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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31
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2answers
26k views

What exactly is hydrogen bonding (and does it really need fluorine, oxygen, or nitrogen)?

I'm not satisfied with the rationale for the intermolecular attraction known as hydrogen bonding. In my book, it states that Hydrogen bonding is a special type of intermolecular attraction between ...
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1answer
13k views

How do equilibrium shifts affect solids?

Say we have the following reversible reaction: $$\ce{NaOH{(s) }<=> Na^+{(aq) }$+ $OH^{-}{(aq)}} +10.6 \mathrm{kcal}$$ If we add $\ce{OH^-}$ and equilibrium shifts to the left, does that affect ...
4
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1answer
981 views

Effect of vibration on wine quality

Most people that have ever visited a viticulteur (winemaker) will know that it is common wisdom that vibrations are bad for the wine quality. When I was just a kid the winemaker always told me that we ...
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3answers
319k views

Why is it important to use a salt bridge in a voltaic cell? Can a wire be used?

I was learning about voltaic cells and came across salt bridges. If the purpose of the salt bridge is only to move electrons from an electrolyte solution to the other, then why can I not use a wire? ...
9
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2answers
7k views

How to determine the empirical formula for an unknown iron oxide?

I have to determine the empirical formula of an oxide of iron which has $69.9\%$ iron and $30.1\%$ dioxygen by mass in it. I have started as follows: For our convenience we take $\pu{100 g}$ of that ...
5
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1answer
12k views

What is the effect of adding salt to ice?

What will happen to the melting point temperature of ice if some common salt is added to it? How to justfy the answer?
22
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1answer
440 views

Molecular Imaging - Any surprises to be had?

Molecular imaging, using STM and AFM technologies, appear so far to visually prove most of what we already know about chemical structures, such as VSEPR theory. For instance, here are the ideal ...
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2answers
1k views

Chemical Equilibrium - Why does changing the temperature shift the equilibrium?

I know that raising the temperature in a reversible chemical reaction causes the equilibrium to shift to the endothermic side. I know that $\Delta G = \Delta H − T\Delta S $ but I don't know how to ...
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3answers
3k views

How can I measure the pH of high-viscosity fluids like dough?

I would like to measure the pH value of dough with a pH-Meter, to ensure a pH value of 4.1 (max). Because of the high viscosity I would thin down the dough with demineralized water for two reasons: I ...
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0answers
34 views

Chemical Equilibrium - Why does changing the temperature shift the equilibrium? [duplicate]

I know that raising the temperature in a reversible chemical reaction causes the equilibrium to shift to the endothermic side. I know that $\Delta G = \Delta H - T \Delta S$ but I don't know how to ...
5
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3answers
3k views

Why do you need a strong base for a titration

For a titration, if you want to know the concentration of an acid you use a strong base such as $\ce{NaOH}$. My question is; why you can't use a weak base for this purpose? Is it because the ...
7
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1answer
161 views

Interpretation of Figure - biofunctionalization

In this paper on page 1985 they say: Fluorescence analysis confirmed that enhanced protein binding at higher electron dose depends only to a small extent on the slight increase in the lateral ...
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2answers
6k views

How does the volume of a mole of ideal gas relate to pressure and temperature?

The volume of a mole of gas is: $\ce{22.4 l}$ directly proportional to pressure and Kelvin temperature. directly proportional to pressure, inversely proportional to Kelvin temperature. inversely ...
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2answers
6k views

How many valence electrons does the azide ion have?

We had a test and and we had a question asking the number of valence electrons in an azide ion, $\ce{N3^{-}}$. I find out that the answer is 16 but I could not understand how. I saw the structure. ...
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3answers
9k views

Can you heat water with additives?

I have been curious about this question for a while. If you want to warm up a large amount of water, is it feasible to do this by adding a substance that has an exothermic reaction with the water? ...
2
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2answers
197 views

Helium empirical wavefunctions

I know that analytic forms of the Helium wavefunctions are not known. However, are there empirical expressions for the low-lying states of Helium? I'd like to use them to calculate some transition ...
5
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2answers
3k views

Finding Pv in Pv=nRT for molecular hydrogen

I'm attempting to write a small program that will calculate the values for the Ideal Gas Law, specifically for molecular Hydrogen (H2) in space. Eventually, this will grow to be a sort of "simulator",...
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4answers
5k views

Chemical properties of isotopes

I've always heard that different isotopes of the same element have exactly the same chemical properties. But for example, I've read that some compounds which have hydrogen atoms interchanged by ...
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3answers
2k views

On the scale of stuff with a pH

I am a confused (and somewhat anal retentive) non-chemist who is trying to understand what pH is. I have come to understand it is a measure of acidity, but I have not yet figured out what that means ...
9
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4answers
84k views

Why calcium chloride is used to melt ice over sodium chloride

I'm not quite sure I understand this. My question is: Calcium chloride is a salt used widely to melt ice on sidewalks and roads. Explain why one mole of $\ce{CaCl2}$ would be more effective than ...
6
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2answers
238 views

Why does the Förster resonance energy transfer scale as 1/r⁶?

The title more or less contains my question - why does the Förster resonance energy transfer (FRET) (photon?)-based energy transfer mechanism, which folks sometimes use as a spectroscopic ruler to ...
0
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1answer
4k views

Find the volume of the carbon dioxide product

Below you can see the chemical reaction I'm working with: $$\ce{CH_3CH_2OH(l) + 3O_2(g) -> 2CO_2 (g) + 3H_2O(g)}$$ I've got so far: How can I calculate the volume of the product of carbon ...
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2answers
7k views

Butane Burning Color

Why does Butane burn blue? From my (very) basic understanding of chemistry, carbon and hydrogen do not burn blue, their radiation spectra are different-so why should the combination burn blue? Please ...
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2answers
40k views

What does it mean for a reaction to favor the reactants/products?

(This example is purely hypothetical.) You have the reaction $$\ce{H2(g) + O2(g) <=> H2O2(g)}$$ at $T = 500\ \mathrm{K}$. The reaction reaches equilibrium at the following concentrations: $$\...
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3answers
7k views

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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1answer
3k views

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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2answers
177 views

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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2answers
619 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 ...
5
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1answer
20k views

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
262 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}, ...
5
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1answer
245 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 $\ce{CO2}$ ...
0
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1answer
350 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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2answers
2k views

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
90 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 ...
3
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2answers
335 views

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?
4
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1answer
507 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 ...
2
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1answer
146 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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1answer
1k views

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: \...
3
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1answer
5k views

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 ...
5
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1answer
2k views

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 ...
2
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2answers
13k views

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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2answers
493 views

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
636 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
9k views

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 ...
2
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1answer
556 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
118 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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2answers
630 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 ...
3
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1answer
3k views

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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5answers
89k views

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?