# Questions tagged [units]

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

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### What's with the weird definition for the Calorie? [duplicate]

I came across this definition for the calorie, which I found... strange: "The (15°) calorie is the heat required to raise the temperature of one gram of water from 14.5°C to 15.5°C." Subsequent ...
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### 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 ...
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### What is meaning of unit (mg-Pt/cm^2) in relation to fuel cells?

I do not understand what the meaning is of the unit mg-$\ce{Pt}$/cm$^2$ which I have come across in reading about the theory of fuel cells. In simple terms, what does this unit mean?
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### Where does the label ‘Dq’ to denote the field split in coordination compounds come from?

As established in a previous question, coordination compounds typically have a field split between the $\mathrm{t_{2g}}$ and the $\mathrm{e_g}$ d-orbitals. This energy difference can be explained ...
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### Is mg/m^3 same as ppmw (CO2)?

I'm trying to convert $\ce{CO2}$ ppm (volume) in the air into ppm (weight) in the water. I found this formula $\mathrm{mg \ m^{-3}}$ = (ppmv)(MW) / [($0.08205$) (¦$\mathrm{K}$)] where, ppmv = ...
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### ppm as hexane/propane/methane etc

This question is primarily related to stack testing data. Whenever stack testing is being conducted and the parameter being measured is VOC concentration, it is often reported as ppm (as hexane) or ...
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### How to convert metre to ångström? [closed]

In my book it says Convert $1.213\cdot10^{-11}$ metre into Angstrom. The answer is $12.13\cdot10^{-2}$ Angstrom. How is it possible? I thought of multiplying the metre with $10^{-10}$, but I ...
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### How is the unit of measure “equivalent” abbreviated?

I usually use "eq" but I have been told to use "equiv." in order to avoid confusion with "equation". Is there a reference I can check?
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### Is 44.5 mg/L a measurement of weight or volume?

I have a test result that I am trying to fully grasp. I need to determine how much of a contaminant I need to reach that amount in 4,800 gallons. Making the conversion to liters is no problem, nor is ...
The modified Arrhenius equation is used to express the rate constant in a chemical mechanism model I'm working with. The equations is as follows: $$k_\mathrm{f} = A\times T^b\times\exp\left(-\frac{E_\... 1answer 1k views ### Units of mass on the atomic scale what are the systems of recording atomic masses and their units? I know that the nucleon number is the number of protons and neutrons I also know that the mass number on the periodic table is the ... 6answers 4k views ### May I treat units (e.g. joules, grams, etc.) in equations as variables? Suppose I have two units, Tonks, T, and Borks, B, as well as the equation \frac{50T}{20B}=x. May I manipulate the equation as though T and B were the kinds of symbols that we learned to ... 1answer 861 views ### Unit of time used in rate law equations If doing a problem where you use the integrated rate law for a second order reaction (\frac{1}{[\mathrm{A}]}-\frac{1}{[\mathrm{A_0}]}=kt) to first find k, then use that to find the half-life. ... 1answer 30k views ### What is the difference between the units 'torr' and 'mm of Hg'? According to present definition torr and mm of Hg differ though slightly. I am still confused over it. Can someone write an explicit answer differentiating between them and the reason for ... 3answers 3k views ### Why does 1 cubic meter = 1000000 cubic centimeters? I have the SI chart in front of me. It states that a centi is 10^-2 and since they are already designated cubic meters why does 1 cubic meter equal 1000000 cubic centimeters? 4answers 11k views ### Why is the units of kcat 1/s? I understand that k_\text{cat} measures the turnover number of an enzyme. This measure is therefore a quantity of molecule conversions per unit of time. I suspect that my problem is more that of a ... 4answers 41k views ### What does the ‘−1’ superscript mean in units? For example, the rate of a chemical reaction can be expressed in \mathrm{mol}/\mathrm{L}^{-1}/\mathrm{sec}^{-1}. Why is it ‘−1’ and not, say, ‘−2’? Does it change the meaning if the minus is removed ... 2answers 4k views ### SI Units for Isothermal Incompressibility and Specific heat capacity at constant volume I am looking for the SI units of isothermal incompressibility and specific heat capacity at constant volume. What I found was that isothermal compression was measured in " \mathrm{m^2}\ \mathrm{N^{-... 1answer 1k views ### Using Conversion Factors In Europe, gasoline efficiency is measured in \mathrm{km/L}. If your car's gas mileage is 35.0~\mathrm{mi/gal}, how many liters of gasoline would you need to buy to complete a 142-km trip in ... 2answers 2k 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 ... 1answer 457 views ### How to convert cm² to km²? I have a problem that says "a small hole is 20.7~\mathrm{cm^2} wide. What is the area in \mathrm{km^2}." I started doing it like this: 20.7~\mathrm{cm^2} \cdot 0.01~\mathrm{m^2} / 1~\mathrm{cm^... 2answers 5k views ### Why is it written as “joule” and “J”? [closed] Why is the unit joule is written as "joule" and "J", I mean what does "J" mean and what does "joule" mean. 1answer 938 views ### Molarity to Percent, Mole Fraction, Molality I have a series of questions similar to the following. I want to make sure I do this correct before I do the others. I am in need of a hint. ... 2answers 526 views ### What are the units of P and V when R is expressed in terms of J/(mol K)? I have learnt the ideal gas equation PV=nRT. However, I have some doubts regarding the units of the quantities in this equation. When we take the gas constant R=8.314\ \mathrm{J\ mol^{-1}\ K^{-1}}... 1answer 1k views ### What units do I use for energy in the Boltzmann distribution? I am a mathematics student doing a course in thermodynamics and am working on the basics now. The question I am asked is: Suppose two states differ in energy by one electronvolt. What can be said ... 1answer 1k views ### How to find the units of the rate constant? Consider the reaction$$\ce{2A + B -> Products},$$when the concentration of \ce{B} alone was doubled, the half-life did not change. When the concentration of \ce{A} alone was doubled, the rate ... 1answer 174 views ### What does this scale mean?$$\log_{10}~\mathrm{pg}/\mathrm{ml} I do not understand the scale. How do I convert this to a standard concentration (without the logs)?
I'm given a flow rate of a gas mixture of $3\ \mathrm{Nm^3/h}$ in standard cubic meters through a cylinder of length $50\ \mathrm{cm}$ and cross section $70\ \mathrm{cm^2}$, can I compute how long a ...