Questions tagged [units]

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

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How do I convert salinity measured in mg/L Cl into parts per thousand as NaCl?

I have collected water data using a refractometer that records salinity in ‰ (parts per thousand) as $\ce{NaCl}$. I want to compare my data to other data but every journal article I read has the ...
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1answer
812 views

Are atomic mass unit (amu), unified mass unit (u) and Dalton (Da) the same unit but with different names? [duplicate]

I recently read up on some chemistry and found these units of mass: atomic mass unit (amu) unified atomic mass unit (u) Dalton (Da) Are they all the same unit but just with different names or are ...
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1answer
128 views

Up to which dilution can one assume that 1 mg/l = 1 ppm?

For aqueous solutions it is often assumed that 1 milligram per liter equals 1 ppm. Are there any guidelines up to which concentration this shortcut is admissible? Are there any formulas which ...
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1answer
138 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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1answer
65 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 ...
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1answer
231 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 ...
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1answer
78 views

What is the 'electronic energy', in units of 'negative' hartrees, of a molecule?

On a NIST (National Institute for Standards and Technology) page on the molecular details of water and it's various isotopologues, I came across this: Electronic energy: -76.4089547246 hartree There ...
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1answer
511 views

Units of the rate constant for reactions whose order is greater than 1

In many textbooks, it is written that: The value of rate constant depends on the nature of the reactants, temperature and catalyst. It is independent of the concentration of the reactants. However,...
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1answer
535 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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20 views

Thermochemistry: On Units of Enthalpy Change and Implicit Given of Initial Temperature of Calorimeter

I would like to ask two separate questions. But first, so that we are on the same page, the experiment used a constant-pressure calorimeter, and it is a neutralization reaction of 1M of HCl and 1M of ...
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25 views

Interlayer binding energy: how to convert to meV/Ų?

I've noticed that in most papers the interlayer binding energy is given in units of meV/Ų. But how do I get these units? Do I divide the interlayer binding energy I get by the area of my unit cell?
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116 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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104 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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3k 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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494 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
169 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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44 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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6k 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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1k 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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62k 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
32 views

What is the use of both mol and mass representing the same thing i.e. amount of something? [closed]

A mol is a unit but also has a magnitude. How can a unit have a magnitude? And if mole is used to measure an amount of substance than what does mass measure?
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1answer
126 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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3answers
2k 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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2answers
57 views

Is 1% m/m the same as 1% w/w?

Hydrocortisone cream seems to be sold as 1% w/w, but my cream is 1% m/m, which is confusing. Are they the same units or the same concentrations?
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1answer
1k views

Meaning of mass concentration “units” % m/v [closed]

In a saline solution that is 5.750% (m/v) potassium iodide $(\ce{KI}),$ there are: a) 5.750 mL $\ce{KI}$ per 100.0 mL $\ce{H2O}$ b) 575.0 mg $\ce{KI}$ per 10.00 mL of $\ce{KI}$ solution c) 5.750 g $\...
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90 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
369 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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2answers
102 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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2answers
13k 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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1answer
20k 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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1answer
87 views

Is it accurate to describe these radiation measurement units in terms of where-it's-measured and relationship-to-time?

I'm reading about measuring radiation and trying to come up with a more intuitive retelling for an audience familiar with math but not necessarily chemistry or any physical science. Is any or all of ...
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1answer
40 views

Question on notation

Here, when they write $v/(\mathrm{mol\ s^{-1}\ (kg\ \text{protein})^{-1}})$ is $0.30$ for the first row first column, do they mean that the value is $0.30\ \mathrm{mol\ s^{-1}\ (kg\ \text{protein})^{-...
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1answer
255 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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3answers
1k 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
36 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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1answer
65 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
964 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. ...
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33 views

Why isn't entropy dimensionless?

The usual units are J/K, however the only reason temperature has different units than energy is because we didn't used to know temperature was just the average kinetic energy of atoms/molecules. ...
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0answers
27 views

When multiple sources give the content of an ingredient my an unqualified percentage, can I safely assume this is by weight? Or; it's not a wart

I am trying to make a simple scented shampoo and body soap for my own use and for Christmas gifts. I statr with a base of 32 oz – (a bit under a kg) of a mild commercial castille soap (Dr. Bronner’s ...
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1answer
470 views

Why is gas pressure measured in mmHg?

Pressure can be measured in $\pu{mmHg}$ which is equal to the pressure exerted by a “column of mercury” of a height expressed in $\pu{mm},$ at $\pu{0 °C}$ under Earth’s normal gravity. Does the ...
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1answer
29 views

Difference between “physical quantities” and “units”?

My textbook, Atkins' Physical Chemistry, 11th edition, by Atkins, de Paula, and Keeler, says the following: Although it is good practice to use only SI units, there will be occasions where accepted ...
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1answer
70 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
46 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
87 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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1answer
4k 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 = $\frac{\kappa}{M}$. S.I. unit of : $...
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1answer
5k 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
638 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
20k 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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3answers
13k 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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2answers
45 views

Calculating the molar volume of a perfect gas under standard ambient temperature and pressure [duplicate]

I am trying to calculate the molar volume of a perfect gas under standard ambient temperature and pressure, but I'm having problems with the units. $$\begin{align} V_m = \dfrac{RT}{p} &= \dfrac{(8....