Questions tagged [units]

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

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6
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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?
3
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2answers
53 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 ...
-4
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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
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0answers
38 views

Units for R when relating delta G and Kp

I'm looking into the effect of temperature $T$ and pressure $p$ on the position of equilibrium in the Haber process and ran into a problem with units. Currently I have: $$\exp{\left( -\frac{\Delta G}{...
-1
votes
1answer
51 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 ...
5
votes
1answer
145 views

Dealing with different units (psi) in ideal gas equation

My assignment is as follows: A sample of air is trapped in a container at $\pu{16.2 psi}$. If the initial temperature is increased from $\pu{10 ^\circ C}$ to $\pu{32 ^\circ C}$, what is the final ...
2
votes
3answers
303 views

Is the amount of cubic inches in a cubic foot exact up to infinite significant figures?

I know we are supposed to use SI units and I know they are defined very well. But does that mean that Imperial units of length/volume aren't exact? Don't they have infinite significant figures in the ...
1
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0answers
19 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 ...
9
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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 ...
0
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1answer
3k views

Unit conversion of Molar Conductivity

I know this might be a silly question but I am confused with different answers of internet. My question is: Molar conductivity is given as: Molar conductivity = $\frac{\kappa}{M}$. S.I. unit of : $...
7
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2answers
1k views

Why a specific temperature for value of 1 calorie?

Everything in chemistry often comes with a strong reason and facts behind it, but I stuck in definition of $1$ calorie. My reference book define one calorie as amount of heat required to raise the ...
4
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2answers
2k views

Freundlich Equation's dimensions

So, we have the Freundlich's Isotherm equation as $\frac{x}{m} = kp^{\frac{1}n}$, where the LHS measures the extent of adsorption with the corresponding pressure $p$ in the RHS with the two constants, ...
18
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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 ...
2
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2answers
173 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
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1answer
455 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?
6
votes
1answer
543 views

Which unit should I use for m/z?

I've written m/z 300 - 1100 Thomson in my paper, but the reviewer said that the Th is already deprecated. So in this case, should I write m/z 300 - 1100 Da? or something else?
0
votes
1answer
64 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
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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
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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....
2
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2answers
37k views

How to convert g/L to mol/L

I know I should divide $\pu{g L-1}$ by the molar mass of the substance, but I don't seem to find the specific answer on Google. So just to be sure: If I have $10^{-5}~\pu{g L-1}~\ce{Cu^2+}$ solution, ...
3
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2answers
3k views

Atomic Mass of ¹²C before 1961

Question: Prior to 1961, the atomic mass unit was defined as 1/16 the mass of the atomic mass of oxygen; that is, the atomic mass of oxygen was defined as exactly $\pu{16 amu}$. What was the ...
-1
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1answer
29 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
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1answer
78 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 ...
2
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3answers
165 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
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})^{-...
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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
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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
24 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 ...
4
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1answer
1k views

How to find the units of the rate constant?

Consider the reaction $$\ce{2A + B -> Products},$$ when the concentration of $\ce{B}$ alone was doubled, the half-life did not change. When the concentration of $\ce{A}$ alone was doubled, the rate ...
1
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1answer
219 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,...
4
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1answer
2k views

Do all conversion factors have an infinite number of significant figures?

I believe conversion factors have an infinite number of significant figures. They should be exact values; but not all of them are, are they? I looked at a pdf document (via the Internet Archive) and ...
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3answers
563 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
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1answer
48 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
47 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
488 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 ...
9
votes
3answers
223 views

Amount of substance expressed in yoctomole units

I stumbled upon a couple of papers where amounts were expressed in yoctomoles ([ymol], $\pu{10^{-24} mol}$) and I find it somewhat bothersome as 1 ymol would correspond to about 60% of atom/molecule/...
4
votes
2answers
122 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 ...
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
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1answer
180 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
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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
105 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 ...
4
votes
1answer
4k views

How is sulfuric acid with a concentration of 12 M also 72% w/w?

According to LabChem, $12~\mathrm{M}$ of $\ce{H2SO4}$ is $72~\mathrm{w/w\%}$. When I try to convert $12~\mathrm{M}$ of $\ce{H2SO4}$ to $\mathrm{w/w\%}$ I get $117\%$ which is obviously wrong. $$\...
4
votes
2answers
505 views

Relation of pressure and temperature for an ideal gas at constant volume

I have a tank of ideal gas with volume $3.1\ \mathrm L$; I have $R = 8.314$, and 11 moles. The tank is filled at $23\ \mathrm{^\circ C}$ (not sure why that's relevant) and the tank will rupture at $P =...
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
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2answers
434 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
134 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
112 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 ...
1
vote
2answers
915 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
157 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?
8
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6answers
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How to correctly write “7 apples” according to the international system of units (SI)

According to international system of units (SI), we can write "7 kg of apples" to refer to the mass of these apples. However, if we want refer to the amount of apples, that is, the number of ...