CHEMISTRY COMMUNITY

Mackenzie Van Val 3E

Hi! For help on determining whether a molecule is an acid or a base, here are some tips. An acid will contain an H atom somewhere in its formula that it can lose to form H3O+ when combined with water. For example, when combined with H2O, H2SO4 can lose an H to form HSO4- and H3O+. Other acids that f...

Mackenzie Van Val 3E

Typically, the units of the gas will be given in the equation. Bar is the SI unit for gas, so if it does not say a unit, it may be safe to assume that the gas is measured in bar. However, atm and bar are so close that the professor said we can typically trade them out without consequence.

Mackenzie Van Val 3E

The difference between Q and K is that K is only measured at equilibrium, and, if the reaction quotient is calculated at any other point during the reaction, it is called Q. K can come at different temperatures, because equilibrium occurs at different temperatures, but K will vary from temperature t...

Mackenzie Van Val 3E

Dr. Lavelle said that x must be smaller than 10^-3, but, to be safe, you should make sure that x is significantly smaller than the molar concentrations- as other commenters have said, 5% is a really good benchmark to use (because 10^-3, in some situations, could be large enough to not be negligible).

Mackenzie Van Val 3E

Hi! As the commenter above said, the H+ or OH- comes from the dissociation of the strong acid or the strong base. Just like the dissociation of NaCl in water would cause both Na+ and Cl- ions to form, when a strong acid like HCl dissociates in water, both H+ and Cl- ions are formed, so H+ has a mola...

Mackenzie Van Val 3E

We should use Kelvin for all of chemistry. Many temperatures will be given in Celsius, so to convert from Celsius to Kelvin, you add 273 to the Celsius temperature.

Mackenzie Van Val 3E

Taylor Newville 1C wrote:You can use either K or 1/K for this question and get the same answer. If you use K, make sure you set it equal to [NO]^2/[N2][O2]. And if you use 1/K, make sure you set it equal to [N2][O2]/[NO]^2.

Thank you so much! I'll try again, this time with 1/K, and hopefully it'll work.

Mackenzie Van Val 3E

Some of my favorite TV shows at the moment are Schitt's Creek and Community (which are both on Netflix) and Psych (which is on Amazon Prime)! Also, if you're into history, Band of Brothers is a really great miniseries about a troop throughout World War 2, and that is also on Amazon Prime. I'll defin...

Mackenzie Van Val 3E

Hi! For this problem, you only need to include equations that involve the reactants and products in some way. If there is a reaction mechanism that does not include the products and reactants, it will not be useful for you, so you can disregard it. For my problem, I only used two of the intermediate...

Mackenzie Van Val 3E

Hi! Can you post the specific reaction that you have for question #3? That would make it easier to describe and help you!

Mackenzie Van Val 3E

Hi! The total pressure is the combined pressure of all of the gases in the mixture within the vessel. The partial pressure of each gas is the portion of this pressure associated with each gas in the mixture. So, if the partial pressure of one gas was 0.75 atm, and another gas had a partial pressure ...

Mackenzie Van Val 3E

Hi! You want to pick only the equations that allow you to end up at the overall reaction at the top. Therefore, you only need the equations involving the products and/or reactants of the overall reaction in some form. For example, in this problem, you would not need to use the reaction with Kd at al...

Mackenzie Van Val 3E

Hi! I'm really struggling on this question. I swear I've followed all the correct steps, and I've redone the problem completely just to double check, and somehow I just keep coming up with the wrong answer. Just to clarify, as I have seen it both ways on here: should I be setting the new Kc equation...

Mackenzie Van Val 3E

There is definitely no curve in terms of a 'bell curve', which I am very grateful for. As the other commenters have said, though, he did kind of 'curve' our final by adding points.

Mackenzie Van Val 3E

No, which is why strong acids have extremely weak conjugate bases and strong bases have very weak conjugate acids. It is very difficult for conjugate bases of strong acids to accept a proton or donate a lone pair because of the high dissociation of the strong acid.

Mackenzie Van Val 3E

I would say that an indicator with a smaller range that includes the predicted pH would be the best to use. I assume that because the range is more narrow, it would be easier to find if the predicted pH really does fall into that range. Like Gwendolyn said, though, it would also be good to run the e...

Mackenzie Van Val 3E

Yes, the ions in the strong acids and bases will completely dissociate and the pH of the end solution will be neutral. As Brandon said, you can basically think of it as the two cancelling each other out.

Mackenzie Van Val 3E

I might rewatch the video modules from some of the earlier concepts in 14A because I feel like I won't retain that information as well since we learned it a while ago (although, knowing me, I'll likely forget to do that). Other than that, I'll probably just keep my 14A notebook handy as we start 14B.

