Questions tagged [physical-chemistry]

The study of chemical systems using the laws and concepts of physics. This usually requires the techniques of thermodynamics, statistical mechanics and quantum mechanics.

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How can the equilibrium shift, while Kc remains constant?

Consider the following reversible reaction. $$\ce{Cr2O7^2-(aq) + H2O(l) <=> 2 CrO4^2-(aq) + 2 H+(aq)}$$ What will happen to the position of equilibrium and the value of $K_c$ when more ...
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How do I know if concentration of Ammonia affected my absorbency measurements?

This is for determination of copper in a penny Lab. After making standard solutions we made a solution with a penny, dissolving it in nitric acid and adding ammonium hydroxide to form a more ...
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Lewis model and modern physics

Electron affinity of chlorine = -349 kJ per mole. And, first ionization energy of sodium = 496 kJ per mole while energy required for combining= 147 kJ per mole. Isn't it a contradiction against ...
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606 views

What are the main axis of research in Chemistry? [closed]

I would like to know what are the main problems currently studied in Chemistry. For some reason, it seems that there is far less vulgarisation in chemistry than the other fields, and it's hard to ...
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Calculating volumes of acid/base required to make buffer using pH and pKa

How would we use our $\mathrm{p}K_{\mathrm{a}}$ of the acid component of the buffer to calculate $\large \frac{[\ce{A-}]}{[\ce{HA}]}$? I know $$\mathrm{pH} = \mathrm{p}K_{\mathrm{a}} + \log\left(\...
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1answer
258 views

Partial pressure of water in a submerged balloon

A balloon is half filled with water at sea level ($P = \pu{1 atm}$, $T = \pu{298K}$) such that the partial pressure of water is $\pu{24 mmHg}$. It is then submerged to a point where $P = \pu{2 atm}, ...
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1answer
222 views

Equilibrium constant expression for heterogeneous equilibrium

Consider this reaction: $$\ce{CaCO3(s) <=> CaO(s) + CO2(g)}$$ for this, $K_c=\left[\ce{CO2(g)}\right]$ For a given temperature, $K_c$ will remain fixed. That means, the amount of $\ce{CO2}$ ...
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342 views

Ratio of concentrations

I'm studying for my chemistry final and having a hard time with this question on one of the practice exams. Here's the question: At $20$ degreees Celsius, Henry's Law constant for Ar is half of that ...
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Empirical formula when given percentages?

A binary compound of lead and oxygen contains $90.66 \% \ce{Pb}$. What is the empirical formula for the compound? How do I use the percentage to get to the normal formula, then how do I change the ...
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1answer
88 views

How to find partial pressures of gases only given their percentages?

How do I find partial pressures if I am only given their percentages? Where do I begin? Example: Assume $\ce{O}=21\%$ , $\ce{N}=78\%$ , $\ce{Ar}=1\%$ by volume. How would I calculate the partial ...
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Equilibrium constant

I know that equilibrium constant depends on temperature and pressure. But according to Le Chatelier's principle when we change pressure of reagents the equilibrium is the same. Why?
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1answer
468 views

Why do aqueous solutions always have to be electrically neutral?

I was reviewing some analytical chemistry and stumbled upon a section that explained the imperfection of using a salt bridge. It said that the using dissimilar ions is a problem because in, for ...
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1answer
136 views

Are there any free electrons in metal vapor?

Metals contain free moving electrons, in both the solid and liquid state. Does this happen when it is in vapor form?
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I have to write a Python program relating with molecular weight. Any insight?

Write a program that determines the molecular weight of a chemical compound based on the number of hydrogen, carbon, and oxygen atoms in the compound. You should use the following atomic weights: \...
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1answer
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Diamagnetism in sulfur

Sulfur is a diamagnetic element, meaning that it has no unpaired electrons. But its electronic configuration for valence shell is just like oxygen because they are in the same group i.e ns2, np4. My ...
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1answer
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Removing wine stains from a decanter

I have a decanter which I use fairly often to pour out wines. I typically rinse it with hot water after the last wine has come out, but I cannot prevent it from forming 'residual' wine stains over ...
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2answers
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When is it okay to use $Q=mc\Delta T$, is this equation only for calorimetry questions?

