Questions tagged [units]
For questions about the definition and usage of units of measurement.
162
questions
3
votes
1answer
40 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 ...
0
votes
1answer
18 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 ...
6
votes
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?
-4
votes
1answer
20 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 ...
-1
votes
1answer
57 views
What is the dimension of a mole? [closed]
The dimension of a mole is $\mathbf{N^{-1}}$. A mole is defined as:
number of moles= (mass of substance)/(molecular weight of substance)
The dimension of the mass of a substance is M the dimension of ...
1
vote
0answers
20 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 ...
3
votes
2answers
55 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 ...
18
votes
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 ...
3
votes
2answers
216 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 ...
7
votes
1answer
538 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?
0
votes
1answer
88 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 ...
4
votes
1answer
129 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 ...
-1
votes
2answers
29 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....
0
votes
1answer
98 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 ...
-1
votes
1answer
30 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
38 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
votes
1answer
33 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
votes
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
votes
1answer
26 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
votes
3answers
621 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
51 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
52 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
votes
3answers
491 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
140 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 ...
2
votes
3answers
185 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
votes
1answer
32 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
votes
1answer
276 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 $\...
1
vote
0answers
23 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
146 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
236 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
224 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
votes
2answers
474 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
votes
0answers
148 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
119 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
votes
3answers
4k 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
vote
2answers
1k 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
161 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
407 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
votes
2answers
852 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
votes
2answers
4k 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
votes
1answer
254 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
541 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
80 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 \times 10^-37 ...
-1
votes
1answer
1k 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
votes
2answers
65 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
votes
1answer
216 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!
-3
votes
2answers
213 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 $...
-3
votes
1answer
103 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
votes
3answers
864 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 ...
