48
votes
Accepted
How to interpret parts-per notations? Is 1 ppb equal to 1 μg/kg?
You are correct suggesting that 1 μg/kg implies 1 ppb, however the reverse is not true. For instance, 1 ppb can also be 1 nmol/mol, and the reader will never have a chance to deduce which one is it ...
- 37.4k
36
votes
Accepted
Is it possible to achieve a level of "truly zero" concentration?
It depends how you dilute it. If you take an aqueous solution of A and just add pure water (absolutely 100% water), the concentration of A will never quite be null. In this case however, you will ...
- 1,618
20
votes
What is the point of using molality when we have molarity?
One of the key attractions of molality is that changing the temperature of a solution does not change the molality, while it may change the molarity. This is because the volume of the solution changes ...
- 1,121
16
votes
What is active mass?
The term "active mass" is a historical term.
The concept of an equilibrium constant was developed by Cato Maximilian Guldberg and Peter Waage. The Law of Mass Action has also been referred to as the ...
- 40.2k
16
votes
Accepted
What is the difference between chemical equilibrium and dynamic equilibrium?
Chemical equilibrium is a type of dynamic equilibrium, but not every dynamic equilibrium is a chemical equilibrium.
In a chemical equilibrium there is no change on the macroscopic scale. That means ...
- 331
15
votes
Accepted
What do curly brackets {} mean?
The curly brackets denote "activity of" the species therein.
See the Wikipedia section: Basic definitions and properties of Equilibrium constant
- 40.2k
15
votes
Accepted
Why proton concentration is divided by 10⁻⁷?
The textbook is precisely correct.
The equilibrium constant $K$ which the logarithm is taken of is dimensionless, and includes activities or fugacities, and not concentrations and pressures.
In ...
- 37.4k
15
votes
Accepted
Is there a name for this algorithm to calculate the concentration of a mixture of two solutions containing the same solute?
This is a so-called "Pearson's square" or "Box method" of balancing ratios, originally used extensively in dairy industry (at least since 1920s judging from Google Books search).
Earlier the similar ...
- 37.4k
15
votes
What is the difference between chemical equilibrium and dynamic equilibrium?
A chemical equilibrium concerns chemical reactions. There should be at least a forward- and backward reaction between two species but more complex systems with multiple individual reactions may occur. ...
- 67.3k
15
votes
Is it possible to achieve a level of "truly zero" concentration?
For an analytical chemist, the concept of zero concentration does not exist. The concentration cannot be exactly 0! Only a limit of detection can be developed in terms of statistics. This is why a ...
- 39.1k
14
votes
Accepted
How to increase the concentration of HCl
Add Calcium Chloride to destroy the azeotrope. Then distill and capture the vapor phase.
See the second method "Extractive Distillation" here:
http://www.jacobs.com/uploadedFiles/wwwjacobscom/...
- 40.2k
12
votes
What is the point of using molality when we have molarity?
In addition to JSK's excellent answer, I'd like to point out that there's a common pitfall related to molarity (which JSKs answer might have slipped on): molarity is defined as amount of substance of ...
- 3,373
11
votes
What is an equivalent?
Equivalent concept is an archaic unit of measurement that was used in chemistry and the biological sciences in the era before researchers knew how to determine the chemical formula for a compound.
An ...
- 629
11
votes
Accepted
Discrepancy when calulating mol weights with ChemSketch and Python RDKit
In first place, you must clarify with the users of the database the purpose of the entries about molecular weight because average molecular weight and exact molecular weight differ in their meaning ...
- 28.4k
11
votes
Best regression model for a sigmoidal pattern
It looks like your model is not complete. The current shape of your model makes the function go to zero for small values of $x$. There is nothing to indicate that the function should go to zero, ...
10
votes
Accepted
Why is Ka constant when volume is increased?
$$K_\mathrm{a}=\ce{\frac{[A-][H3O+]}{[HA]}}$$
The above equation defines the $K_\mathrm{a}$, or acid dissociation constant, of an acid. The reason it is even a value worth measuring is because it is ...
