CHEMISTRY COMMUNITY

Julia Mazzucato 4D

Hi, this was one of the questions I messed up on on the midterm. I'm redoing the entire test right now to study for the final and I'd love if someone can check my answer for question 4. Question: Light with wavelength 102.557 nm excites a hydrogen atom gas sample. Is the change in energy of a hydrog...

Julia Mazzucato 4D

I've heard in the past Dr. Lavelle has curved the class so the average is a B-, but it's important to note that the class structure has changed since a few years ago (there used to be two midterms, now there is one, etc.) so this could or could not be a thing. I would say don't rely on a curve, prep...

Julia Mazzucato 4D

I did the Marshmallow review packet and I'm going to the review session for that tonight. I'm also going to be redoing the homework problems, redoing the two tests and the midterm's problems, and possibly watching some Khan Academy videos to fill in extra knowledge. Memorize relevant formulas (from ...

Julia Mazzucato 4D

I asked my TA, apparently it's calculated online by the Chemistry Community system. It does track how many you did each week, and each week "closes" on Sunday at midnight. So if you forgot to post one week and made up for it the second week with ten posts, it won't count those 5 extra post...

Julia Mazzucato 4D

Just to make sure, the final is going to cover all topics we learned this quarter including high school review? Just a prediction given how our tests have been laid out: The test is likely to be heavy on the acids and bases questions and lewis structures (drawing dot structures and doing VSEPR). If...

Julia Mazzucato 4D

Will there be another practice packet for the final review session given like the midterm? Does anyone know when the answers for this packet will be given? The review session is tonight in Haines 39 from 7:30-10:30. That's where answers will be given in depth. The pdf of the answers will be posted ...

Julia Mazzucato 4D

PS. this is from 6C.21

Julia Mazzucato 4D

I thought because acetic acid has more electronegative atoms in its compound it was the stronger acid. Why is formic acid stronger?

Julia Mazzucato 4D

KBELTRAMI_4I wrote:For number 4 how does the hybridization become sp2 if there are 4 p electrons and 2 s electrons?

Because you need three hybrid orbitals for this compound with three areas of electron density, there are 6 electrons to place in total.

Julia Mazzucato 4D

Based on the Lewis Structure of CH₃S^-, is this compound more likely to be a Lewis acid or base? This is from TB question 6A14.

Julia Mazzucato 4D

If you draw the Lewis structures, look at which have lone pairs. This will give you an indication of which will donate electrons (lewis base) and which will accept electrons (lewis acid)! For H2O2 and SO3, both have lone pairs on them. Would SO3 be the lewis acid because of its resonance structures?

Julia Mazzucato 4D

6.5 Hydrogen peroxide, H2O2, reacts with sulfur trioxide to form peroxomonosulfuric acid, H2SO5, in a Lewis acid–base reaction. (a) Write the chemical equation for the reaction. (b) Draw the Lewis structures of the reactants and product (in the product, one OOH group in sulfuric acid is replaced by ...

Julia Mazzucato 4D

I know that polydentate ligands must have more than one bonding site (and thus more than one lone pair), but do all ligands with more than one lone pair qualify as polydentate? How can you tell?

Julia Mazzucato 4D

Yes, it's just based off the number of bonds, not lone pairs.

Julia Mazzucato 4D

You should memorize a few of the common ligands for naming purposes (my TA recommended that we memorize I-, Cl-, Br-, OH-, CN-, C2O4-, H2O, NH3, CO, en). You should know which coordination compounds/ligands can be polydentate due to structure. For coordination number you just need to know how many b...

Julia Mazzucato 4D

Doing practice problems (particularly (9C9 from the TB) and came across the ligands en and edta a few times. What are these, how do we name them, and how would we draw them in a lewis structure?

Julia Mazzucato 4D

2F.15 Noting that the bond angle of an sp3 hybridized atom is 109.58 and that of an sp2 hybridized atom is 1208, do you expect the bond angle between two hybrid orbitals to increase or decrease as the s-character of the hybrids is increased? The answer is increases. Does anyone know why/can explain ...

Julia Mazzucato 4D

jvera4b wrote:Does anyone know if the HW for both weeks is due on week 10 for all discussions or only those after Wednesday?

I believe I was told it's for those after wednesday. I have a wednesday discussion so I'm gonna turn 5 in today and 5 in week ten just to be safe.

Julia Mazzucato 4D

I emailed my TA and he said that would be fine, just draw a line or label what is for week 9 and what is for week 10.

Julia Mazzucato 4D

I've found conflicting answers on the internet so I wanted to ask here. I was always taught throughout high school that covalent bonds are stronger than ionic bonds. But I came across a question in the textbook asking which of two substances would have a higher boiling point and I know that it has t...

Julia Mazzucato 4D

I actually got C4H8O. First, I used the grams of CO2 given to determine the grams of carbon. Next, I used the grams of H2O to determine the grams of hydrogen. Then, add these values together and subtract from 2.00 g of the sample compound to determine how many grams of oxygen there are. Afterwards,...

Julia Mazzucato 4D

Doing a problem from a Test Bank exam, and I just wanted to check my answer. A 2.00 g sample of a compound containing only C, H, and O undergoes complete combustion and gives off 4.86g CO 2 and 2.03g H 2 O. What is the empirical formula of the substance? My answer was C 2 H 4 O 5 but i just wanted t...

