CHEMISTRY COMMUNITY

AnnikaMittelhauser4E

For future reference regarding the bases, I think BeO (despite being a group 2 oxide) is actually an amphoteric oxide, not a base.

AnnikaMittelhauser4E

I thought it had 6? When you look it up, images like this come up:

So 5 nitrogens, 1 oxygen.

AnnikaMittelhauser4E

It's a little late for this final - but for future ones, you can try avoiding or reducing caffeine (if you aren't already). Caffeine definitely increases anxiety. If you need the caffeine to study, make sure you don't take it on an empty stomach.

AnnikaMittelhauser4E

I drew the Lewis structure of SO3 with the S in the middle and double bonded to each oxygen atom. Each oxygen atom then has two lone pairs, and everything has FC = 0. But, in 6.5, the textbook has the lewis structure of SO3 with S double bonded to one of the oxygens and single bonded to the other tw...

AnnikaMittelhauser4E

6B3 says "A careless laboratory technician wants to prepare 200.0 mL of a 0.025 M HCl(aq) solution but uses a volumetric flask of volume 250.0 mL by mistake. (a) What would the pH of the desired solution have been? (b) What will be the actual pH of the solution as prepared?" The textbook s...

AnnikaMittelhauser4E

I think you might be misreading the answers. Aqua always refers to water, which can be written as either H2O or OH2 (which I think you're mistaking for OH-). The textbook writes water as OH2 to emphasize the oxygen is doing the bonding I believe.

AnnikaMittelhauser4E

They both have resonance I think. But, an acid is stronger if its conjugate base is more stable. Conjugate bases are stabilized by electron attracting atoms (delocalization of electrons) and destabilized by electron releasing groups. CH3 is more prone to electron donation/release than H is, so the c...

AnnikaMittelhauser4E

Organic acids contain carbon whereas inorganic ones do not.

AnnikaMittelhauser4E

The oxidation number of an atom in a compound is the charge that atom would have if the compound was separated into ions. In general, to figure out oxidation numbers, you look at the overall charge and at atoms that always have the same oxidation number to figure out the ones that can vary. There ar...

AnnikaMittelhauser4E

I think all carboxylic acids are weak acids whereas some oxyacids are strong acids (H2SO4 and HNO3 for example),so maybe, in general, oxyacids are stronger. They're also inorganic whereas carboxylic are organic and I think Prof. Lavelle said inorganic acids are usually stronger.

AnnikaMittelhauser4E

He said he chose to write it that way so he'd remember to tell us that the order doesn't matter.

AnnikaMittelhauser4E

They exist because not all the p orbitals have to be hybridized. For example, Lavelle talked about C2H4 in class. Each carbon has a 2sp2 hybridization because they each have 3 regions of electron density. One electron stays in a non-hybridized 3p orbital. This makes sense because then the electron i...

AnnikaMittelhauser4E

I think the easiest way to think of it is that hybridization comes from the distribution of the electrons (looking at lone pairs and bonds). So, to figure out the hybridization, you'd first want to look at the electron density. Ex: CH4 and H2O both have 4 regions of electron density, so the hybridiz...

AnnikaMittelhauser4E

VB theory uses orbitals to describe covalent bonding. Each unpaired valence electron can form a covalent bond. It needs hybridization to adequately do so because otherwise, the bonding tendencies of elements like carbon would not make sense. Ex: Carbon has 2 unpaired valence electrons in the 2p shel...

AnnikaMittelhauser4E

I think the steric number is just part of the thought process for determining the hybridization, so you should give both or just the hybridization.

AnnikaMittelhauser4E

I think it's polar because acids tend to be and it looks like this: H2SeO4.jpg My guess is you'd draw the negative dipole moment arrows going towards the various oxygens, from both the Se and the H. So the oxygens bonded to both the Se and H would have a negative dipole moment in both bonds and be m...

AnnikaMittelhauser4E

The textbook does mention a pentagonal bipyramidal structure which is 7 regions of electron density, but I don't see anything past that. Maybe molecules past 7 regions of electron density don't really exist due to too much electron repulsion or something - the bond angles get too small? Or, they're ...

AnnikaMittelhauser4E

The textbook said you can just consider each central atom individually. So, both carbons have 4 bonds (3 C-H and 1 C-Be) and no lone pairs. This makes them both have tetrahedral structure and bond angles of 109.5. The Be has two bonds (both Be-C) so this gives it the linear structure and a bond angl...