Mackenzie Van Val 3E

Well, you can see from my post time that I'm not doing too well on the "get some sleep" thing... oops! I really appreciate these words of encouragement, Aliya! Good luck everyone. We've got this!

Mackenzie Van Val 3E

I assumed that the reason for writing out the dissociated version of H3PO4 was to see where the H+ goes when it becomes a product- basically, to help us visualize it better. However, looking at it now, I see that the H+ transfers from HBrO2 to H2O in part (c), but the HBrO2 is not written out in a w...

Mackenzie Van Val 3E

I believe that the more moles of OH- produced, the stronger a base will be. I remember that being alluded to on the Sapling for this week; when we had to order bases in order of strength, it told me that the more OH- produced, the stronger of a base it is. Therefore, I think you're right in saying t...

Mackenzie Van Val 3E

I do think it's best to know oxidation numbers of common atoms and the overall way of assigning them, simply because it will make answering questions involving oxidation numbers a lot faster. Some key oxidation number trends to remember are that elements in the first column of the periodic table hav...

Mackenzie Van Val 3E

Hi! I also tend to lack organization while studying, so I'm going to attend lots of review sessions for this final so that the studying is structured for me! I also plan on completing all the textbook questions before the final, which is a lot of work, but has proven to be worth it on the two midter...

Mackenzie Van Val 3E

I can't believe it's the end of Week 9. I feel like the quarter has flown by. I'm hoping for the best for this last week of class and for the final. Good luck everyone! Hope you're doing alright!

Mackenzie Van Val 3E

If you think about it, the points on a hexagon are much closer to each other than the points on a three-dimensional figure with 6 vertices (I can't remember what that shape is called) Just to clarify this, this shape for a molecule with six bonded atoms is an octahedral structure. To add on to what...

Mackenzie Van Val 3E

NH3 is a neutral molecule on its own; therefore, it acts as a neutral ligand. I had this same question, and I found that drawing out the Lewis structure and calculating the formal charge on each atom (which will all be 0) to be helpful in determining an answer, as it made it easy to see that NH3 has...

Mackenzie Van Val 3E

One of the few focusing tips that I have is to drink lots of water as you're studying. A lot of times when we are feeling sleepy or cloudy-headed, it is because we are dehydrated, so drinking water can help you wake up/stay alert! Plus, getting up to go to the bathroom after you drink a ton of water...

Mackenzie Van Val 3E

AsO43- has a symmetric distribution of O around the central As, and all four atoms surrounding As are the same and thus will have the same electronegativity (and the same dipole moment with As). Therefore, the dipole moments between the As and each O will cancel out. Drawing the dipole arrows on the...

Mackenzie Van Val 3E

Hi Karina! I look at this when looking at electronegativity: Cl is much further right on the periodic table than Be, but Be is only one row higher than Cl. Therefore, it makes sense that Cl has a higher electronegativity than Be. Hope this helps!

Mackenzie Van Val 3E

Hi Claire! I'm not sure if Dr. Lavelle has said whether or not there will be partial credit for the final yet. I'll ask my TA during my discussion this week. This also would be a good question to ask at Dr. Lavelle's office hours. I hope we can get the answer to this soon!

Mackenzie Van Val 3E

Single bonds are sigma bonds. Double bonds contain a pi and a sigma bond, and a triple bond has one sigma and two pi bonds. Therefore, all three contain sigma bonds. Something that helps me remember the number of pi bonds in a bond type is that for double and triple bonds, the number of pi bonds is ...

Mackenzie Van Val 3E

what determines how strong london dispersion forces are again? Large, heavy atoms have stronger London Dispersion forces than smaller, lighter atoms because they are more polarizable and have more electrons. On a related note, the larger the surface area of an atom, the stronger its London Dispersi...

Mackenzie Van Val 3E

As other commenters have mentioned, looking at the difference in electronegativity is a great way to determine covalent vs. ionic character. I'd also like to point out that highly polarizable ions that are large and rich in electrons (such as I-) lead to more covalent bond character, which is why th...

Mackenzie Van Val 3E

If a molecule has a symmetric structure with all the same atoms around the central atom (for example, SF6), you can immediately tell that the molecule is non-polar, With molecules that have an asymmetric structure, you can look at the atoms within the molecule and use periodic trends to help you det...

Mackenzie Van Val 3E

Ni should be written [Ar] 3d8 4s2 and not [Ar] 3d10. The reason that Cu and Cr are exceptions is that they are only 1 electron away from filling the d-orbital, and thus it makes them more stable to fill up that one electron in the d-orbital rather than the s-orbital. However, I strongly believe that...

