X?
Thank you in advance!
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votes
2answers
207 views
What will be unit of volume in V=nRT/p if p is in Torr instead of atm? [closed]
$V=nRT/p$, where $n$ is amount of gas in $\mathrm{mol}$, $R$ is gas constant, $V$ is volume in $\pu{dm3}$, and $p$ is pressure in $\pu{atm}$.
What happens to $R$ if $p$ is in $\pu{Torr}$ instead of $...
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1answer
101 views
How to actually calculate work from force times distance?
I understand that work = force $\cdot$ distance, but I'm not sure of the units and conversions to use.
What I know is that I have a piston head of area $\pu{28cm^2}$ (~$\pu{4.4in^2}$) traveling a ...
0
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3answers
559 views
Conversion from a PPB value to µg/m3 of Isobutylene
I have a quite simple problem, found a lot of information about it, but I am not sure anymore if I do my calculations right.
I own a sensor, which reports measurements in isobutylene units as PPB. So ...
0
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1answer
30 views
Combination of several unit symbols when finding new pressure from ideal gas law
As a car is driven, the tires heat up and the volume and the pressure change. Calculate the new pressure of a 35.0 L tire initially at 235 kPa that expands to 36.5 L when it heats up from 15.0 °C to ...
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0answers
75 views
Is the negative excess volume of ethanol/water the cause of my math error?
I have a binary mixture, say ethanol at $90\%wt$ ethanol, $10\%wt$ water. According to Perry's Chemical Engineers' Handbook 9th Ed., that mixture has a density of $0.81797g/cc$ (at $20^{\circ}C$). ...
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3answers
5k views
What are the units of Kc and Kp?
They are both equilibrium constants as far as I know.
Kc is in terms of molarity and Kp is in terms of pressure. Also both of them are ratios of respective quantities [ ratio of molarity(s) in Kc and ...
0
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1answer
59 views
Confusion between moles and molarity units
I am a total newbie in the field and i would like some kind of guidance.
So, I have been recently given 2 datasets with measurements regarding several phospholipid fatty acids of plasma HDL. Both ...
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1answer
22 views
Rate constant converion [closed]
I need to calculate the half life for a first order reaction for which I need the rate constant $k$. From literature I observed that for a very closely simulated experiment I obtained the numerical ...
0
votes
1answer
61 views
Converting pounds to grams [closed]
I wanted to figure out how many grams were to a lb and instead of looking it up I used my textbook and found 1 kg = 1000 g and 1 kg = 2.205 lb. I set up a cross multiplication equation to solve for x. ...
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votes
1answer
1k views
What does atm•L mean? [closed]
In my chemistry book I always see a dot for units. For example, atm•L. Does this mean “1 atm per Liter?” By Atm I mean atmospheric pressure, not atom.
4
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1answer
71 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. ...
3
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1answer
174 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 ...
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5answers
9k 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 ...
asked Jul 12 '18 at 0:26
Pritt says Reinstate Monica
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0answers
101 views
Why was the unit micrometers spelled backwards in old research papers?
I have recently been doing a literature research project on polymers and have been reading a lot of papers from the 1950s and 1960s (mostly in American journals from American authors). I saw this ...
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1answer
77 views
Units for density in nanorod image
I am counting the number of nanorods contained in a sample (see picture below). The length scale is 1 µm. What units does the density have? Is it µm-2?
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0answers
102 views
How to convert from ppm to ug/m3 in case of PMx?
For $\text{PM}_{10}$, $\text{PM}_{2.5}$ sensors, sensitivity, minimum and maximum recordable concentrations are generally expressed in $\pu{ppm}$ (parts per million) while legislation uses $\pu{\mu g /...
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0answers
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Conversion of a permeability value from barrer to SI unit
I have done a permeability experiment on LDPE in the presence of air. At $\pu{30^\circ C}$, the permeability was found to be 1.846 barrer.
Now I am trying to apply this permeability value to a ...
