CHEMISTRY COMMUNITY

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Chem_Mod wrote:
705573518 wrote:Is there an answer key for the first half of the worksheet? Thanks!

answers are posted under the week 9 post!

Thank you so much for the amazing workshops these past two quarters, Matthew!! You will be missed :)

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Make sure to convert to seconds, not minutes. This is done by multiplying 3.9 by 3600.

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Is there an answer key for the first half of the worksheet? Thanks!

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I keep on doing this problem exactly how it is explained above and keep getting the wrong answer. Did anyone else have this issue?

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Hannah Kim 3C wrote:This is a question for anyone, but how would you know that the rate of H2O is constant?

Because H2O is the solvent, the concentration of H2O is a constant.

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oliviacarnot wrote:the final is cumulative and it covers each topic based on how much time we spent on it
- but either way I would make sure you have a good understanding of everything:)

Yes, it is cumulative. See the most recent email from Dr. Lavelle...he breaks down which topics are more heavily focussed on. :)

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Patrick Herbig 1E wrote:Can delta U be zero? Conceptually, is this possible in all systems? Also, what would this indicate about the initial and final internal energies?

Correct me if I'm wrong but delta U will be equal to zero for isolated systems.

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After going through my midterm with my TA I realized that a key concept for this problem that I personally messed up on it that E naught is equal to 0 in this case. Then to understand what E cell is you need to use the given proportion (ex: 20/1 or 1/20...sorry I don't remember the exact problem) fo...

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Going through the derivation of the equations will be really helpful for understanding where the positive/negative is coming from. Just take your time to go step by step and you will end up seeing why the resulting equations (after taking the derivatives) end up negative or positive. Reviewing Lavel...

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I'd recommend approaching this problem with dimensional analysis. This helped me visualize which units were needing to be cancelled in order to end up with a rate in M/s.

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Steps: 1) Identify what is being oxidized and what is being reduced. 2) Split the original reaction into half reactions (one for reduction and one for oxidation reaction). 3) Balance all the elements except for Oxygen and Hydrogen. 4.1) If the solution is BASIC, First balance the Oxygens with H2O. ...

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So, EMF/ E-naught is standard conditions where as E is non standard?

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degeneracy is the W term of the boltzmann equation, right?

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Can we still use the first equation with standard conditions?

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I spent nearly an hour trying to figure out why I kept getting the balancing part of the question incorrect just to realize I put an incorrect phase and that was the incorrect part. That said make sure you are careful with the phases for this problem because its such a silly mistake that will result...

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Hi! you have the right coefficients for Cl2O7 and H2O but not the rest. Here's my general thought process for doing this problem: So first, you separate the compounds and determine which one is the oxidizing/reducing reagant by seeing how the charges change with Cl. Then, you balance the non Oxygen...

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Emma Kelly 3L wrote:There isn't one equation that can do both. Rather, you calculate ΔS = n*R*ln(V2/V1) and ΔS = Cv*ln(T2/T1) and add both answers. Hope this helps!

What units do we use for the T1 and T2? kelvin? or does it not matter?

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Just wanted to clarify...the lectures next week are not covered on the midterm, correct?

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There are 3 steps for this problem: First, we should use the equation ∆S = Cpm ln (T2/T1) using the temperature change from 39 °C to 100 °C, using 75.3 J⋅K−1⋅mol−1 for the Cpm. Second, we need the entropy of vaporization of water at 100 °C, which is 109.0 J⋅K−1⋅mol−1. Third, we use ∆S = Cpm ln (T2/...

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Would someone mind elaborating a little/giving a step by step approach to do this problem?

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Would someone mind elaborating a little/giving a step by step approach to do this problem?

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I saw this question too and was a little confused. From my understanding this is not possible but maybe there is some exception?

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Does anyone know if we are able to review the answers we missed on the midterm?

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VeronicaShepherd3B wrote:What does it mean to have crystalline form ? What does T stand for?

In this question, T just stand for temperature. The temperature is 0 K which is absolute 0.

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Chem_Mod wrote:B) C) D) are weak so you can calculate their pH using a regular ICE box.

