Questions tagged [units]

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

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3
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1answer
162 views

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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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 ...
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1answer
341 views

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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3answers
2k views

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.[1] This energy difference can be explained ...
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1answer
188 views

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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0answers
472 views

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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0answers
168 views

Rate constants using Arrhenius equation

If I'm trying to use the Arrhenius equation in this form: $$\ln\left(\frac{k_2}{k_1}\right) = -\frac{E_\mathrm a}R\cdot\left(\frac1{T_2}-\frac1{T_1}\right)$$ And I am given the activation energy ($...
2
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1answer
3k views

How does 1 mole of a compound = the molecular weight of the compound?

I am reviewing moles, and came across this sentence in my book: One mole of a compound has a mass in grams equal the molecular weight of the compound expressed in amu and contains $6.022\times10^{...
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2answers
562 views

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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1answer
4k views

Dividing different units of measurement? [duplicate]

1) $$\frac{35 \; \text{cm}^2}{0.62 \; \text{cm}} = 56.4516$$ For the units in the answer, do you put $56.4516$ cm$^2$ or just $56.4516$ cm? 1) $$\frac{0.075 \; \text{g}}{0.0003 \; \text{cm}^3} = 250$...
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When to use Da or u for mass units

Following up on the question Units of mass on the atomic scale, Daltons (Da) and unified mass units (u) are equivalent, but which is used seems to depend on context. For example, when discussing ...
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2answers
80 views

Should you work in metres-cubed or litres in chemistry? [closed]

Generally, is it better to work in metres-cubed or litres in chemistry? My undergraduate degree was in civil engineering and we always worked in $\mathrm{m^3}$. However, I notice the gas constant $R$...
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3answers
6k views

How to retrieve Dalton value from m/z value?

As I understand, Dalton (Da) is the standard way for representing mass unit. As for $m/z$, which is a specific notation for mass spectrometry, quoting for wikipedia: This notation eases data ...
2
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2answers
326 views

Why are osmoles not considered SI units?

The Wikipedia article for osmoles is pretty clear on what they are and how they are used. Right off the bat, though, it states that the osmole is not an SI unit, and indeed the usual lists of derived ...
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2answers
10k views

Convert pg/mL to/from pmol/L?

In this article, 148 pmol/L is said to be equivalent to 200 pg/mL. But this unit conversion site says that 200 pg/mL are equivalent to ~734 pmol/L, and this one gives 44 pmol/L for the same value. ...
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2answers
54k views

Conversion of a value between g/gmol and lb/lbmol

I am doing some air emissions calculations and I really just have a bad feeling about my results. By definition, molecular weights are given in $\pu{g/gmol}$. For the substance I have, I was told by ...
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1answer
18k views

What does the unit M stand for? [closed]

What does $\mathrm{M}$ signify in this pH calculation? To me, it just seems to be a random variable. For example: $\ce{[H+]} = 1.0\times10^{-7}~\mathrm{M}$ I am not sure what the $\mathrm{M}$ is ...
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0answers
43 views

How much CO2 is produced by plant litter?

I made measurements of CO2 formed during decomposition of dead plant material using gas spectrometer. It gives the results of CO2 in ppm. How do I convert it to $\large \rm \frac{mg~CO2}{\text{1 g ...
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3answers
564 views

Units of rate constant (cycles per second vs radians per second) in Eyring equation

In the Eyring equation (EE), $$k = \frac{k_\mathrm B T}{h} \exp\left(\frac{-\Delta G_{\mathrm f}}{RT}\right),$$ the units of $k$ are $\mathrm{s^{-1}}$. However, in general rate constants are usually ...
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0answers
551 views

Calculating volume ethanol in an fermented aqueous solution of sucrose, water, and ethanol

I've been working on a biology experiment where I calculate the fermentation yield of yeast under various light induced stress responses inhibiting the translation of various fermentation enzymes. My ...
2
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1answer
105 views

Pressure change of a gas upon increasing the temperature

My assignment is as follows: A sample of air is trapped in a container at 16.2 psi. If the initial temperature is increased from 10°C to 32°C, what is the final pressure? I am really confused as ...
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1answer
872 views

Reaction rate constant conversion [closed]

I'm flustered, I'm not into chemistry and need to convert $\mathrm{(nM\cdot s)^{-1}}$ to $\mathrm{(M\cdot s)^{-1}}$. Is it correct to multiply with $10^9$?
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1answer
6k views

How dangerous is a megacurie (MCi)?

I was researching the Kyshtym (nuclear) Disaster, and it said that it released 20 MCi (20 megacuries) of radiation into the air in a dust cloud. I read from this Yahoo Answers answer that: The ...
3
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1answer
19k views

Convert from Pa to Pa to J/dm^3 [closed]

Given the value 380 PaK^-1, how do I convert Pa to Jdm-3? I know that 1 joule = 1 Nm, and that 1 pascal = 1 N/m^2. But where does the dm^-3 (or L) come into this? I don't know where to start.
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3answers
625 views

What is the physical quantity/magnitude of pH?

I have a database to classify variables/parameters in water resources as follows: Variable: Precipitation (P); Quantity: Length; Units of measurement: mm (millimeters). Following the example, what ...
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1answer
228 views

Find the displacement of mercury in a vertical tube

So we have a 1m tube sealed at both ends. It is initially horizontally placed and the middle 0.1m contains mercury and the two ends contain air. Now the tube is turned vertical, we have to find the ...
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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1answer
3k 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. $$\...
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1answer
7k views

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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3answers
1k views

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 ...
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1answer
1k views

Units in modified Arrhenius equation?

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_\...
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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 ...
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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 ...
2
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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. ...
14
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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 ...
2
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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?
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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 ...
4
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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 ...
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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^{-...
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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 ...
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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 ...
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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^...
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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.
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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. ...
5
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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}}...
6
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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 ...
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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 ...
2
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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)?
4
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1answer
774 views

How to compute the residence time for given flow rate in standard cubic meters?

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 ...

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