CHEMISTRY COMMUNITY

Andre Fabian 1F

why do we have to check whether K is less than 10^-4 in order to approximate? what does that mean? and if that condition is not met, what happens? If K is less than 10^-4, we can omit it from the K equation. This is because if you would subtract such a small value from a much larger value, the diff...

Andre Fabian 1F

Hi everyone! I had trouble with the problem that had multiple given equations to solve for K. I was able to solve the problem, but it took me a lot of tries and a lot of time. Can someone explain how they worked through the problem with the steps and what worked best for you in terms of how you sol...

Andre Fabian 1F

HClO is a weak acid ( Ka=4.0×10−8 ) and so the salt NaClO acts as a weak base. What is the pH of a solution that is 0.040 M in NaClO at 25 °C? First, you have to get the Kb value of ClO- (Na is a spectator ion) by doing 10^-14/Ka Then, create an ICE table using the provided moles of NaClO and setti...

Andre Fabian 1F

At equilibrium, the concentrations in this system were found to be [N2]=[O2]=0.200 M and [NO]=0.600 M. N2(g)+O2(g)↽−−⇀2NO(g) If more NO is added, bringing its concentration to 0.900 M, what will the final concentration of NO be after equilibrium is re‑established? I keep getting 0.75 my K is 9 and ...

Andre Fabian 1F

From my notes it didn't seem like he was necessarily adding the two values, but instead demonstrating how extrapolation of the Hydronium ion concentration could lead to miscalculations. In the end he told wrote that if [H3O+] < 10^-7, then the solution would be considered neutral because we know tha...

Andre Fabian 1F

Yeah! So whenever there's a coefficient in an equation, it would correspond to a power in the K equation!

Andre Fabian 1F

It's exactly as the person above said! If you're given partial pressures, you would substitute them into an ICE box, plugging in what you know for the Initial, using the given chemical reaction coefficients to write out the change, then using those values to write the equilibrium partial pressures. ...

Andre Fabian 1F

Hi guys can someone please help guide me through this problem because I cannot seem to get the correct answer, thanks! At a certain temperature, the given reaction has an equilibrium constant of Kp=425. PCl3(g)+Cl2(g)↽−−⇀PCl5(g) PCl5 is placed in a sealed container at an initial pressure of 0.0290 ...

Andre Fabian 1F

For the Sapling question where they had us determine the equilibrium constant given two or more of the given equations, why would the equilibrium constant of the hydrogen iodide reaction be cubed before finding the product of the two equilibrium constants? It would be cubed because the reaction for...

Andre Fabian 1F

Hello, I've been having trouble trying to work this problem. I don't know if I'm doing it right but I multiplied the given equilibrium moles of O2 by 2 to get 0.240 mol for SO2. I then added the equilibrium moles of SO2 and O2 to get 0.360, which I then subtracted from the 0.820 initial moles of SO...

Andre Fabian 1F

At equilibrium, the concentrations in this system were found to be [N2]=[O2]=0.100 M and [NO]=0.600 M. N2(g)+O2(g)↽−−⇀2NO(g) If more NO is added, bringing its concentration to 0.900 M, what will the final concentration of NO be after equilibrium is re‑established? Hi. I know there's already a post ...

Andre Fabian 1F

What is the relationship between pH and pKa, and how does this factor into negative charges/neutral compounds? pH and pKa are similar in as both values decrease, the acid is stronger. I'm not sure what you mean by your second question, but these are the formulas to calculate each: pKa = -log(Ka) pH...

Andre Fabian 1F

Hi everyone, How can you tell that a substance is amphoteric? For instance, referring to the Sapling Week 10 HW Question 2: How are you able to tell that H2PO4- can act as a Bronsted acid and a Bronsted base? An amphoteric substance is one able to act as both an acid and a base. In this case, a sub...

Andre Fabian 1F

So electron diffraction is a phenomenon observed when a beam of electrons passes through a very small opening, and as a result of constructive/destructive interference (a property of waves only), a pattern of electron density is generated that shows high/low presence in unexpected places. ie, becau...

Andre Fabian 1F

I realize from the photoelectric effect that the electrons are emitted when each photons have enough energy to meet the threshhold. And I thought professor said that if each photon energy does not meet the quota, then they just go through the atom. Then, how do electrons get excited? Excited electr...

Andre Fabian 1F

Adding on to Hannah, elements in period 3 and beyond all have the ability to hold an expanded octet given their d orbitals electrons.

