CHEMISTRY COMMUNITY

Melanie Lin 3E

If I can move into dorms, I will definitely take that opportunity! I just feel like it's an experience worth having and since I missed it for freshman year, it wouldn't hurt to try it for sophomore year. Regardless, I hope I can meet you all in-person soon ;-;

Melanie Lin 3E

n is the number of electrons transferred in the reaction. You will find this by writing out the half reactions.

Melanie Lin 3E

Hi! I actually just used the equation sheet to compare the relationship between [A]initial and their half life. Since the half life decreases when [A]initial increases, you can tell it's 2nd order because the equation is t1/2=1/(k[A]init). Hope this helps!

Melanie Lin 3E

Like everyone who has already replied, I suggest using Lewis structures to make sure what type of molecule it is.

Melanie Lin 3E

Hi! You basically use the equilibrium equation from the first step! So K=[HClO][OH-]/[ClO-] so you isolate [HClO] to get K[ClO-]/[OH-]. This would be used to replace [HClO] in the rate law. I'm pretty sure water isn't included in the answer (if I remember correctly). Hope this helps!

Melanie Lin 3E

Hi! I honestly suck at explaining but I'll do my best. I usually just remember its always (-1/a)*(d[A]/dt)=rate or any of the reactants (make sure to have that negative sign because they technically lose reactants as the forward reaction goes). As for products, it'll be (1/c)*(d[C]/dt)=rate (it'll b...

Melanie Lin 3E

Hi! I think K+ and Cl- aren't used in this case because they are more like spectator ions (so their oxidation states don't really change throughout the reaction nor do they really affect what's going on at all). As for why Fe2+ becomes Fe3+ I'm not totally sure but that's how it's written in the sta...

Melanie Lin 3E

Hi! You can pretty much find everything you need to know on Lavelle's Chem 14B site. There's a linked Exam Schedule doc and it will detail the time for the final, which should be Sunday March 14 9:30am-11:00am.

Melanie Lin 3E

I have definitely been feeling burnt out these days. My biggest factor for relieving stress has actually just been regular exercise. Just like Lavelle always says: Exercise your mind AND body. As for just resting I also just watch some Youtube/Twitch.

Melanie Lin 3E

I am probably going to shoot that question to my TA but if anyone has already asked/knows please let us know!! (usually i just do all/sample the questions from each section for review anyways)

Melanie Lin 3E

Hi! I was actually stuck on a question for a while because I didn't add the states so just make sure to check the problem. I think some specifically say adding the states are optional.

Melanie Lin 3E

n is the number of electrons transferred, which you can find once you create the half reactions. Hope this helps!

Melanie Lin 3E

I don't think I've ever had to memorize oxidation numbers aside from Oxygen (-2) and Hydrogen (+1) which I think should be pretty easy to remember once you've done enough redox problems.

Melanie Lin 3E

I also have a LS7B midterm tomorrow :(( I haven't started WandaVision but I might. Is it really good though? I've heard some gooood things about it. OMG is Wandavision AMAZING! I love all the marvel movies and am a huge fanatic, but even my mom who hasn't seen any superhero movies ever is enjoying ...

Melanie Lin 3E

Hi! Since lnQ is doubled (because reactants and products are all squared) and n is doubled (because of the coefficients), E stays constant because both cancel each other out.

Melanie Lin 3E

I will be continuing with the 14 series (to 14C), but BL and CL I might take over the summer.

Melanie Lin 3E

Hi! When Q<K, reactants are greater relative to it's proportion at equilibrium, so the reaction will go towards products. It will be the reverse for Q>K.

Melanie Lin 3E

I also have a LS7B midterm tomorrow :(( I haven't started WandaVision but I might. Is it really good though? I've heard some gooood things about it.

Melanie Lin 3E

Thank you so much for telling us about your experience! It was super helpful and inspirational! I'm so happy for your success and I hope everyone here can find success (in this field or any other path)!

Melanie Lin 3E

Hi! You should always take the context of the problem that you have. If you have the equation down and everything, the constant will be there to cancel all the units out. This is a little vague so let me know if you need more explanation/examples!

Melanie Lin 3E

All of those values can be found on the equations sheet, so just become familiar with how/when to use them and you'll be fine! (you might even end up remembering some if you practice them a lot)

Melanie Lin 3E

It'll depend on the given choices or units for the answer. With sapling, the units are always listed after the blank so just pay attention to that and convert units accordingly.