AnnikaMittelhauser4E

The chart is great but to understand it you could think of a seesaw molecule like SF4 like this: 1. SF4 has 5 regions of electron density because the lone pair is kinda like another S-F bond. So, I think of trigonal bipyramidal as the "template" shape because that's what's formed by centra...

AnnikaMittelhauser4E

You really can't tell just from the formula unless it's something really simple like a diatomic molecule (H2, O2, etc are always non polar because their only bond is non polar). Additionally, Lewis structures can be helpful, but they can also be misleading. For example, you could draw the Lewis stru...

AnnikaMittelhauser4E

A dipole-dipole moment will always occur between two elements of different electronegativity bonded together. It will be more extreme between atoms of very different electronegativity (like Cl and H).

AnnikaMittelhauser4E

Can you give an example molecule? Generally I don’t think whether the molecule has single/double/etc bonds has much to do with whether it’s polar or not.

AnnikaMittelhauser4E

Yes, if the elements have different electronegativities, you should consider the bond to have a dipole moment. This is because the element with the higher electronegativity is pulling the electrons closer to it, making it slightly negative and the other element slightly positive.

AnnikaMittelhauser4E

Also, the more polarizable an atom or molecule is, the greater the intermolecular induced-dipole induced-dipole forces. Since those forces are greater, it takes more energy to reach the liquid or gas phase. Ex: Cl2 is a gas at room temperature because the interactions between the Cl2 molecules are w...

AnnikaMittelhauser4E

Question 2C3 asks for the lewis structure of HClO3. I came up with the structure below in which I think everything has a formal charge of 0, and there are no expanded octets. IMG_1948.jpg But in reality, it has an expanded octet on chlorine and resonance as its most stable structure (furthest right)...

AnnikaMittelhauser4E

The least electronegative atom (in this case, nitrogen) being the central atom is also important.

AnnikaMittelhauser4E

To figure out this lewis structure... 1. Count the valence electrons. Each F has 7 and Br has 7 as well, so there need to be 28 in total. 2. Determine Br is the central atom because it has the lower electronegativity. 3. F is not flexible octet wise, so we know each one needs a single bond with the ...

AnnikaMittelhauser4E

That is confusing. One explanation I found online says "although the energy of empty 3d-orbitals is ordinarily higher than that of the 4s orbital, that difference is small and the additional d orbitals can accommodate more electrons." Or vaguely, on another site, that somehow excitation of...

AnnikaMittelhauser4E

As mentioned above, the elements in group 3 can break the octet rule likely by forming an expanded octet. This is because they have access to the unfilled 3d orbitals for bonding. I think other groups above n=2 can also access their respective unfilled d-orbitals to have expanded octets. Additionall...

AnnikaMittelhauser4E

I'm a little confused about when d-orbital electrons count as valence electrons. For example, in problem 2A1, Sb is considered to have 5 valence electrons. I assume those electrons are the 3 in the 5p shell and the 2 in the 5s shell. If that is the case, the 10 4d electrons are not considered valenc...

AnnikaMittelhauser4E

The shared electrons are the ones involved in bonding (the lines in Lewis structures). Each line counts as two shared electrons (so a double bond is 4 shared electrons). For example, in H2O, the oxygen has 4 shared electrons because it has a single bond with each hydrogen. It then has 4 lone electro...

AnnikaMittelhauser4E

Also, when adding up the FCs to get the overall molecule charge, it is important to remember that the negative formal charges should be on the more electronegative elements and the positive formal charges on the less electronegative elements.

AnnikaMittelhauser4E

I don't think so since even checking formal charge I don't think can tell you if you're correct - it can just tell you which potential lewis structures are better/more likely to be correct. But, when it comes to figuring out a good lewis structure that is likely to be experimentally correct, you can...

AnnikaMittelhauser4E

Expanded valence shell = element having more than octet. They occur for elements n=3 and above because the element can make use of empty d orbitals for bonding. For example, sulfur is [Ne]3s^2 3p^4 in its ground state. So, you might expect it to only “have room” for two more electrons to fill the 3p...

AnnikaMittelhauser4E

To add on, partial bonds have bond lengths that are also a blend b/w the two structures. So, since the nitrate bonds can be thought of as a blend between double and single bonds, the length is between that of a double (shorter) and single bond (longer) involving nitrogen and oxygen in a non-resonanc...

