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37

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 unless you explicitly define the usage of the "parts per something" in the text. This clutters the manuscript with redundant notes and causes overall confusion. ...


31

Sigma-Aldrich gives a very useful table outlining what the different purity levels are and suggested applications. I was less successful at finding equal documentation from some of the other suppliers, but the analysis for the purity of the chemicals they sell is in the catalog and on the bottle. In general, technical grade or laboratory grade are the ...


19

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 as a result of expansion or contraction of the solvent upon changing the temperature, and thus the molarity changes, since $$M=\frac{n}{V}.$$ The downside, of ...


15

You correctly point out that the number of molecules in a solution is finite and constant, however the volumetric concentration (that is, how many molecules per litre) changes upon dilution. If, for instance, you take one liter of a 1 mol/L solution of ethanol in water ($\approx{6.02\times{10}^{23}}$ ethanol molecules per liter) and add 9 liters of distilled ...


15

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 practice this is achieved by using standard states which refer to the pure materials: standard concentration $c^⦵$ and standard pressure $p^⦵$. One must be very ...


14

The curly brackets denote "activity of" the species therein. See the Wikipedia section: Basic definitions and properties of Equilibrium constant


13

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 presence of oxygen $\ce{O2}$ and water $\ce{H2O}$. $$\ce{4Fe +3O2 +6H2O->4Fe(OH)3}$$ Corrosion (rust) is a 'redox' reaction, which means it involves ...


13

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 approach has been used in sugar industry by using "Cobenz diagrams" aka spider diagrams. Widely popularized in Soviet books for analytical chemistry at least ...


12

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 solute per liter of solution, while molality is amount of substance of solute per kilogram of solvent. This might not seem like a huge difference, but if you ...


11

What you witnessed was probably something similar to superheating/bumping. Essentially, when a solution is heated too rapidly, large pockets of vapor can form beneath the surface of the liquid that have an internal temperature higher than the expected boiling temperature of the liquid. In those situations, even small physical disturbances and/or further ...


11

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 consumed, but the ratio of the two is constant. If the reaction causes a temperature change then there are small changes in density, but that would also alter the ...


11

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 skip over the fact that adding solute to a solution increases its density, because it makes life more complicated. As you just found out though, sometimes you ...


11

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/20_Learn_About_Us/25_Products/252_Chemetics/Hydrochloric%20Acid%20Concentration.pdf (Obviously, proper equipment and safety precautions are essential.)


11

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 that if you look at the system it seems like nothing is happening, but at molecular scale there are reactions going on and the rate of forward reaction = rate of ...


10

$$K_\mathrm{a}=\ce{\frac{[A-][H3O+]}{[HA]}}$$ 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 an intrinsic property of the acid, i.e. it's value does not change from sample to sample (temperature aside). While it is true that as a solution of acid ...


10

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, distillation will not result in a higher concentration of acid. Hydrogen chloride and water form a constant-boiling mixture (azeotrope) at ~20% HCl, which is what you ...


10

A chemical equilibrium concerns chemical reactions. There should be at least a forward- and backward reaction between two species but more complex systems (polygons, circles, …) with multiple individual reactions may occur. The important observation is that there is no macroscopic change to the chemical constituents of the system, i.e. the concentrations of ...


9

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 kilogram of solvent). Molarity is just one way to express concentrations, which, more specifically, is the number of moles of solute per liter of solution. But ...


9

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 around $40~\%$ and commercial concentrations ranging from 30 to 35% for convenience in transportation infrastructure. For nitric acid — $\ce{HNO3}$ the $68~\%$ ...


9

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 Law of Guldberg and Waage, historically. Guldberg and Waage defined the term "active mass" in the 1867 Études sur les affinitès chimiques. l'on peut ...


7

The short answer is because $\ce{HF}$ doesn't dissociate as much as $\ce{HCl}$. Let's examine why this is by introducing some terms like $\ce{pH}$ and $\ce{pK_a}$. $\ce{pH}$ does not measure the strength of a given acid, but rather the acidity of a given solution. $\ce{pK_a}$ does measure the relative strength of an acid, look at the following ...


7

Your concentration is 0.615 M. The M stands for molar concentration, which is the number of moles per liter of solution. Thus, your concentration of 0.615 M (mol/L) means that in one liter of solution there will be 0.615 moles of solute (KCl). Similarly, a solution with concentration of 0.615 g/L would have 0.615 grams of solute per 1 liter of solution. A ...


7

The correct and standardized quantity name is amount-of-substance concentration. In chemistry, however, the name is generally abbreviated to the single word concentration, it being assumed that the adjective ‘amount-of-substance’ is intended. The quantity symbol is a lower case $c$. The quantity ‘amount-of-substance concentration of $\ce{B}$’ is defined as $...


7

To convert from molal to molar, one generally needs to know the density of solution as well as the formula weight of solute. Suppose there is a single solute $x$ in water, and that $x$ has a formula weight of $f_x$ g/mol. Suppose also that the solution of $x$ has a density of of $\rho$. Let the molarity of $x$ in the solution be $C_x$ mol/L. Then: the ...


7

Correct me if I'm wrong, but don't you need at least 2 solutes to make a solution? You are, unfortunately, incorrect. In wet chemistry, you need just a solvent$^\dagger$ and at least one solute to make a solution. While this does mean that you need at least two constituents to make up a solution, you do not need at least two solutes. ... is it saying ...


6

They don't vanish. It's just that if there are X molecules and Z bottles with Z > X, part of the bottles will not have even one molecule.


6

They can. Vinegar is typically 4-18% acetic acid by mass, but of course concentrated acetic acid also exists with a concentration >90% by mass. It is corrosive and flammable, but it surely exists.


6

What the question is asking is the molar concentration of the given substance after dilution with the help of values given.The first step of the answer is converting the given weight of $\ce{Na2CO3 * 10H2O}$ into no of moles.For that we have a formula as \begin{align} \text {no of moles} &= \frac{\text{Weight of substance}}{\text{Molecular weight of the ...


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