Questions tagged [units]

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

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2
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65 views

TINKER molecular modeling extra forces - energy unit

I use the TINKER molecular modeling software package (version 7.1.2) for computational physical chemistry. There is an extra file (extra1.f) included for the user to edit and incorporate their own ...
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1answer
38 views

What would be the unit for the calculation of the rate of reaction?

In a reaction between magnesium and hydrochloric acid, I measured the change in mass of the magnesium strip (Initial weight: 0.03g). I calculated the the time for the reaction to reach completion. A ...
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1answer
279 views

What's an “amagat”? How is it different from the Loschmidt Number?

At class, we were (briefly) taught about the Loschmidt Number/Constant ($n_0$), which isn't really different from the Wikipedia definition, The Loschmidt constant or Loschmidt's number (symbol: $...
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78 views

Why does mmHg persist in organic chemistry? [closed]

In most areas of chemistry, standard units are adopted - we never would think to use ounces (even American chemists resist) BUT even in modern journals or textbooks, we commonly still see the unit <...
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3answers
476 views

What term should I use when I am quantifying the amount of matter, mass or weight?

I am writing an academic article but I got confused as I am writing about the weight a specific metal got by using balance scale. Should I use the term mass, which has a unit of kg, or weight, which ...
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2answers
3k views

How to convert ppm to g/m3?

I'm not very comfortable with chemistry and I need to convert $\ce{CH4}$ concentration values in $\pu{ppm}$ to $\pu{g/m3}$. Is that possible? I've already researched a bit and realised that for water ...
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1answer
4k views

nM or μM, which is more concentrated? [closed]

A study found that a particular herb fueled cancer at nM concentrations but actually inhibited cancer in μM concentrations. Which is the higher concentration, nM or μM?
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2answers
683 views

Is zero-point vibrational energy an intensive or extensive property?

The zero-point vibrational energy (ZPVE) is the non-zero amount of non-electronic energy that molecules have even at zero kelvin and is purely quantum mechanical in nature. For a molecule with $c$ ...
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3answers
971 views

What does the universal gas constant represent? [closed]

In the formula $$V=\frac{nRT}{P},$$ when we say $R$ is $\pu{0.082 L atm//mol K}$: What do we mean? Specifically what does $\pu{K}$ mean here? Also $\pu{mol k}$? and $\pu{L atm}$? Can you simplify it ...
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195 views

Difference between change in energy level and hyperfine energy level?

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. --> This ...
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1answer
50 views

Expressing solubility in binary solvents with non-additive volumes

I have experimentally obtained data that shows the molarity of a saturated solution in mixed solvents, with different proportions of solvents. The issue here is that the two solvents (water/ethanol)...
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2answers
142 views

Force Fields and units

I am a math student trying to implement a simple force field in python for determining the energy of a small molecule and trying to minimize it in order to find a stable configuration. I was wondering ...
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941 views

What do we have both r.a.m. and r.m.m.?

Read Orthocresol's answer here first: What is the difference between "molecular mass", "average atomic mass" and "molar mass"? Orthocresol, in his answer, says that ...
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1answer
1k views

Why are absolute entropies given the units J mol−1 K−1?

I've just started looking at the concept of entropy and can't understand why absolute (or standard molar entropies) are given the units $\pu{J mol^−1 K^−1}$. Could someone explain the rationale for ...
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1answer
2k 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 = $k÷M$. SI unit of : k is S/m & ...
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1answer
12k views

How to use C1V1=C2V2 How do the units work?

I am trying to do a dilution where I know I have 0.749 g/ mL and I want to use this concentration to calculate the grams of aspirin in each 100 mL solution. I am specifically worried about the units. ...
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3answers
267 views

Is the amount of cubic inches in a cubic foot exact?

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 ...
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1answer
44 views

How to convert the D'Arcy and Watt model from activity to concentration in mol/m^3

The D'Arcy and Watt 1970 model in "Analysis of sorption isotherms of non-homogeneous sorbents" for water adsorption is as follows: $$W = \frac{KK'a}{1+Ka} + ca + \frac{kk'a}{1-ka},$$ where: $W$ is ...
2
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2answers
198 views

When you report Einstein units do you capitalize it?

When you report Einstein units do you capitalize it? Since we're using it in the context of a unit (and not a name) is capitalization necessary? Example: The measured value was reported as 10 ...
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2answers
415 views

Number of picoseconds in a week is exact? [closed]

My chemistry textbook states that the number of picoseconds in a week is not exact because a week is defined by the Earth's rotation. Does that also mean that 3600 seconds in an hour also isn't exact ...
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1answer
9k views

What does mol L−1 mean? [closed]

I am researching how to Determination of Vitamin C Concentration by Titration for my science fair project and it says Titrate the sample with $0.005~\mathrm{mol~L^{−1}}$ iodine solution. The ...
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1answer
346 views

Reaction rate unit conversion: how to convert from molarity to ppm

How can I convert a reaction rate $k$ with units of $\mathrm{1\over M~~s}$ to $\mathrm{1\over ppm~~s}$? For context: I want to make a numerical simulation of a system of reactions that happens in air ...
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3answers
163 views

What is a mole?

Is my understanding of the concept of a mole correct? The mole is a simply a number ($6.023 \times 10^{23}$). It's just like what a dozen would be like for eggs, except that the mole applies to atoms ...
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1answer
966 views

Why does Avogadro's number have the value it has? [duplicate]

As I was learning about Avogadro's number and I was wondering why is Avogadro's number equal to $6.02214\cdot 10^{23}$? I mean, how did chemists come up with this particular number?
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1answer
110 views

What advantage is there in reporting the Lennard-Jones well depth in wavenumber units?

The Lennard-Jones well depth $\epsilon$ is typically given in energy units ($\mathrm{kJ}$ or $\mathrm{kcal}$; sometimes per mole). Jasper and Miller, however, reported $\epsilon$ values in reciprocal ...
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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 ...
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4answers
1k views

Is the ideal gas constant not a constant if Celsius is used instead of Kelvin? [closed]

For example, Charles' law does not work with Celsius. Why is this? I can only think that the gas constant is not a constant when Celsius is used rather than Kelvin.
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2answers
480 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 =...
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1answer
2k views

When was the amu renamed the dalton? [closed]

I taught chemistry for 30 years and never encountered the unit until I saw it on a juice bottle!
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1answer
769 views

What does 10 Vols mean?

I am currently working on a project and the protocol says the following, "Wash resin 3 times 10 vols of the extract buffer." This is very confusing to me and I am not exactly sure what the 10 vols ...
2
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1answer
366 views

What does the word carat really mean?

For gold, the word carat defines its purity whereas in diamonds it indicates its weight. This seems inconsistent -- what does the unit 'carat' actually refer to?
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1answer
541 views

Effects of Changing Avogadro's Constant

The Avogadro project suggests that we redefine the Avogadro constant to be equal to our best known estimate, $N_\mathrm{A} = 6.02214179 \times 10^{23}$, and redefine the kilogram based on the Avogadro ...
3
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1answer
159 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
899 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
273 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
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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.[1] This energy difference can be explained ...
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1answer
127 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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416 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
163 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 ($...
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1answer
2k 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
414 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
3k 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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2answers
2k views

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
71 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
5k 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 ...
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2answers
245 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
7k 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
44k 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
14k 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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42 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 ...