45 votes
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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 ...
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34 votes
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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 ...
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  • 1,538
21 votes
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Why is active mass of a pure solid or liquid always taken as unity?

Does the activity of a solid or liquid change over the course of a reaction? The density of a solid or liquid reactant doesn't change over the course of a reaction. The mass and volume do as it is ...
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19 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 ...
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  • 1,080
16 votes
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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 ...
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  • 331
15 votes
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Does concentration of salt increase or decrease rate of rusting?

In general, salt (particularly NaCl) will increase the rate of corrosion (rusting). To understand why, consider metallic iron $\ce{Fe}$ which rusts (oxidises) to iron(II) oxide $\ce{Fe2O3}$ in the ...
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  • 2,182
15 votes
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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
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15 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 ...
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15 votes
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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 ...
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  • 35.2k
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. ...
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14 votes
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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/...
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13 votes
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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 ...
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13 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 ...
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  • 32.7k
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 ...
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  • 3,333
11 votes

How is sulfuric acid with a concentration of 12 M also 72% w/w?

Your problem is that you've assumed the density is 1 gram/mL. Remember that a molar is defined as a mole of solvent per liter of solution, not solvent. Usually, in introductory chemistry classes, we ...
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  • 3,333
11 votes
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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 ...
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10 votes
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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 ...
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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, ...
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10 votes
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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....
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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 ...
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  • 18.7k
9 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 ...
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9 votes
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How do I calculate the concentration of sulphuric acid by a titration experiment with sodium hydroxide?

Molarity of diluted $\ce {H2SO4}$ (solution 2): $\pu{0.0013798 mol}/\pu{0.025 L} = \pu{0.054172 M}$ (I may be using the wrong volume, is it possible that I have to add the $\pu{25 ml}$ to the $\pu{...
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  • 7,902
9 votes
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What is the difference between molarity and concentration?

Concentration is any measurement of the quantity of a solute that is present per unit of solution (in general, there are exceptions like moLality, which measures the number of moles of solute per ...
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9 votes

How to interpret the instruction of the dilution of hydrogen peroxide?

Take $\pu{200 mL}$ of the $30\%$ solution, add water to a final volume of $1$ liter. Then you have a $6\%$ solution. Next, take $\pu{59 mL}$ of the $6\%$ solution and add water until it is $1$ liter. ...
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9 votes
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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 ...
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  • 636
9 votes
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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 ...
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8 votes
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How to convert g/L to mol/L

Yes. $$n = \frac{m}{M}~~~~~~~~~~n = cV$$ So $$c = \frac{m}{VM}$$ Given $\frac{m}{V}$ you can work out $c$ (in $\mathrm{mol~dm^{-3}}$).
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8 votes
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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 ...
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  • 16.8k
8 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 ...
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