AnnikaMittelhauser4E

I think Wesley is correct because if higher than needed energy photons were absorbed, I think that would mean that material that absorbs red light (λ = 700) would also absorb all other colors of light since they are higher energy. But that isn't the case because we can see colors like blue being ref...

AnnikaMittelhauser4E

Are you referencing the Ep = Ek + ϕ equation where Ek = 1/2mv^2? If so, it's helpful I think to compare the electron removal to rolling a ball. It takes a certain amount of your energy (pushing) to overcome the initial friction and get the ball moving. That is the ϕ for the electron. Any extra energ...

AnnikaMittelhauser4E

I'm sure it's different for everyone, but personally I find it easiest to just convert to meters in the middle since we know the conversion factors relating to meters. Ex: Convert 400 nm to pm. (400 nm) x (1 m/10^9 nm) x (10^12 pm/1 m) = 400,000 pm You can tell that all the units except pm cancel. A...

AnnikaMittelhauser4E

This is the link to the equation sheet Dr. Lavelle has on his website: https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14A/constants_equations.pdf I think it has all the necessary quantum equations. If the link doesn't work, you can find it under the "constants and equations"...

AnnikaMittelhauser4E

The standard unit for wavelength is just meters since it is a distance. You should have wavelength in meters when doing calculations so the units will cancel properly. However, often we are given the value in nanometers (in which case you'd want to convert the value to meters as your first step) or ...

AnnikaMittelhauser4E

Well, it is relevant in that visible light corresponds to the Balmer series. So, when we are given the wavelength of emitted light (and it is in the visible portion of the spectrum) and we have to find the initial and final n, we know that the final n is 2. And we can get the energy and frequency fr...

AnnikaMittelhauser4E

I'm not sure how to know when to use each model, but my guess is he doesn't expect us to be experts at that. I think understanding the two models and the experiments he talks about (like photoelectric effect experiment) is a good enough place to start. If we understand the models well, we can probab...

AnnikaMittelhauser4E

Catalysts can work in a variety of ways. One way is as a reactant in an intermediate reaction. A lot of the chemical equations we see are simplified and don't include middle steps. An example is O3 + O --> 2O2. That reaction is catalyzed by Cl and in full it is actually two steps 1. Cl + O3 --> ClO ...

AnnikaMittelhauser4E

Tin is a transition metal, and like most of the transition metals (Ag and Zn are the common exceptions, forming only 1+ and 2+ ions respectively), it can become differently charged ions. In particular, tin can either become Sn 2+ or Sn 4+. Therefore the compound has to be named Tin (IV) Oxide to tel...

AnnikaMittelhauser4E

I'm not entirely sure what you were trying to multiply by 2, but I'm guessing you may be combining NaCl and KCl too early in your calculations. You can't add the masses together and divide by a combined molar mass and then multiply by 2, and you can't just find the # of moles of one of the compounds...

AnnikaMittelhauser4E

Yeah, nitrogen is diatomic so it rarely is seen as just "N." It is usually N2 gas, but that isn't very relevant for finding the empirical and molecular formulas (just for writing the balanced equation). To find the empirical formula, you can start by calculating how many moles of CO2 were ...

AnnikaMittelhauser4E

I agree with the above commenter. You just need to find how many moles of NaCl and KCl were added by dividing each compound by its respective molar mass. Then, since the ratio is 1:1 (cation : Cl-) in both compounds, the number of moles of compound = the number of moles of Cl- in each. To find the t...

AnnikaMittelhauser4E

In both ionic and molecular compounds, the ending of the second element is changed to -ide. In ionic compounds, the second element is the one that acts as an anion. Generally in molecular compounds, the element further to the right on the periodic table is the second one. So, in this case, chlorine ...

AnnikaMittelhauser4E

To add on to what the others said, it is true you can generally base your answer off of the smallest number of sig figs in the original numbers. However, you have to be careful not to count definitions like metric conversion factors (as mentioned) or numbers that could have been garnered from counti...

AnnikaMittelhauser4E

I think that yes, the answer would be limited to 4 sig figs. The molar mass of carbon is an imprecise, rounded measurement (that's why it could also be considered 12.011 g/mol). Unlike, exact definitions (e.g 1000 mL = 1 L) or counted numbers (e.g 4 beakers), the sig figs of all measurements must be...

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