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Questions tagged [units]

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

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1answer
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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?
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1answer
287 views

Barron states that 4.18×10⁸ joules equal 1 kcal, is this correct?

Here is a paragraph in Barron's SAT chemistry prep book: Work itself is measured in Joules, and so is energy. In some problems, however, energy may be expressed in kilocalories. The relationship ...
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2answers
74 views

How are lowercase k and uppercase K used in general chemistry courses? [closed]

Where do lowercase k and uppercase K make appearances in general chemistry, and what do they signify?
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2answers
167 views

How are lowercase m and uppercase M used in general chemistry courses? [closed]

Where do lowercase m and uppercase M make appearances in general chemistry, and what do they signify?
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1answer
45 views

IUPAC Recommendations for Concentration Units (ppb, ppm)

In a recent interesting post, (Is 1 ppb equal to 1 μg/kg?) it was pointed out that IUPAC advises to abandon parts per million, parts per billion quantities and instead it suggests to employ micromole/ ...
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1answer
2k views

Is 1 ppb equal to 1 μg/kg?

In an article I recently submitted, a reviewer asked that I provide a concentration in μg/kg instead of ppb (parts per billion), and mentions that the later is not correct. I am not a chemist, and I ...
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2answers
526 views

How to determine omitted units in a publication

I have found a 1954 paper in J. Am. Chem. Soc. which gives various temperatures in degrees but does not specify units. A snippet of the introduction is: Titanium reacts appreciably with fluorine ...
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2answers
70 views

Does the prefix of an inverted unit also get inverted?

What is the energy of radiation that has a frequency of $\pu{2.51 \times 10^11 ms-1}$? (a) $\pu{1.66 \times 10^-19 J}$ (supposedly correct) (b) $\pu{1.66 \times 10^-22 J}$ (c) $\pu{7.92 \...
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1answer
40 views

Standard litres to kg conversion for Hydrogen [closed]

When looking at hydrogen fuel tanks, they often give the capacity in terms of "standard litres". However, I can't find a definition of what this is. I assumed this may mean a litre of hydrogen gas ...
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2answers
32 views

Unit analysis for neutralization reaction

What volume of $\pu{0.500 mol L-1}$ $\ce{HCl}$ (in $\pu{mL}$) is required to neutralize $\pu{2.02 g}$ of solid $\ce{NaOH}$ pellets? So far I have tried regular unit analysis: 2.02 g NaOH * 1mol ...
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1answer
30 views

Types of concentration units

I would like to ask about a concentration unit. What does it exactly mean when stock concentration of something is, for example, 5X? Thank you in advance!
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2answers
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What will be unit of volume in V=nRT/p if p is in Torr instead of atm? [closed]

$V=nRT/p$, where $n$ is amount of gas in $\mathrm{mol}$, $R$ is gas constant, $V$ is volume in $\pu{dm3}$, and $p$ is pressure in $\pu{atm}$. What happens to $R$ if $p$ is in $\pu{Torr}$ instead of $...
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1answer
95 views

How to actually calculate work from force times distance?

I understand that work = force $\cdot$ distance, but I'm not sure of the units and conversions to use. What I know is that I have a piston head of area $\pu{28cm^2}$ (~$\pu{4.4in^2}$) traveling a ...
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3answers
181 views

Conversion from a PPB value to µg/m3 of Isobutylene

I have a quite simple problem, found a lot of information about it, but I am not sure anymore if I do my calculations right. I own a sensor, which reports measurements in isobutylene units as PPB. So ...
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1answer
22 views

Combination of several unit symbols when finding new pressure from ideal gas law

As a car is driven, the tires heat up and the volume and the pressure change. Calculate the new pressure of a 35.0 L tire initially at 235 kPa that expands to 36.5 L when it heats up from 15.0 °C to ...
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0answers
19 views

Is the negative excess volume of ethanol/water the cause of my math error?

I have a binary mixture, say ethanol at $90\%wt$ ethanol, $10\%wt$ water. According to Perry's Chemical Engineers' Handbook 9th Ed., that mixture has a density of $0.81797g/cc$ (at $20^{\circ}C$). ...
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3answers
934 views

What are the units of Kc and Kp?