Melanie Lin 3E

I'd say your best bet is to always convert to Kelvin. I don't think you get the correct answer with Celsius.

Melanie Lin 3E

deltaSvap is deltaHvap/(boiling point temp). I hope this helps!

Melanie Lin 3E

Hi! deltaG is just Gibbs free energy while deltaG knot is standard Gibbs free energy (so at standard conditions). The equation to convert between the two is deltaG = deltaG knot * RTlnQ and it'll be on the equations sheet so no need to memorize it!

Melanie Lin 3E

Overall I am really confused about this question. I was confused as to how to use the values in the table and apply it to the problem. I know which formula to use but how do you apply the values in the table? You decide which value to use on the table depending on the molecule/atom in the problem. ...

Melanie Lin 3E

hi! since the volume is constant, work will equal 0. With work equal to 0, delta U (or internal energy) will be the same as the q value.

Melanie Lin 3E

Hi! I think most times, if not all, you will be using q=mC(delta T). All of the calorimeters/molecules in the problems I've done have a given mass or mole.

Melanie Lin 3E

Hi Lucy! For starters, you can calculate q using the equation q=n*(delta T)*Cp. The Cp (or molar heat capacity at constant pressure) given is 4R. You would just plug and chug with the values given. As for the second part, delta U can be calculated with the equation delta U = n*(delta T)*Cv. Cv is th...

Melanie Lin 3E

Hi Lucy! The heat capacity of a calorimeter shows how much heat must be taken in by the calorimeter to change its temperature by a unit (K or Celsius. You can use this to calculate how much energy was given off by the reaction that occurred inside it. And every calorimeter has different heat capacit...

Melanie Lin 3E

Lavelle mentioned that Ka or Kb should be less than 10^-4 for x to be negligible. I actually had a previous teacher say that K needed to be less than 10^-3 of the original concentration but it's definitely best to listen to Lavelle. Hope this helps!

Melanie Lin 3E

Hi! There will likely be phase changes that must be accounted for since he does cover it and there are homework problems that also cover it. Hope this helps!

Melanie Lin 3E

Hi! Through the balanced equation, it shows that 1 mol of S8 will mean an absorption of 358.8 kJ. Since there was 1.31 mol of S8 reacted, you can multiply that by 358.8 kJ/mol to get the total amount of heat absorbed. Hope this helps!

Melanie Lin 3E

Hi! Midterm scores usually come back around a week after you take it. However, you will likely not be able to see what you got wrong (unless you contact your TA about reviewing it).

Melanie Lin 3E

Hi! Most if not all the values you need are either provided in the problem or on the constants and equations sheet! Hope this helps!

Melanie Lin 3E

Hi! Not sure if this has been mentioned yet but in addition to UA sessions and textbook problems, I recommend redoing the Audio Visual Focus topics too! They're a nice review and can give you an idea of how Lavelle likes to write his questions.

Melanie Lin 3E

I just memorize some easy mnemonics . For strong acids, it's "So I Brought No Clean Clothes" (H2SO4, HI, HBr, HNO3, HCL, HCLO3, HCLO4". For strong bases, it's Lily and Nate RoBbed a Bank and Killed a Cessium of Strong Cats" (Li, Na, Rb, Ba, K, Cs, Sr, Ca) this was super helpful ...

Melanie Lin 3E

Percent ionization tells us how much an acid or bases dissolves/ionizes in solution. Knowing this shows us the relative strength of an acid or base and can also be used to solve for certain concentrations before or after equilibrium.

Melanie Lin 3E

Hi! I was worried about it too but Lavelle usually specifies if he wants us to memorize values. Otherwise you can pretty much expect the values needed for the problem to be provided during the exam

Melanie Lin 3E

pKa is -log(Ka). It's often used to compare the strength of an acid (the smaller the pKa the stronger the acid). Keep it in mind for the midterm! Hope this helps :)

Melanie Lin 3E

Hi! A change in pressure will only affect the equilibrium if there's a difference in number of moles of gas on either side of the chemical reaction.

Melanie Lin 3E

Hi! When Q<K, it means that the denominator (or reactants) increased and vice versa. So when there's more reactants, the equilibrium will shift towards the right to form more products. Hope this helps!

Melanie Lin 3E

Hi! I'm not sure what the chemical equation was but if an increase in pressure didn't affect the equilibrium, then there were probably an equal number of moles of gas on each side of the equation. Hope this helps!

