Questions tagged [units]
For questions about the definition and usage of units of measurement.
194
questions
1
vote
0
answers
32
views
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^{\...
0
votes
0
answers
53
views
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 ...
2
votes
1
answer
121
views
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 ...
- 133
-2
votes
1
answer
5k
views
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!
7
votes
1
answer
723
views
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”?
...
- 231
0
votes
4
answers
368
views
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}\...
- 168
2
votes
2
answers
614
views
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, ...
- 95
6
votes
1
answer
801
views
What is the correct unit of a RI detector used for HPLC analysis?
I analyze sugar compounds in soft drinks with HPLC using refractive index detection (RID) on Agilent 1260 Infinity Binary LC (System User Guide (PDF)). I am not sure which unit to put next to the $y$-...
- 79
2
votes
1
answer
1k
views
What is the unit of molar density?
I'm reading Unit operations of Chemical Engineering, 5th edition by Warren McCabe.
The authors defined the molar density as $\rho_{M} = \frac{n}{V}$. Here, $n$ and $V$ represent the number of moles ...
- 67
0
votes
1
answer
46
views
Masses of atoms and molecules [duplicate]
This is my understanding so far:
Average atomic mass is the mass of an element considering isotopes and is measured in atomic mass units. However relative atomic mass is the average mass of an atom ( ...
1
vote
1
answer
371
views
Definition of Avogadro's constant
I've read this excerpt from Wikipedia countless times, but I'm still confused:
The Avogadro number is the approximate number of nucleons (protons or neutrons) in one gram of ordinary matter.
Shouldn'...
2
votes
1
answer
330
views
What is the correct method for converting a concentration given in ppm to molarity?
I need to convert a concentration of dissolved boric acid $(\ce{H_3BO_3})$ in water (at high temperature and pressure) from ppm to mol/L.
Concentration of boric acid: $c_{ppm}=1500\ \ce{ppm}$
Molar ...
- 137
0
votes
0
answers
41
views
How to rewrite kinetic mass balance to get to proper SI units?
Suppose from kinetic mass balance I get the following differential equation of the molarity substances $A$,$B$, and $C$ in a reactor, with molar in- or outflow rate $\phi$, and rate constant $k$:
$$
\...
- 11
1
vote
1
answer
222
views
How do you rewrite molarity to weight percentage or volume percentage?
Suppose I have a molar concentration $[C]$ in $mol/m^3$ in a continuous reactor, from which the differential equation is as follows:
$ \frac{\partial [C]}{\partial t} = k[A][B] + \phi C_{,in} - \phi ...
- 11
-1
votes
1
answer
182
views
Convention for writing concentration units
1st part of my question:
Q) The concentration of an NaCl solution is 1M. Express this in percent wt. by vol. unit.
Ans:
$$\text{In %(w/v) unit}=\frac{58.5\pu g}{1000\pu{mL}}\times \frac{100}{100}=5.85\...
- 201