Questions tagged [units]
For questions about the definition and usage of units of measurement.
202 questions
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What is the evaluation order of sign and unit in chemistry quantities?
I am curious about the evaluation order when a chemistry quantity has both sign and unit, such as $+3.4\,\text{kJ}/\text{mol}$.
Because there is space between number and unit, I think $e = (+3.4) \, (...
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What does “Gm. Moles per 1000 gms. H2O” mean?
I was looking for the solubility of sodium carbonate in Atherton's Solubilities of Inorganic and Organic Compounds, section Solubility of sodium carbonate in water (Kobe and Sheehy, 1948) [1, p. 442], ...
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Significant figures when converting temperature units
Say I am solving a gas law equation using
$$
\frac{p_1}{T_1} = \frac{p_2}{T_2}
$$
where the givens are
$p_1 = \pu{91 atm}$ (2 sig figs)
$T_1 = \pu{467 °C}$ (3 sig figs)
$T_2 = \pu{300 °C}$ (1 sig fig)
...
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Does the Value of ka change if concentration values are above 1?
I might be overthinking, but take this example:
Ka=[A-]*[H3O+]/[HA]=0.1(M)*0.1(M)/0.02(M)=0.01/0.02=0.5
The concentrations here have the unit (mol/liter).
But if we switch the same concentrations to (...
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Are you allowed to cancel masses of different materials in chemistry to create a dimensionless quantity?
My roommate and I are debating unit cancellation. He’s taking a hazardous materials class, and one of his homework quantities boils down to mg of contaminant/(kg of body mass * days). I believe ...
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Why does this method for finding the amount of atoms given a known mass of a pure sample work?
Say I have this problem, for example:
How many atoms are in 39.6g of pure tungsten?
So what I would know here is that first off the molar mass of tungsten is $183.84\,\text g/\text{mol}$. While I ...
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Why is 1 ppm considered equal to 1 mg/dm3?
I usually work with percentages, so am not familiar with parts per million (ppm) fraction (ratio) method.
My teacher taught me that $1~\pu{ppm} = 1 ~\pu{mg/dm^3}$ without proof, so I looked in the ...
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Electrons Transferred in an Electrochemical Reation
I was reading Bard's book, I noticed in Chpt 2, he defines the change in Gibbs free energy of a cell as:
$$\Delta G = nFE_{rxn}$$
He describes $n$ as the number of electrons, and $F$ as having units ...
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Why isn't the rate of an elementary reaction expressed in activity?
Why don't chemists use $r_\ce{X}=\frac{\text{d}a_\ce{X}}{\text{d}t}$ where $a_\ce{X}$ is the chemical activity of species $\ce{X}$ and corresponding rate law for elementary reactions as $r=ka_1^{\...
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Comparison of oxygen and carbon-12 as standards for mass
While studying about the standards of atomic and molecular mass, I have learnt that while initially hydrogen was used as a standard of mass (its mass taken as unity), according to my book, oxygen was ...
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How do I find the amount of moles from the amount of units?
The problem:
Find out the amount of moles of a certain substance knowing that the amount of units of said substance is $\pu{1.8E23}$. I have to find how many moles of carbon, hydrogen, and oxygen that ...
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Convert daltons (Da) to molar mass (g/mol) [duplicate]
This is probably a stupid question...
In converting Da to molar mass (g/mol),
Since 1 Da = 1 g/mol, is it logical that 64,776 Da = 64,776 g/mol or am I missing an intermediate calculation?
Thank you!
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Uncertainty when beaker goes by 50s
I am helping my daughter do her chemistry homework and am stumped by this one. I have a beaker that seems to count by 50s.
That makes uncertainty calculations awkward. What is the “next unit down”?
...
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How to obtain the value of the gas constant R = 8.20574 × 10⁻² L atm K⁻¹ mol⁻¹
While reading a chemistry book, I saw a table of values of the gas constant for different units.
For example,
$$R=8.20574\times 10^{-2}\ \text{L}\cdot\text{atm}\cdot \text{K}^{-1}\cdot \text{mol}^{-1}\...
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What do the units of enthalpy kJ/mol represent?
Enthalpy is making me slightly confused.
Here $\Delta H$ refers to the change in enthalpy. Does this refer to the enthalpy change of reaction? Are they the same? It seems that change in enthalpy, ...
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