Melanie Lin 3E

Hi! You use inverse Kc (1/Kc) when you want to find the equilibrium constant of the reverse reaction.

Melanie Lin 3E

Hi! When Q > K, the chemical reaction is not in equilibrium and there is an influx of products. As the reaction returns to equilibrium, the reverse reaction will occur more because there is more products. When Q < K, the chemical reaction is not in equilibrium and there is an influx of reactants. In...

Melanie Lin 3E

I usually convert to atm because of the given R constant (that uses atm). Since it will probably be given on the constants sheet, it'll be helpful converting to atm.

Melanie Lin 3E

Hi! The K of the reverse reaction will be the reciprocal of the K of the forward reaction (1/K).

Melanie Lin 3E

I was in 14A last quarter and I actually didn't even realize the "Quick Links" menu was that helpful LOL Thanks to Kiana for starting this thread and everyone else for mentioning this. I will be definitely using that tip.

Melanie Lin 3E

Kp is used when the reactants and products are in gaseous form (make sure they're all in the same units before you input into the formula!). Kc is used usually when the reaction occurs in solution, so you use the concentrations of aqueous reactants and products. Be sure not to include liquid or soli...

Melanie Lin 3E

Hi Rachel! Same as Sara, the coordination compound has an overall negative charge of -4 in my textbook. And to add, I'm pretty sure you can still call a negatively charged compound as an ion as well, not just positively charged. Hope this helps!

Melanie Lin 3E

Thank you for everything Dr. Lavelle! Your lectures and teaching methods have really opened my eyes up to the world of chemistry more so than AP Chemistry ever did for me! Chem 14A has been amazing with you and I'll see you in 14B!! <3

Melanie Lin 3E

CH3COO- is a polyatomic ion so it's not exactly a hydroxide ion with CH3O. So CH3OOH is actually a CH3COO- bonded with a H+ ion.

Melanie Lin 3E

Hi! NH3 is a weak base but NaCl isn't. I think the best way to sort of reason that is because when it is put into aqueous solution, NaCl dissociates into Na+ and Cl-. Both of these ions stay that way and don't contribute or take any H+ protons in the solution so pH does not change. Hope this helps!

Melanie Lin 3E

Hi David! I usually try to draw out the Lewis structure to see if there are any lone pairs on the molecules that could show which one is donating and which one is accepting. Otherwise, I try to see which is more electronegative and sort of kind of build from there. Hope this helps!

Melanie Lin 3E

I'd tell you a chemistry joke....
But all the good ones Argon :(

Melanie Lin 3E

Hi! I essentially drew the Lewis structure with double bonds between the first two carbons and another between the third carbon and the oxygen. The hydrogens were attached to their respective carbons according to how the molecule was written. As for hybridization, you would identify them at each are...

Melanie Lin 3E

Hi Monica! When I answered this, I just put slightly less than 109.5 as well. I remember both Lavelle and my TA mention that the specific angle was not required since it does differ between different atoms.

Melanie Lin 3E

Hi Jessica! When naming coordination compounds, you put the ligands in alphabetical order from their names and not the Greek prefixes so you would put ammine then chlorine. Hope this helps!

Melanie Lin 3E

Hi Ruth! I also had trouble with naming coordination compounds and I honestly don't have any shortcut methods. The way I have been learning is just LOTS of practice to get the rules in my head. Good luck!

Melanie Lin 3E

Hi! I just took a look at my textbook and the formulas for each compound were provided so I don't think you really need to learn how to get the molecular formulas for organic compounds yet. So you can probably go ahead and search them up.

Melanie Lin 3E

Hi! If you saw Lavelle's lecture 11/18 (Wednesday of Week 7), the VSEPR formula probably refers to the AXE notation. If not, A is the central atom, X represents the surrounding atoms, and E represents lone pairs. Like with sulfur tetrachloride, its Lewis structure has 4 bonded atoms and 1 lone pair ...

Melanie Lin 3E

I can sort of see what you guys mean. Can someone explain square pyramidal and square planar so I can better understand this? With square pyramidal and square planar, their base shape (without lone pairs) is octahedral so the angles all around are 90 degrees. So when you take out that first lone pa...

Melanie Lin 3E

Hi! So this basically meant finding the empirical formula and using the molar mass to find the molecular formula of the molecule. Hopefully you know how to get an empirical formula by now so you should've gotten CH4O, which also matches the given molar mass. That means that CH4O is also the molecula...