Mackenzie Van Val 3E

I definitely realized some mistakes I made right after I submitted my midterm, but hopefully what we learn from this midterm's mistakes will help us succeed on the final (fingers crossed!).

Mackenzie Van Val 3E

The positive and negative identifiers that Dr. Lavelle used in his lectures for charge distribution seemed to be based more off of the electronegativity of an atom than the formal charges found in Lewis structures. For example, in KCl, Cl would carry the partial negative charge because it has a high...

Mackenzie Van Val 3E

Hydrogen is an exception to the octet rule, as it only needs 2 electrons to be satisfied. Atoms in the 3rd row of the periodic table and below can sometimes take on extra electrons and therefore exceed the octet rule. I read this in the textbook, but wasn't sure if it mean only p-block elements bel...

Mackenzie Van Val 3E

Hi! My TA said that because we have not gotten to the naming section of this class yet, we should not be expected to know the names of the various ions for this midterm, but we will likely need to know it for the final. However, as the other commenters have said, it definitely wouldn't hurt to revie...

Mackenzie Van Val 3E

The bonds do not have to be the same length because the bond length is dependent on whether H is bonding with N, O, or F. From a table on chemistryreference.com, I found that the typical H-F bond length is 92 pm, whereas O-H is 96 pm and N-H is 101 pm. Therefore, I believe that even if two molecules...

Mackenzie Van Val 3E

From Dr. Lavelle's lecture, it seemed like Lewis Acids and Lewis Bases are different from cations and anions. For example, BF3 was considered a Lewis acid, but it does not have a positive or negative charge, so it is not a cation or anion. I believe the important distinction to make is that cations ...

Mackenzie Van Val 3E

H is a smaller atom than O, which means that the bond between it and N will be shorter. In addition, there is a higher difference in electronegativity between N and H than N and O, which means that N and H will pull closer together.

Mackenzie Van Val 3E

Hi! In addition to the size of the atoms, the size of the overall molecule will affect the LDF; the larger the molecule, the stronger the LDF. This typically means that molecules with longer chains of atoms will have stronger London Dispersion Forces than molecules with shorter chains. In addition, ...

Mackenzie Van Val 3E

I believe that in our class, we will write 3d before 4s in all circumstances. As a previous commenter pointed out, it is important to remember that the 4s orbital should always be filled before the 3d, even though the 3d orbital is written first. The 3d and 4s orbitals are very close in energy, whic...

Mackenzie Van Val 3E

Hydrogen is an exception to the octet rule, as it only needs 2 electrons to be satisfied. Boron is another exception that can have less than 8 electrons around it (typically, it works with 6 electrons). Beryllium can also be satisfied with less than 8. Atoms in the 3rd row of the periodic table and ...

Mackenzie Van Val 3E

Hi! I really enjoy Crash Course videos on YouTube. Here's the link for a playlist with a ton of their chemistry videos: https://www.youtube.com/watch?v=uVFCOfS ... aX9mQQ8oGr.

Mackenzie Van Val 3E

Hi! A molecule with resonance structures has more than one valid Lewis structure that can represent that molecule. For example, NO3-, the nitrate ion, has 3 valid structures that can represent it. These are called the resonance structures. The resonance hybrid can be thought of as the average of the...

Mackenzie Van Val 3E

Hi! Most atoms want to have 8 electrons attached to them in some ways, either through lone pairs of electrons on the atom or through bonds, in order to get a full octet. There are some exceptions, such as H, which needs only 2 electrons. Single bonds (one line) mean 2 electrons are being shared, dou...

Mackenzie Van Val 3E

Hi! I would guess that we would use the periodic trend concerning atomic radii to determine which atom has the larger radius. It seems that the atomic radii trend tends to be very consistent across the periodic table, which would probably make it easier to use than looking at the number of valence e...

Mackenzie Van Val 3E

Mathematically, the units of uncertainty in position must be meters in order for the other units in the Heisenberg equation to cancel out correctly while you are solving for a value. Conceptually, the reason that m is to the first power and is not m^3 is because we assume that the electron's possibl...

Mackenzie Van Val 3E

Hi Marco! I didn't have this exact question, but the steps should be the same from the one I had. Step 1: Find the frequency that corresponds to 434.0 nm by using v=c/wavelength. (Remember to convert the wavelength from nm to m.) Step 2: Using the Rydberg equation, plug the calculated frequency in f...

Mackenzie Van Val 3E

n2 represents the excited state the electron is in, while n1 represents the energy level that the electron returns to. n2 is always greater than n1. I may be wrong, but that's the way I think about the Rydberg Equation. Thank you so much for this explanation! I have been confused about what n1 and ...