When is it okay to use $Q=mc\Delta T$, is the equation only for calorimetry questions? When a question talks about heat how do I know if i'm measuring $\Delta H$ or Q?
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Did Mendeleev predict the existence of neutrinos?

According to this Wikipedia page, Mendeleev originally believed that the inert gases belonged in Group 0 (to the left of the alkali metals). Thus, helium would be placed in the second period to the ...
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1answer
596 views

Electrochemical Cell

Now in an electrochemical cell , we make use of the fact that whenever there is a potential drop , electrons will flow from lower to higher potential as for a potential drop , there must be an ...
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2answers
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What are the factors behind making ice faster?

Recently I saw a video in YouTube about how to make ice in seconds. In this video, salt is added to a glass of water and then a straw is closed from one end and the other side is lit using a lighter ...
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1answer
542 views

Salt Hydrolysis

Referring to this, when "hydrolysis of a salt" is its reaction with water: How is it that both the cation and anion of the salt are reacting in an equal amount with water? To state this specifically ...
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0answers
354 views

Polar group strong dipole effect molecular axis

There are some fundamental charactaristics of liquid crystal. One of them is that all liquid crystals are organic compounds and at one side of their long molecular foundation there exists a polar ...
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1answer
110 views

What determines a substance's energy of activation with air?

Ice will melt when heat is applied; paper will catch fire. In trying to figure out why — what the difference is between things that melt and things that catch — I found "Burn, Char, Melt" by Roberto ...
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604 views

Balancing a chemical equation, but with an incorrect mole ratio

I'm wondering if it's possible to balance a chemical equation with an incorrect mole ratio. I have the unbalanced equation $\ce{AgNO3 + Cu -> Ag + Cu(NO3)2}$, I have a $2:3$ ratio of $\ce{Cu}$ and ...
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1answer
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Application of the Arrhenius equation to a parallel reaction

I'm struggling with this homework question: "For a parallel reaction A goes to B with rate constant $k_1$ and A goes to C with rate constant $k_2$, you determine that the activation energies are 71.9 ...
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Why does the melting point get lower going down the Alkali Metal Group with increase in atomic number?

Why does the melting point get lower going down the Alkali Metal Group with increase in atomic number?
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which chemical element has the lowest boiling point? [closed]

which chemical element has the lowest boiling point? Is it Helium(He) or is it Hydrogen(H)? Hydrogen is lighter than Helium.
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Standard electrode potential of a galvanic half cell is zero at equilibrium at standard conditions?

At standard conditions(at 1atm pressure and unit activity(1molal or 1molar concentration)of all dissolved compounds),the electrode potential is equal to the standard electrode potential by the Nernst ...
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2answers
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Why do frying pans stick less with heat?

I know this is simple, compared to most questions asked here, but I notice this several times a week and I'm curious. I notice if I break an egg and drop it into the frying pan, and the pan is still ...
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1answer
456 views

What does under-stoichiometric or stoichiometric mean?

When people talk about oxides like $\ce{SiO2}$ or $\ce{Al2O3}$, they use expressions like stoichiometric or under-stoichiometric. I understand that this refers to the relative composition of materials,...
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296 views

How does caesium’s density change with temperature?

According to Wikipedia caesium’s density is $1.90\ \mathrm{g/cm^3}$ at $\Theta =20\ \mathrm{^\circ C}$. How does this change when $T$ changes? E.g. will it expand when melting?
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How to find the molecular weight of a given gas based on its density?

The density of a gas $\ce{X}$ is $10$ times that of hydrogen. In that case, what is the molecular weight of gas $\ce{X}$? Well what I've done up to now is this: The molecular mass of hydrogen is $M(\...
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1answer
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In acidic solution, what does ClO₃⁻ reduce into?

