Questions tagged [units]

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

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2
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2answers
425 views

What are the units that measure free energy?

Is it $\pu{kJ/mol}$? I have done problems and I always end up with these units, but is this correct?
0
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1answer
95 views

How isotopes atomic mass was defined?

So we say that 1 Amu (u) = 1/12 of the mass of a Carbon 12 atom. Im not sure about this, but I believe that first we measured the mass of 1 carbon 12 atom (This includes the mass defects that occur ...
-2
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1answer
49 views

Convert the lowest IR wavelength to energy in J, frequency in Hz, and wavenumbers in cm-1

Look up the range of wavelengths in nm in the IR region of the electromagnetic spectrum. Convert the lowest IR wavelength to energy in J, frequency in Hz, and wavenumbers in cm-1. I think I’m going ...
6
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2answers
256 views

In biochemistry, when do rate constant units matter in calculating free energy changes?

In my experience, it is always been the ratio of products to reactants that mattered, so as long as your units are consistent; what they are doesn't matter. However, an in-class problem today required ...
1
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1answer
2k views

What pressure units should be used when calculating electrode potential with Nernst equation?

Example 17: Calculate the electrode potential of given electrode $$\ce{Pt,Cl2 (\pu{1.5 bar}) | 2 Cl- (\pu{0.01 M})}\,; \quad E^\circ_\ce{Cl2/2Cl-} = \pu{1.36 V}$$ Solution: The reaction of ...
3
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1answer
140 views

Order of the reaction between hemoglobin and oxygen

I have the reaction: $$\ce{Hb + O2 <=>[$k$][$k'$] HbO2}$$ where $\ce{Hb}$ is hemoglobin, $k$ is the rate constant of the forward reaction, $k'$ is the rate constant of the inverse reaction. On ...
-5
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1answer
525 views

Concentration of ammonium phosphate if dissolved in water [closed]

If $\pu{2.54 mg}$ of ammonium phosphate is dissolved in enough water to make $\pu{300 mL}$ of the solution, what is the molar concentration of the solution? The answer is $\pu{5.68e-5 mol L-1}$. ...
1
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1answer
92 views

Concentration of mercury in body

Symptoms of mercury poisoning become apparent after a person has accumulated $\pu{20 mg}$ of mercury. If a person ingested $\pu{30 mg}$ of mercury, what concentration of mercury in parts per million, ...
10
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2answers
9k views

What is the correct unit for the equilibrium constant?

I have the following equation $$K_a = \dfrac{c\alpha^2}{1-\alpha},$$ with units $c= \pu{mol/cm^3}$ and $\alpha$ is the same. I'm not sure how to get the correct units for $K_a$. What I have come up ...
9
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3answers
185 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/...
3
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0answers
39 views

Could the loss of mass in the kilogram be due to oxidation?

I was wondering about this. I have heard, and known for a while, that the famous and celebrated Prototype Kilogram - the lump of metal whose mass is used to define the standard mass scale worldwide - ...
6
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1answer
604 views

Does adding prefixes to SI units make them no longer SI units?

Our chemistry professor said that "mg for mass, g/m3 for density, ms for time are all SI units". I want to make sure now, even if we added a prefix to the unit, will it still be an SI unit?
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1answer
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, ...
2
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0answers
67 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 ...
0
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1answer
39 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 ...
5
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1answer
355 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: $...
3
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0answers
80 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 <...
5
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3answers
482 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 ...
4
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2answers
4k 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
6k 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?
14
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2answers
745 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 $c$ ...
0
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3answers
1k 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 ...
4
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0answers
210 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 ...
0
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1answer
63 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)...
6
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2answers
151 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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0answers
1k 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
2k 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 ...
0
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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 & ...
0
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1answer
14k 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. ...
2
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3answers
281 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
47 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
225 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
477 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
10k 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
391 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 ...
2
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3answers
225 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
1k 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?
6
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1answer
118 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
3k views

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 ...
4
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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.
4
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2answers
495 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
3k 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
1k 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
524 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
657 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 ...
6
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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 ...
0
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1answer
322 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?
14
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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
159 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 = ...