A) is a strong acid so its pH is simply -log(0.00005)

Just to clarify, we know that c d and b are weak just because they aren'y the strong ones we are supposed to memorize??

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For (a) you would want to: 1) NH3 + H2O <-> NH4+ + OH- then do an ice table I 0.057 M 0 0 C -x +x +x E .057 x x ignore x since Kb= 1.8e-5 is small Then do K = x^2/.057 and solve for x which is equal to the concentration of OH-. then do -log[OH-] = pOH and you can find pH from 14-pOH. Hope you can s...

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Ashley Johnson 2G wrote:What is the relationship between Cv and Cp?

Cv = Cp-R

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I don't think so. Just what is on the outline for midterm 1.

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to find delta U, you have to use the eq nxC(v)xdeltaT; this is because you're given moles and temp and can solve for specific heat at constant volume aka C(v) using C(v)=C(p)-R. you use this because you're now focusing on the internal system, so the volume no longer changes. Does this relationship ...

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I got the same answer too!

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Does anyone know about how many questions are on the exam? I was thinking it is similar to 14A but i think we have 10 extra minutes so maybe there are more questions?

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Does anyone know about how many questions are on the exam? I was thinking it is similar to 14A but i think we have 10 extra minutes so maybe there are more questions?

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Does anyone know about how many questions are on the exam? I was thinking it is similar to 14A but i think we have 10 extra minutes so maybe there are more questions?

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For this type of question remember that breaking bonds requires energy and forming bonds releases energy. For endothermic reactions, there is more input of energy to break the reactant bonds and less released forming the product bonds. For exothermic reactions more energy is released forming product...

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In problem 5I.13 we have to get the k/kc value from the chart. How do we know which value to use... k or kc?

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Hi Carla, You should break that equation into two. The table 5G.2 will give you H2 + Cl2 --> <-- 2HCL K = 4.0 x 10^31 and 2BrCl --><-- Br2 and Cl2 K = 377. The Cl2 will cancel out leaving you with the original equation and then you would multiply the two K's together. I understand how to break the ...

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For number 9 on the achieve week #2, you can compare the two values of pH and pKa to determine the more predominant species. If pH is above the pKa, the deprotonated species of A- will become more predominant in the solution. If pH is lower than pKa, the neutral species of HA is more predominant. p...

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Whenever I am feeling a little negative about my pre med classes, reading through these "Advice from a Medical Student" posts on here really actually help me. It just goes to show where we can all be someday and it is really motivating for me! :)

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I could not find anywhere what sections are covered on midterm 1...has that been announced yet or will we know closer to the exam date?

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When changing temperature, does that mean we need to start considering if it is endothermic or exothermic? Yes. PS. There are tables listing equilibrium constants at different temperatures. Kw has been studied extensively at different temperatures. I was confused about how we are supposed to know K...

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Maggie Clark wrote:
705573518 wrote:Every time I try to solve the quadratic I am getting complex numbers for the answer. Is this happening to anyone else?

Are you getting negative numbers? Because those can be removed, or anything smaller than the initial concentration

Yes, thank you for responding!

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It's helpful to think of this problem as solving it backwards that usual. We can easily get the [OH-] from the pH. Then from there we can plug everything into the Kb equations and rather than unknown x, we have unknown B. Solve for B and then simply use that and [OH-] to answer the question.

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You can think of the general equation for acids: HA(aq) ↽−−⇀ H+(aq)+A−(aq). The initial concentration of HA is given as 0.142 M, and the initial concentrations of H+ and A- are both 0. The change in equilibrium (from left to right) of the ICE table is going to be -x, +x, +x since you're forming pro...

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Do we have to know specific kw at temperatures other than 25C or just be able to find it through calculation?

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If you aren't sure which value to use, I'd suggest just plugging them in to decide what makes sense. If one of the numbers results in a negative molarity, you know that is incorrect and use the other value. If both values work, check your work because you might have made a mistake somewhere.

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I kept on losing a negative in my calculations which resulted in getting imaginary numbers for the value of x. Just make sure you keep track of the negative in your calculations if you are getting imaginary values!