Melanie Lin 3E

Hi! Usually once lone pairs come to play on the trigonal bipyramidal shape, I think of the largest angles possible within it that I can separate the lone pairs with. So with 2 lone pairs, the largest angle that can separate them from other areas of electron density is on trigonal part of the structu...

Melanie Lin 3E

That's correct! With more hydrogen bonds, GC pairs are markedly stronger than AT pairs. So DNA with low GC ratio often times are less stable compared to DNA with high GC ratio!

Melanie Lin 3E

Hi Natalie! You generally look into intermolecular forces to see if the molecule is nonpolar or polar. Once you figure that out, you determine the type of intramolecular force that the molecules have with each other. So the BP and MP are mostly determined by the strength of the intramolecular forces.

Melanie Lin 3E

Since As has 5 valence electrons, it perfectly bonds to the 5 F atoms and leaves no lone pairs, making it a nonpolar molecule. With AsF3, a lone pair is left on the As atom, so the molecule becomes nonpolar. With nonpolar molecules, the IMFs are much stronger than with nonpolar ones so the boiling p...

Melanie Lin 3E

Adding onto what Gina said, you can use the list of IMF's that Lavelle discussed in his lectures to see which molecules have stronger interactions depending the type of interaction.

Melanie Lin 3E

Extra credit would definitely be much appreciated! I'm sure there are many of us out there struggling with classes in quarantine, with all that Zoom fatigue :(

Melanie Lin 3E

Hi Algernon! I think you'll have to find out by drawing the different possible Lewis structures and checking the formal charges. If they have similar formal charges and are only switching bonds/number of bonds, they are generally identified as resonance structures. Sadly, as of what I know so far, t...

Melanie Lin 3E

I'm confused on the hydrogen bonding question, what are the rules that apply to h bonds? Does anyone know how to remember them? Hi Alyssa! To add onto what Gina mentioned, take into account the dipoles of those atoms. For example, in CH4, the H atoms don't have any dipole moments so they can't hydr...

Melanie Lin 3E

Hi Hana! You can find which Lewis structure is more stable by looking at the formal charges of each atom. If the formal charges of each atom are closer to zero and add up to the overall charge of the actual molecule/polyatomic ion, then it is the most similar to the actual structure of the molecule/...

Melanie Lin 3E

Hi Samaya! Regarding the oxidation thing, the problem meant to show you that you rely more on formal charge than oxidation numbers to see which structures are closer to the actual structure of the molecule.

Melanie Lin 3E

The oxidation number problem was meant to show you that it doesn't really show which structure is more stable. You rely more on formal charge to get an idea of which structures are closer to its real form.

Melanie Lin 3E

What do you do with a dead chemist?
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Barium :(

Melanie Lin 3E

As the number of bonds increase between atoms, the pull between them also becomes stronger and they're drawn closer together. This results in a shorter bond length between the two atoms.

Melanie Lin 3E

Hello! Prof. Lavelle actually mentioned this in the lecture about them being the exception but since they're also going to the n=3 shell, they are able to take in more electrons.

Melanie Lin 3E

Hi Noelle! You can't really determine the exact length of a bond given a certain bond unless you memorize it from online, but you can definitely determine its relative size through the bond type/strength. For example, if there's a double bond, the bond length will be shorter than it usually would be...

Melanie Lin 3E

When multiplying two numbers in scientific notation, multiply their coefficients and add their exponents. When dividing two numbers in scientific notation, divide their coefficients and subtract their exponents. If you remember from math where we were taught how to multiply and divide with exponent...

Melanie Lin 3E

So I told Sodium and Neon a super funny joke. Guess what they said:
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Na: That was SODIUM funny!
Ne: I slapped my NEON that one.

Melanie Lin 3E

Hi Jaylin! I actually think all of them increase! Here's my explanation: (a) electron energy will increase as they jump to a higher energy level and stray further from the nucleus. (b) n also increases because the jump goes from n=1 to n=2 (c) l also increases because it goes from s orbital to p orb...

Melanie Lin 3E

Hi Ian! This rule of having an electron in each orbital before filling them is actually the Hund's Rule! It basically states the order in which the orbitals are filled with electrons. Here's a link about Hund's Rule and how it works with orbital filling diagrams if you're curious: [https://courses.l...

Melanie Lin 3E

Hi Brandon! There will be two equations that you have to utilize that can be found on Lavelle's equation sheet. You use ΔxΔp >= h/4π to find Δp with the Δx given to you (13 Å, which you should convert to meters). With the Δp solved for, you can find the uncertainty in velocity using the equation Δp ...