They are both equilibrium constants as far as I know. Kc is in terms of molarity and Kp is in terms of pressure. Also both of them are ratios of respective quantities [ ratio of molarity(s) in Kc and ...
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1answer
45 views

Confusion between moles and molarity units

I am a total newbie in the field and i would like some kind of guidance. So, I have been recently given 2 datasets with measurements regarding several phospholipid fatty acids of plasma HDL. Both ...
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1answer
22 views

Rate constant converion [closed]

I need to calculate the half life for a first order reaction for which I need the rate constant $k$. From literature I observed that for a very closely simulated experiment I obtained the numerical ...
0
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1answer
42 views

Converting pounds to grams [closed]

I wanted to figure out how many grams were to a lb and instead of looking it up I used my textbook and found 1 kg = 1000 g and 1 kg = 2.205 lb. I set up a cross multiplication equation to solve for x. ...
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1answer
461 views

What does atm•L mean? [closed]

In my chemistry book I always see a dot for units. For example, atm•L. Does this mean “1 atm per Liter?” By Atm I mean atmospheric pressure, not atom.
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62 views

Is the kilogram still consistent with the old definition?

My old understanding was that a gram is the amount of mass in a cubic centimeter of water. But then I recently learned that the second was redefined to be some 9 billion cycles of a Caesium atom. ...
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1answer
97 views

Is it possible for me to derive Avogadro's number?

To my understanding the mole is the unit used to translate between mass on the atomic level and mass in the macro level, defined as the number of atoms in $12$ grams of Carbon-$12$, which apparently ...
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5answers
6k views

Why do we still need to know about the Rankine temperature scale?

I am learning process principles in chemical engineering and I was taught various temperature units like Fahrenheit, Celsius, Kelvin, and Rankine. I see the first three being used quite commonly, but ...
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0answers
81 views

Why was the unit micrometers spelled backwards in old research papers?

I have recently been doing a literature research project on polymers and have been reading a lot of papers from the 1950s and 1960s (mostly in American journals from American authors). I saw this ...
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1answer
47 views

Units for density in nanorod image

I am counting the number of nanorods contained in a sample (see picture below). The length scale is 1 µm. What units does the density have? Is it µm-2?
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84 views

How to convert from ppm to ug/m3 in case of PMx?

For $\text{PM}_{10}$, $\text{PM}_{2.5}$ sensors, sensitivity, minimum and maximum recordable concentrations are generally expressed in $\pu{ppm}$ (parts per million) while legislation uses $\pu{\mu g /...
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0answers
660 views

Conversion of a permeability value from barrer to SI unit

I have done a permeability experiment on LDPE in the presence of air. At $\pu{30^\circ C}$, the permeability was found to be 1.846 barrer. Now I am trying to apply this permeability value to a ...
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0answers
71 views

is rydberg constant and rydberg unit are same? [closed]

i want to ask a question that.is rydberg constant and rydberg unit are same. UNIT OF RYDBERG CONSTANT IS m^-1, right? Then what is rydberg unit.
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0answers
1k views

What really are the units of enthalpy change? [duplicate]

A reasonable definition of standard enthalpy change of formation would be: 'The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions' ...
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1answer
108 views

Calculate pressure equilibrium constant from concentration equilibrium constant

$$\ce{N2(g) + 3H2 (g) -> 2NH3(g)}$$ Calculate $K_\mathrm p$ from $K_\mathrm c$, given that $K_\mathrm c = \pu{4.2 M^-2}$ and that there is 0.2 moles of $\ce{N2}$, 0.6 moles of $\ce{H2}$ and 0....
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2answers
238 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?
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1answer
77 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 ...
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1answer
41 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 ...
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2answers
253 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 ...
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1answer
1k 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 ...
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1answer
121 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 ...
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388 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 $\pu{5.68e-5 mol L-1}$. ...
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1answer
78 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, ...
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2answers
5k 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 ...
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1answer
112 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/...
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37 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
574 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, ...
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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
251 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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0answers
75 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
475 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 ...