In acidic solution, what does $\ce{ClO3-}$ reduce into and, most importantly, why? It's $$\ce{6e- +ClO_{3}^- +6H^+ -> Cl^- + 3 H_2O}.$$ Now, with the gift of foresight, why was the equation for ...
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1answer
571 views

Solid hydrophilic transparant materials that don't dissolve in water

I am looking for hydrophilic transparant materials which are solid at room temperature and that don't dissolve in water. I need these for an experiment in which I condense droplets on a plate made out ...
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2answers
517 views

Measuring Thermodynamic Stability

We can predict whether a reaction can occur spontaneously by calculating the change in Gibbs' free energy. I was just wondering whether there is any general measure of the thermodynamic stability of ...
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Strength of intramolecular vs intermolecular hydrogen bonds

Why are intramolecular hydrogen bonds weaker than intermolecular hydrogen bonds?
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Carbon atoms in triple bonds [closed]

I was trying to know what is the probability for the carbon atoms to rotate .I know that in single bond it is 0 % and I think it is 100 % in double bond but how about the triple bond ?
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1answer
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How do I calculate the number of moles in the mixture?

5.5 g of a mixture of $\ce{FeSO4.7H2O}$ and $\ce{Fe2(SO4)3.9H2O}$ requires $5.4 \:\mathrm{mL}$ of $0.1 \:\mathrm{N}$ $\ce{KMnO4}$ solution for complete oxidation. Calculate the number of moles of ...
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1answer
135 views

$K_{eq} $ vs $K_c$

I was wondering if there was a difference between $K_{eq}$ and $K_c$. I think they both refer to the Equilibrium Constant. If I'm wrong, could you please tell me the difference between the two? Thanks!...
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1answer
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Boiling and Melting point of Lithium (or any other element that does not occure in free state)

The melting and boiling point of Lithium as compared to other members of the Alkali group metals is high. But, Lithium does not occur in the free state. So, what does it mean that it has high ...
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Why does oxygen not like to be a double anion?

The electron affinity of a neutral oxygen atom is −142 kJ (it releases this energy). The electron affinity for the now double anion $\ce{O^2-}$ is 710 kJ (work must be done on the atom). My question ...
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Why can't we use fusion to make ourselves rich?

As we know on fusing two hydrogen atoms we can convert it to helium and by breaking them we get two $\ce{H}$ atoms. So why can't we convert mercury to gold by removing 1 proton per atom?
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What reactions will their reaction rates increase with time?

My textbook states that for most reactions, the reaction rate decreases with time. I wonder what reactions will have increasing reaction rates with time. I know that adding a catalyst will increase ...
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1answer
335 views

Unsure how to find the activity, once I found the activity coefficient

Ok, this is a simple problem, but I can't see how to get the answer. I calculated the activity coefficient, $\gamma_\pm$ = 0.389, and I need to find the activity $a_\pm$ for a 0.0120m solution of $\ce{...
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2answers
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Bonding and anti-bonding orbitals in the light of time-dependent Schrödinger equation?

In organic chemistry, people draw 2p orbitals like this: and then they explain how the orbitals combine to non-bonding (π*) or bonding (π) molecular orbitals, like this: depending on whether the ...
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1answer
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The colour of fireworks

When the electrons that are excited by the energy of burning drop back into lower energy levels they emit light at the wavelength of colour. When metals are burnt, metal oxides(some coloured) are ...
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1answer
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How can a group be both a good nucleophile and a good leaving group?

I'm reading about $\:\mathrm{S_N1}$ and $\:\mathrm{S_N2}$ reaction mechanisms, and I have a few questions. My book has a couple of tables, one lists a bunch of substances grouped as good, moderate ...
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Why are these molecular orbitals invalid for hexatriene?

Here are the MOs (more accurately, the breakdown of the MOs) for conjugated hexatriene: Now, when first asked to draw the MOs myself, I drew this one for $\psi_3$: It still has two nodes, and is ...
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2answers
507 views

Effect of light on ionic compounds

If I had an ionic molecule which needed $\pu{4 eV}$ to break the ionic bond, a $\pu{7 eV}$ photon is shot at it. If it is absorbed by the molecule and breaks, where does the rest of the energy go?
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Is it possible to form bubbles of ionized hydrogen and ionized oxygen under the right pressure/temperature conditions during electrolysis of water?

When we run a current through water (pure or not), $\ce{O_2}$ forms at the anode while $\ce{H_2}$ forms at the cathode. If I understand correctly, as water is already slightly ionized, $\ce{H}$'s snap ...