- 23.9k
10
votes
How to increase the concentration of HCl
Irrespective of the potential hazards to an inexperienced chemist attempting distillation of hydrochloric acid, especially absence of adequate safety measures such as a fume hood, in this case, ...
- 9,216
10
votes
Accepted
Why is the concentration of pure water is 55.5 mol/L?
First, you want to work out the number of moles in a litre ($\pu{1000 mL}$) of water:
\begin{align}
n &= \frac{m}{M_r}\\[1ex]
&= \frac{\pu{1000 g}}{\pu{18 g mol-1}}\\
&= \pu{55....
- 17.1k
10
votes
Is it possible to achieve a level of "truly zero" concentration?
Unequivocally yes! If your solute is something of which you can identify a single molecule in a macroscopic solvent sample (let´s say a fluorescent dye), then you can dilute down to zero, and be sure ...
- 12k
10
votes
Quantifying soapiness; there's pH, pKa and pO2, is there a p_soap or p_surfactance?
A concept that captures how effective a detergent is at doings its job is aptly called "detergency." As might be expected this is a complex property and difficult to describe unambiguously ...
- 20.8k
10
votes
Does "brine rejection" happen for dissolved gases as well?
While there may well be counterexamples, a paper in Cryobiology (G. Lipp, Ch. Körber, S. Englich, U. Hartmann, and G. Rau in Cryobiology Volume 24, Issue 6, December 1987, Pages 489-503) titled "...
- 8,289
10
votes
Best regression model for a sigmoidal pattern
Some of the examples on the page EdV's comment points to are programmatic (as in: knowledge of a scripting/programming language is beneficial/a requirement) in addition to statistics.
If you are more ...
- 28.4k
9
votes
Accepted
Why is hydrochloric acid often sold as 33% concentration?
The user ssavec prompted me to research further on my own and post an answer.
It seems the limits are practical ones.
For hydrochloric acid — $\ce{HCl}$ a practical limit is 38% with absolute
limit ...
- 656
9
votes
Accepted
Relationship between molar mass and density in solids
If your sample is solid and crystalline, knowledge of the unit cell's dimension and symmetry (e.g., fcc) and density allows you to determine the molecular mass of your compound.
In fact, in ...
- 28.4k
8
votes
Why is the specific gravity of 0.9 % saline 1.0046?
0.9% medical saline is defined to contain $\pu{0.90 g}$ sodium chloride per $\pu{1000 ml}$ solution at $\pu{22 °C}$. Its density is $\pu{1.0046 g/cm^3}$ at $\pu{22 °C}$.
The density of water is $\pu{...
- 7,142
8
votes
Accepted
How to calculate the molarity of a gas?
Molarity is defined as "moles of solute per volume of solution", which implies that the system is in the liquid state.
As such, molarity is undefined for anything in a gaseous state -- there is no ...
- 17.3k
8
votes
Best regression model for a sigmoidal pattern
I am answering to the comment that you made to the @Buttonwood's great answer.
Clearly the $y$ values are the $\text{pH}$ of the solution and the $x$ values the volume of base added. If you don't mind,...
- 2,246
7
votes
Accepted
What was the lithium concentration in 1940's 7-Up?
Given that the formula was a trade secret and that lithium was removed from the product ~70 years ago, the likelihood of this question being answered by providing a detailed formula seems remote. Note ...
- 20.8k
7
votes
Accepted
How to convert from w/w% to molarity?
Weight percent is essentially the same as (I arbitrarily picked grams as the mass unit):
$$
\%~\mathrm{w/w~X} = \mathrm{g~X\over g~solution}
$$
In order to get to the $\mathrm{mol\over L}$ units of ...
- 17.3k
7
votes
Accepted
Temperature dependence of molarity and molality
Recall that molarity $c$ is
$$c=\frac{n_\text{solute}}{V_\text{solution}}$$
whereas for molality $b$ is
$$b=\frac{n_\text{solute}}{m_\text{solvent}}$$
While in the extreme case you mentioned, both ...
- 698
Only top scored, non community-wiki answers of a minimum length are eligible