Hope this helps!

Andre Fabian 1F

To add on to what Alice said, HClO3 can also be considered a strong acid, but not in every situation. For the strong bases, it's pretty much a "b" shape on the left side of the period table (LiOH, NaOH, KOH, RbOH, CsOH, CaOH, SrOH, BaOH) Why does Mg(OH)2 not fall into the strong base cate...

Andre Fabian 1F

Resonance structures are ones with delocalized electrons, meaning electrons flow freely throughout the system. Because of this, the actual bond length is equal throughout the system, calling for a more stable overall structure.

Hope this helps!
Andre

Andre Fabian 1F

In Dr. Lavelle's lectures he stated that H+ and H3O+ for the purposes in this class can be used interchangeably, but he mentioned that in later chemistry classes, in chemical equations we should write it out as HA + H20 = A +H30. In addition, pH can be calculated as pH = -log10[H+] Hope this helps! ...

Andre Fabian 1F

Strong acids are characterized by their ability to dissociate completely into H+ ions. HF has an extremely short bond because of its small atomic radius (thus the nucleus of Fluorine has a greater pulling power), as well as it's high electronegativity. Because of HF's properties, HF is unable to com...

Andre Fabian 1F

Because of a higher electronegativity, Chlorine can pull electrons away from other atoms, delocalizing them. And as such, it's a more stable acid because that molecule has resonance structures.

Hope this helps!
Andre

Andre Fabian 1F

Hi there! Another way of thinking of it is how ionic bonds aren't really 'bonds.' The ions are attached together in a lattice connected by their charges, with electrons flowing throughout them. As such, it can be said that bonds with a higher difference in electronegativity can be classified as weak...

Andre Fabian 1F

Hi there! We can determine relative melting and boiling points by looking at bond strength within a molecule. For example, a molecule with a single bond is much weaker than the same molecule harboring a double bond. As such, (given that as the bond order increases, the length of the bond decreases, ...

Andre Fabian 1F

Hi there! To determine the relative ionic character of an atom, we could do one of two things: generalize based on their positions on the periodic table, or subtract the electronegativities of the constituent atoms in the compounds. Using the former justification, we can identify that Bromine is hig...

Andre Fabian 1F

I agree with what everyone has stated beforehand. Adding on, electronegativity tends to increasing going up a group or across (left to right) in a period. As such, we could infer that the more "negative" pole of a molecule tends to center around atoms found in the upper right corner of the...

Andre Fabian 1F

Hi there! Octet exceptions occur for elements that contain d-block electrons (i.e. in and past the third period on the periodic table). My interpretation of "accommodating" would mean when would we use this rule? Normally, we would use the expanded octet in order to ensure the stability of...

Andre Fabian 1F

Nevermind, I just missed a question at the bottom, my bad.

Andre Fabian 1F

Dr. Lavelle mentioned in one of his lectures that that 4s 3d rule only applies on element 20 and before; after that it's 3d 4s as per usual.

Hope this helps!

Andre Fabian 1F

Polar molecules contain polar bonds which contribute to the formation of dipoles. A good rule of thumb to keep in mind is that if a molecule has a symmetric structure (with the same atoms surrounding the core atom), then chances are it does not have a dipole moment, while polar molecules do have dip...

Andre Fabian 1F

In determining melting and boiling point, we would look at intermolecular forces, given that these are formed between molecules. In order to change the state of a species, we must first excite or slow down the individual molecules in order for them to separate (i.e. increasing the temperature to exc...

Andre Fabian 1F

The lowest energy structure would be the one where the most atoms have a FC of 0 (or the lowest FC). For example, while comparing two resonance structures of the same species, if one has atoms with formal charges of +1, -2, +1 while the other has the formal charges of +1, -1, 0, the latter would hav...

Andre Fabian 1F

Hydrogen, like in everything else, is an exception to most rules.

A hydrogen atom has an electronegativity of 2.2, carbon 2.55, and fluorine 3.98.

2.55(C)-2.2(H)= 0.35
3.98(F)-2.55(C)=1.43

As 1.43 > 0.35, CF4 has a higher ionic character.

Hope this helps!

Andre Fabian 1F

https://imgur.com/I3yLLry
I'm really sorry for spamming this post but here's the imgur link^

Andre Fabian 1F

I'm hoping this time it's going to load

Andre Fabian 1F

https://imgur.com/a/OfSIUrC (since the photo isn't loading for me in the first post)

Andre Fabian 1F

Hi there! I'm stuck on question 17 of the Sapling weeks 7 & 8 homework.