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Every time I try to solve the quadratic I am getting complex numbers for the answer. Is this happening to anyone else?

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The prefixes are only used for the ligands attached to the TM. No need to put di- in front of chloride as long as it is outside the brackets.

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I am still having trouble getting achieve to accept my formatting. If anyone has additional tips for making it work please help. Thanks!

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Correct. And it is easy to just remember the stronger the acid, the lower the pH and the stronger the base, the higher pH.

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When NaCl dissociates in water we end up with HCl and NaOH. Since this is a strong acid and a strong base as the products, NaCl is neither an acid nor a base.

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Does anyone know the grading scale for this class. Ex: what % is A vs A-??

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Like Marlenna explained, I basically just eliminated all the other answers. So, I'd suggest process of elimination because the actual answer was not very obvious, however, the other answers were easily proven false.

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Leah Wildmann 1F wrote:To find the coordination number, look at the number of atoms other than the metal atom within the brackets. This will give you the number of atoms that are bonded to the central atom.

Do we always ignore what is outside the brackets?

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Hi! Yes, theoretically what you're saying makes sense, however, you must realize that the question asks which is the "most common" and we were always told in class to regard 4 ligands as a tetrahedral or square planar and 6 ligands as octahedral. I noticed this to. The question does say &...

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Looking at the periodic table, we see that iodine is in period 17, so it will have an oxidation state of -1. Since there are 3 iodine atoms in the compound, the individual charges will add up to -3. Overall, the compound has a charge of -1. This indicates that Hg has to have an oxidation state of +...

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When finding oxidation state, do we ignore anything outside of the brackets?

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Meagan Kimbrell 1F wrote:http://www.chemistry.wustl.edu/~edudev/LabTutorials/naming_coord_comp.html

This link has a really helpful step-by-step format!

If you are having trouble, definitely look at this! It was really helpful for me.

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Does anyone know if there is discussion monday-wednesday?

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Does anyone know if the last few weeks are more heavily focussed on for the final?

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So we just need to determine the molecular formula and use that compound for the rest of the problem?

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I confirmed with my TA that the posts must be 5 weekly.

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I am still a little confused about delocalized pi bonds. Is it always delocalized if the structure has resonance?

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To start off, understanding the definition of dipole moments is pretty important. Dipole moments are basically vectors, which means that we must consider both factors of magnitude and direction. That's why dipole moments would determine the polarity of a molecule by the net dipole moment. Take for ...

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Yes, I can confirm. My TA said that the A will have no subscript.

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To complete this problem I first drew all of the lewis structures. Then I looked at the types of bonds in each one. Single bond is the longest bond, then double is slightly shorter, and then triple is the shortest.

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When we know a bond angle is for example <109.5, do we need to specify how much smaller or is <109.5 a sufficient answer?

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Ivan Huang Dis 2B wrote:the odd-numbered answers on the back matter say that A and B are the only radicals. Is this answer wrong because I also only got B and C are radicals

I got the same answer as you. I think it is just a mistake in the solutions.

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The double bonded oxygen has a formal charge of 0, not -1.

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London dispersion forces are stronger for bigger, heavier atoms.

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Does the H have to be between to electronegative atoms? It that the key characteristic to look for?

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Just to clarify, increased atomic radius means increased bond length and thus a weaker bond?

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TJ Lai 2H wrote:
FionaHunter21 wrote:Are london dispersion forces present in all nonpolar molecules?

London dispersion forces are present in all molecules, but they are the only intermolecular force present in nonpolar molecues.

So, if the molecule is non-polar we can conclude it only has london forces?

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Thank you so much for sharing with us! Recently I've been on the fence about my choice to be pre med but this reminded me of all the reasons I should sick with it! :)

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Utilize youtube videos! Khan academy videos are super helpful for me personally, but there are many others who post videos on general chem topics.

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Janna Shakiba wrote:
Mari Williams 1K wrote:My brother and I once shattered a glass pan of brownies after putting it on a cold metal table straight out of the oven...truly tragic

Lmao I've done this too

lol I have definitely done this too. maybe if I took chem14a sooner I'd understand why that was a bad idea and not do it ahhaha

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I was having trouble with this too. Thank you for your explanation it really helped!