Melanie Lin 3E

Hi Karina!
Like the previous replies, you likely forgot to use parenthesis to separate one value when dividing or some similar typing error on the calculator. It happens to the best of everyone so just make sure you double check your entries during exams!

Melanie Lin 3E

Hi Bethany! Wavelengths in the visible light region (which are the ones usually used in problems) are usually in nanometers but you will often have to convert it to meters if you need to use it in a formula (especially those containing Planck's constant). Frequencies will generally be in Hz (or s^(-...

Melanie Lin 3E

For problem #11 on the Sapling Homework (A violet line is observed at 434.0 nm in the spectrum of atomic hydrogen. Determine the values of n for the beginning and ending energy levels of the electron during the emission of energy that leads to this spectral line), why is it that 5 is N2 and 2 is N1...

Melanie Lin 3E

idella 1F wrote:Where do you get the 9.33x10^-6 from?

Hi Idella! The problem said that the wavelength was 9.33x10^-4 cm so you have to convert it to meters before plugging it into the E=hc/wavelength. In meters, the wavelength would be 9.33x10^-6.

Melanie Lin 3E

Hi all! I got super stuck on this problem for a while too! Turns out my biggest problem was mixing up my constants so maybe try making sure you have the correct units being cancelled out and having the correct numbers? Idk but aside from what Sophia said those two were what got me wrong answers.

Melanie Lin 3E

Can someone explain why you can't approach this problem the E = (0.5)mv^2 way? I feel like there are so many equations where things are equal to E but I'm unsure if the E is for the energy of photons or the energy of electrons. I am not a hundred percent sure but I think that the E= (0.5) mv^2 calc...

Melanie Lin 3E

I'm sure you'll be fine since you'll still have the information/work you need to show there. Though I'm not entirely sure if listing it that way may affect future work (outside of class) like in research or showing others your work (as in it might throw people off(?)).

Melanie Lin 3E

Mai V 4L wrote:Does anyone have a video they thinnk is a good reference for this topic?

Hey Mai! I was actually recommended this by my high school chemistry teacher but I think anyone should appreciate our Chemistry Jesus: [https://youtu.be/Aoi4j8es4gQ]. Feel free to check it out!

Melanie Lin 3E

HI! Sophia and Vince pretty much have it all down but if you want a super simple one/two-sentence explanation: atoms go along the octet rule where they want to complete their outer shell (8 electrons) so as they get closer and closer to getting 8 electrons, the more they want/have stronger affinity ...

Melanie Lin 3E

On the test, will we need to format our answers in meters or nanometers when solving for wavelength? It'll definitely depend on the question (whether it asks for a certain unit or not). Otherwise, I think answering in meters isn't wrong. It's just that often times the wavelengths are usually done i...

Melanie Lin 3E

My TA said we can just set up our phone and join zoom a second time, but it might be different for your section. The quiz ran smoothly for us and I just propped my phone up on a mug so she could see my desk. I think Lavelle said somewhere that if the TA has a way of seeing our workspace through zoo...

Melanie Lin 3E

I would first try to find the moles of each solute first and convert to the moles of Potassium in each. For example, with K2S you'd double the number of moles of K2S since there are 2 K+ ions in a molecule of K2S. From there you can add up all the moles of potassium and divide it by 0.500 L (500. mL).

Melanie Lin 3E

This problem is a bit wordy so I definitely understand why this problem would be difficult to piece together. If you've already found out the empirical formula, you just need to divide the given molar mass of the acid by the molar mass of the empirical formula you got.

Melanie Lin 3E

Does anyone have a link for the recommended periodic table mentioned? On Lavelle's Chem 14A site, he linked pdfs of a periodic table and constants so it might be better to base your numbers off that. Otherwise, most periodic tables vary between 3 to 4 decimal places, which I don't think will make a...

Melanie Lin 3E

Hi Isabel! The first thing you'll need to do is find the moles of CuNO3 by dividing the 2.92 g by the molar mass of CuNO3. With the M=N/V equation, you can put 0.260 M for M and the moles of CuNO3 for N. With two of the variables substituted with values, you can find the volume in liters. From there...

Melanie Lin 3E

Thank you so much for the advice! Chemistry has definitely been a concern to study and prepare for the next few years (especially the retention of material for MCAT) and your experience and anecdotes have definitely eased my worries. Since this post is sort of old, I hope you're still doing well and...

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