Mackenzie Van Val 3E

Hi! The 3.29e15 Hz is the version of the Rydberg constant that is on Dr. Lavelle's Constants and Equations sheet, so I'd say that at least for now, that is definitely the more convenient/recommended version of the constant to use for this class. It is also the constant that I assume works with the v...

Mackenzie Van Val 3E

I think that the reason that it is not outright false is because in a traditional wave model, increasing the intensity (amplitude) would result in an increase in energy. It's like the analogy Dr. Lavelle gave of standing in the ocean and being hit by a 2-foot wave versus a 10-foot wave. Even at the ...

Mackenzie Van Val 3E

Hi! This happens to me quite frequently, too. Sometimes, the big numbers with scientific notation just don't load in the answer key. I know that Sachi has already provided the answer key PDF, which is awesome; I just wanted to add this for anyone else who has this issue, because I don't think the lo...

Mackenzie Van Val 3E

The main takeaway from the Heisenberg Indeterminacy Equation is that we can never measure an electron's exact momentum and position because, by measuring these values, we change them. Therefore, there is always some element of uncertainty in an electron's measured position and momentum. Heisenberg's...

Mackenzie Van Val 3E

I always try to keep all the digits throughout the calculation to the best of my ability, but I know that on a previous post on Chem Community, Dr. Lavelle said that a little bit of rounding is okay- for example, if the final answer has 2 sig figs, then it's okay to only keep 4 sig figs throughout y...

Mackenzie Van Val 3E

On the practice quiz that I did with my TA, there was a question that had us choose between two answers with different sig figs, but that doesn’t necessarily mean a question like that will be on the midterm. If the other commenters’ TA’s have said that sig figs won’t be on the midterm, I’d go with t...

Mackenzie Van Val 3E

Hi! To echo what the other commenters have said, delta X stands for the uncertainty in position. There are many different ways this uncertainty can be shown. If the diameter of the electron's "container" is given, then that will be the uncertainty in position. For example, in a problem tha...

Mackenzie Van Val 3E

Hi Nicolette! I'm also a freshman, and definitely having trouble adjusting to the online college set up/workload. I wanted to pass along some study advice I got from my New Student Mentor, which I am trying to put into practice myself. My mentor suggested starting homework as soon as you are done wa...

Mackenzie Van Val 3E

Hi! As many other commenters under this post have said, I think it is best to try and keep as many places after the decimal as possible while you are doing your work, and then round for sig figs only at the end. However, I also saw a post by Dr. Lavelle on Chem Community about how many digits to kee...

Mackenzie Van Val 3E

Hi! To solve (b), all you need to do is plug in the given frequency (2.50 x 10^16 Hz) into E=hv to find the energy necessary to emit an electron. Since the lowest frequency of light that will emit an electron is 2.50 x 10^16 Hz, we can assume that at this frequency, the KE of the electron will be 0....

Mackenzie Van Val 3E

These videos give some background information and explanations of the Heisenberg Uncertainty Principle. I found them helpful, so hopefully someone else will, too! https://youtu.be/qO_W70VegbQ - This is a Crash Course video my TA showed us that I found very helpful for understanding the background be...

Mackenzie Van Val 3E

Is there a way to get closed captioning on the pre-recorded lectures? I checked on the BruinCast site and could not find any information regarding captions. Thank you for any help!

Mackenzie Van Val 3E

I agree with what Catherine said; I think that creating a mini chart that shows the quantity of each element as a reactant (for example, C: 1, H: 3, O:2) and then as a product (which could be something like C:4, H:5, O:1) and then finding a way to make those numbers match is a super helpful tip. I a...

Mackenzie Van Val 3E

Is this question asking us to calculate the number of moles of the compound then?? No, the question is asking how many many molecules of the compound are present in 5.15 g of the compound. In order to find that number, you do need to find the number of moles of the compound by using its molecular w...

Mackenzie Van Val 3E

I would definitely say to make all your stoichiometric coefficients into integers. I know Dr. Lavelle mentioned to do so in a lecture, and the UA who hosted tonight's Workshop also said to be sure to convert them into whole numbers, so I would put in that extra step just to be safe. You can always a...

Mackenzie Van Val 3E

Hi! I know there are a lot of questions about #10, so I hope mine hasn't been asked before. When given a question like this, do we just assume that the equation is balanced before doing our limiting reagent calculations? I know that on other questions where it was not explicitly stated whether an eq...

Mackenzie Van Val 3E

Here's the link for Lecture 3 Discussion 3G. :)

https://groupme.com/join_group/63036960/cUWMMQ0G

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