I have to draw 3 resonance structures for C3H4. Can someone tell me what I'm doing wrong? I don't understand the feedback Sapling's giving me.

Thanks!

Andre Fabian 1F

The equation depends on the variables given in the question, as well as what you're trying to solve for. You can combine/relate these two equations with each other, in order to solve for particular values.

Hope this helps!
Andre

Andre Fabian 1F

Following what the above two people said, you should be able to work out the appropriate answer (using E = hc/lambda). However, I found something really weird with the problem where the first slot only accepted the really specific answer 1.03, and that when I rounded any number before that in my cal...

Andre Fabian 1F

In this context, a "Hamiltonian" represents a double derivative. Thus, the Schrodinger equation represents that the double derivative of the wave function (orbital) representing e- is equal to the Energy of the wave function (orbital) representing e-.

Hope this helps!
Andre

Andre Fabian 1F

My TA said that if there's any calculations, the required information to do such calculations would be provided, but as long as you know the general trends, you should be fine. I don't think Dr. Lavelle would try to pull anything tricky like that; in my experience any conceptual questions related to...

Andre Fabian 1F

Hi there! I'm not sure if this was the answer you're looking for, but when electrons absorb energy from outside sources (i.e. an incoming photon), they can jump to higher energy levels, such as from n=1 --> n=2. This topic was covered in the atomic spectra unit, so I wouldn't worry about it too much...

Andre Fabian 1F

Chi squared represents the probability of finding e-, and nodal planes are the possible locations for an e-.

Hope this helps!
Andre

Andre Fabian 1F

I'm not sure if this answers your question, but I remember hearing from the lecture that Group 13 elements need 5 valence e- to complete their octet. Since it's more difficult for them to gain that many valence electrons, they participate in coordinate covalent bonds, where the Group 13 element form...

Andre Fabian 1F

1/s = Hz

1 J = kg.m^2/s^2

Hope this helps!
Andre

Andre Fabian 1F

I believe so; atoms with similar numbers of valence electrons are organized together in groups (alkali metals, alkaline earth metals, halogens, noble gases, etc.); each group has similar properties (alkali metals being highly reactive with water, noble gases being extremely stable and unwilling to t...

Andre Fabian 1F

While acknowledging certain trends in the periodic table, such as going across a period and up a group are indicative of higher electronegativity levels, can be helpful, Lavelle mentioned that memorization of such values are not necessary. I believe we will be provided with the appropriate resources...

Andre Fabian 1F

To find the mass of a single molecule, you would divide its molar mass by Avagadro's constant (6.022 x 10^23). In addition, the masses found underneath each element in the periodic table are in amu, but we use them to calculate the molar mass of a certain molecule, in which we divide the grams used ...

Andre Fabian 1F

As the student above me mentioned, the wavelength and frequency of a certain photon are specific values that can be derived from each other. As each photon would have the same wavelength and frequency, their values would not be changed. However, as increasing the number of photons increases the inte...

Andre Fabian 1F

Atomic radii increases going down a group (additional shells are further from the nucleus), and decreases across a period (e- are in the same shell and the increasing nuclear charge pulls them in). Ionic radii demonstrate the same trends (increases in size down a group and decreases in size across a...

Andre Fabian 1F

Yes, the Lyman series always ends in n=1 (ground state), while the Balmer series always ends in n=2.

Hope this helps!
Andre Fabian

Andre Fabian 1F

Yes! This is true. Wavelengths and frequencies are inversely proportional to one another, as indicated by the following equation: c = (lambda) v. Since c, the speed of light, is a constant at 3.00 x 10^8 m/s, then it can be derived that if the wavelength of a photon increases, the frequency of the s...

Andre Fabian 1F

Hi I'm still confused over the difference between electron affinity and electronegativity. Do they effectively go hand in hand? Electron affinity is the amount of energy released after an atom or molecule gains an electron, and electronegativity is the ability of an atom to attract an electron. Hop...

Andre Fabian 1F

Photons have a constant speed, and their wavelength and frequency are related to each other through the equation c = λv, where c = speed of light (constant 3.00 x 10^8 m/s), λ = wavelength (in meters/nanometers), and v = the frequency (in Hz). E=pc is used when finding the energy of a particle in re...

Andre Fabian 1F

He briefly went over black bodies in his slides, but he mentioned that these topics would be covered in a more major-specific class. So thankfully, they're not going to be on the midterm.