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Above the actual achieve homework for the first week there is a bunch of practice assignments that I did that really helped me figure out how to draw the bonds. Definitely look over that stuff, it will help a lot!

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I actuallly dont read the textbook and whenever I have a question I either ask the UA's or go to the professor's office hours. Does this work better than reading the text book? Reading the textbook and doing the textbook problems was really helpful for me personally. Going to UA sessions/TA office ...

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Reading through the textbook and completing the problems on the syllabus is a must to do well in this class. It helped me be well prepared for the midterm and also just furthered my understanding of the topics.

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Ally Mosher wrote:Will the grades be posted on CCLE?

Yes. I think my TA said that is where the grades will be posted.

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Dr. Lavelle also mentioned some exceptions for electron configurations. Could someone help me out and remind me what they are? Thanks!

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I am having trouble determining which can and cannot exist. If someone could walk me through this I would really appreciate it.

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I am confused why in the first part we find 4.73x10^-19 but then use 4.73x10^-10 in the second part. why -10 instead of -19?

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Can someone please explain what keV means. Thank you.

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To clarify, when multiplying the moles of the entire compound by avogadros number we get the number of formula units rather than number of atoms?

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Basically, this question wants you to solve for the molar mass of 'M' and identify it by finding the correct element on the periodic table with this molar mass. You do this by subtracting the molar mass of the known elements from the total molar mass. Looking at the periodic table, M is revealed to...

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Is it correct to assume that since astatine is three times as massive as gallium (120 vs 70), that we always need astatine times 3 amount of gallium to balance the two on a scale?

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I am having trouble finding the solution manual to help me check this problem. Can anyone direct me to where I may find it? thanks!

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The final answer was really confusing me too. This is because I disregarded unit conversion when comparing my answer. This is why it is so important to keep track of units throughout the problem!

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It is SO important to be careful and take your time calculating molar masses. Many times I have done it too quickly and ended up messing up the entire problem just because incorrect molar mass calculations in the first steps of the problem led to incorrect mole calculations and so on.

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I completely forgot about learning these acronyms in high school! Thank you for refreshing my memory because they are super helpful!

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I am having trouble finding a thorough periodic table online. I just wanted to print one to use for hw and in class, etc. Does anyone have a good recommendation for one with plenty of info that is also readable? Thanks!

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I always used to forget which symbol was wavelegth and which was frequency. The way I remembered was that greek letter lambda looks like a wave so that it used for wavelength, then v is for frequency. I know it's kind of cheesy and not that hard to remember but this helped me in high school when I w...

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Pretty sure that Sapling is mandatory, and will actually be collected midnight of the following Sunday, however, is this the only "homework" due?? Yes sapling is the only homework that is taken for a grade, but the extra homework problems that Dr. Lavelle lists on the syllabus and chapter...

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Hi everyone, I'm having some trouble understanding the steps needed to get through this problem. I was able to figure out the molecular formulas for the reaction shown in the problem and convert the given quantities of 2-butanone and 3‑methyl‑3‑hexanol to moles, but I'm not entirely sure where to g...

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Hi, I had a question on the Achieve homework question 9: Caproic acid has the odor of goats. (In fact, Capra is the genus of the domestic goat.) The compound contains only C, H, and O and was experimentally found to have a molar mass of 110±10 g/mol . When a 1.000 g sample of caproic acid is burned...

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The target analogy is helpful. Accuracy is hitting bullseye or close to it. Precision is if you hit far away from bullseye but continue to hit that same place over and over again.

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The target analogy is helpful. Accuracy is hitting bullseye or close to it. Precision is if you hit far away from bullseye but continue to hit that same place over and over again.

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Trailing zeros count as sig figs as long as they are both after an actual number and after a decimal. For example, there are 3 sig figs in 2.00 because the trailing zeros are after the number 2. There are also 3 sig figs in 0.0400 because only the zeros after the number 4 count, even though there i...

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