Hope this helps!
Andre

Andre Fabian 1F

I agree with what everyone above said--the equation you listed should be correct. Make sure you have the parentheses at the right locations if you're using your calculator to solve, otherwise I think you're set!

Hope this helps,
Andre Fabian

Andre Fabian 1F

Both are correct, but you would use either depending on the context. A good way to check which one to use is to make sure that the units cancel out in whichever equation you're using, or that you don't have any units leftover that wouldn't apply to the answer.

Hope this helps!
Andre

Andre Fabian 1F

^bump^

Andre Fabian 1F

Hi, I also got around the same number of 1.60 *10^-19 J. You just need to convert that back into eV for eV/atom. Then to find the kJ/mol, you would convert the joules found previously (1.60*10^19) to kilojoules then multiply it by Avogadros's number to get the kJ/mol. Hope this helps! I'm still con...

Andre Fabian 1F

Hi there! To calculate the mass, you would divide the molar mass of N2 (given that nitrogen is only found in pairs in nature) by Avogadro's constant (6.022 x 10^23). This would give you the mass per molecule of N2. Before you substitute this value in the equation, don't forget to convert the g into ...

Andre Fabian 1F

Hi there! A black body is a substance that absorbs and emits all wavelengths. As such, because no light is reflected, black bodies are invisible to the eye. There are substances that exist that are very close property-wise to black bodies, yet there aren't any discoveries of actual substances meetin...

Andre Fabian 1F

Hi there! Protons and neutrons are much similar in mass, and they're present in the nucleus. Protons have a positive charge, and neutrons have a neutral charge (they act as the "glue" between protons in order to keep the electromagnetic positive charges from repelling each other). Electron...

Andre Fabian 1F

Hi there! You'd use the deBroglie relation (λ = h/mv) in order to solve this problem. "h" is Placke's constant, which is 6.626 x 10^-34 J.s^-1. "m" is the mass of an individual nitrogen dioxide molecule (because nitrogen is only found in nature as a pair), which you can calculate...

Andre Fabian 1F

Atomic Spectra can be described as spectroscopic analysis of light given off by excited atoms shows only photons of particular energy (v) are given off. These differing wavelengths of light are then analyzed by a detector, which identifies the substance through a series of colored lines. These lines...

Andre Fabian 1F

Nevermind again, I'm still stuck.

I managed to figure out an error with my calculations, but now my answer is -1.60 x 10^-19. Am I on the right track? I've been stuck on this problem for forever :(

Andre Fabian 1F

Nevermind, I figured it out.

Andre Fabian 1F

Hi there! I'm currently stuck on this problem: The electron affinity of thulium has been measured by a technique known as laser photodetachment electron spectroscopy. In this technique, a gaseous beam of the anions of an element is bombarded with photons from a laser. Electrons from the anion are th...

Andre Fabian 1F

In most of his examples calculating mass percentage, it's easiest to assume the sample contains 100g. That way, you can easily convert the percentages of the product produced into grams, then divide each constituent by its molar mass to get the number of moles of each element, and then divide each o...

Andre Fabian 1F

First calculate the total molar mass of the Epsom salts (246.48 g/mol). Then divide the 5.15g of Epsom salts by the molar mass to calculate the moles of Epsom salts. Then use the molar ratio (11 mol O/ 1 mol Epsom salt). Then multiple by Avagadro's constant to get the number of atoms of O. The units...

Andre Fabian 1F

Hi there! A mole is a unit representative of 6.022x10^23 atoms, which is the calculated amount of atoms in 12 g of a carbon-12 isotope. Moles are used in chemical formulas, molar ratios, and other chemical processes to help break down and quantify otherwise minuscule values. A molecule is a group of...

Andre Fabian 1F

According to this post https://chemistry.stackexchange.com/questions/1239/order-of-elements-in-a-formula , the Hill System is used when determining the order of chemical formulas. In short, carbon is always placed first, followed by hydrogen, and after those two the rest of the elements comprising t...

Andre Fabian 1F

Image doesn't seem to be working, here's the imgur link: https://imgur.com/a/xmBVJjX

Andre Fabian 1F

Hi there! I understand conceptually how to do this problem, but after going over my answer using the answer sheet I was kinda confused. A student prepared a solution of sodium carbonate by adding 2.111g of the solid to a 250.0-mL volumetric flask and adding water to the mark. Some of this solution w...

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