Julia Mazzucato 4D

Do lone pair electrons have a stronger force of repulsion of atoms compared to the strength of atoms repelling against each other? These are the relative strengths of repulsion: lone-lone pair repulsion > lone-bonding pair repulsion > bonding-bonding pair repulsion I've heard from a past student kn...

Julia Mazzucato 4D

Differences in bond angles are dependent on the type of atoms involved in the molecule and their electronegativity and the lone pairs. For example, NH₃ and SO₃^2- are both AX₃E molecules, but the NH₃ has a 107 degree bond angle and SO₃^2- has 106 degree bond angle.

Julia Mazzucato 4D

Wait so regarding the class curve, will that help or hurt our final grades? Does he have a set amount of A's he gives out?

Julia Mazzucato 4D

Crash Course with Hank Green is helpful for a very broad overview of the concepts (plus they're not boring so may be helpful if you get bored with the longer lecture style videos). But I think the Khan Academy videos in their AP chemistry course are really helpful too. Definitely recommend the ones ...

Julia Mazzucato 4D

He confirmed in lecture (2PM) today that the test will not be cumulative, and will cover everything covered after the midterm as well as Monday's lecture. Intermolecular forces, polarity, more lewis dot structures, VSEPR theory.

Julia Mazzucato 4D

For example, a hydrogen bond would have an energy of a greater magnitude (a larger negative number) because they are much stronger and require much more energy to break than a dipole-dipole interaction.

Julia Mazzucato 4D

Is there a certain threshold of electronegativity difference that two atoms in a molecule must have in order to make it a polar molecule with a dipole moment? Also are we going to be given a table with electronegativities on the midterm, or will that info be provided in the question if asked, or not...

Julia Mazzucato 4D

Electronegativity increases as you go right and decreases as you go down. So electronegativity is highest in the upper right hand corner of the PT. Ionization energy increases as you go right and decreases as you go down. So ionization energy is highest in the upper right hand corner of the PT. Atom...

Julia Mazzucato 4D

Equations we need for the midterm: Unit 1 (HS Review) Dilutions: (M i )V i ) = (M f V f ) Percent yield: Actual/theoretical X 100 M = moles/liters Unit 2 - Quantum Photoelectric effect: E=hv E photon - work function = KE electron (energy of the photon minus threshold energy = kinetic energy of the e...

Julia Mazzucato 4D

Hey, does anyone have any good practice problems for quantum numbers? I'm studying for the midterm right now and I've done mostly all of the probs in the Textbook so if anyone knows of another good source to practice lmk.

Julia Mazzucato 4D

If I'm correctly remembering this, any element in period 3 and beyond can have an expanded valence shell. I don't think there is a limit to how expanded the valence shell can be, but generally, most compounds don't really go past 12-ish VE in their Lewis Dot Structure.

Julia Mazzucato 4D

Yes, that process will generally get you the correct result. I think a lot of it is just building an intuition around knowing where to place each atom/bond, which gets easier with practice. It's harder when you're doing Lewis structures for a compound consistent of multiple polyatomic ions like H2CN...

Julia Mazzucato 4D

With cations, the electrons are pulled more closely towards the nucleus because there is less electron shielding occurring. Whereas an anion, because of the additional valence electrons, have a larger outer shell.

Julia Mazzucato 4D

There are so exceptions with the transition metals (they may have different ionic charges depending on what they react with), but in general the elements in groups 1, 2, and 3 will form cations with charges 1+, 2+, and 3+ respectively, while the elements in groups 4,5,6,7 will form anions with charg...

Julia Mazzucato 4D

Also, Lavelle mentioned that most of the High School Review unit will not be on the test, since we already learned it. Expect heavy focus on quantum and chemical bonds up to 2D.

Julia Mazzucato 4D

We will also be able to look at formal charges within the molecule to determine polarity of a covalent bond.

Julia Mazzucato 4D

If you want to be sure, you can think of the electron configuration for the outermost n level. So for an element where the highest energy level is n=2, you can count the number of electrons in the orbitals starting with 2 (in this case 2s and 2p) to get the VE number.

Julia Mazzucato 4D

In most problems, we should be given the charge for transition metals. I think the only type of problems in which we would need to figure out the charge of a transition metal would be if we are given a compound with a nonmetal that we already know the charge of (for example, we'd be given Zn2O3 and ...

Julia Mazzucato 4D

I remember him saying that he only made video modules for the HS review stuff that we were supposed to complete before week zero and that he also made them for quantum topics because those are harder to understand. So my guess is probably not, unfortunately, but I would love to have more video modul...

Julia Mazzucato 4D

I second the above answer. The exception is hydrogen, of course. You never put hydrogen at the center of the molecule when drawing a dot structure.

Julia Mazzucato 4D

Yes, there are a few exceptions to the general trends we see on the periodic table. Electronegativity increases as you move to the right of the PT from the metals up to the halogens, but the noble gases do not have electronegativities (unless forced under very extreme pressure/conditions) because th...

Julia Mazzucato 4D

I believe Dr. Lavelle also said that the electrons choose to occupy different orbitals before pairing up because they are more stable this way.

Julia Mazzucato 4D

To echo some of the above responses, quantum mechanics deals in very extreme values (for example, very large numbers for speed/frequency and very low numbers for mass, momentum, wavelength, energy). In quantum mechanics, because the subjects we are calculating for are so small, there is also a certa...

Julia Mazzucato 4D

Yes, Andrew F is right. Even though the 4s orbital is chronologically before the 3d orbital on the periodic table, an orbital's energy level and thus its order of being filled is determined by its first quantum number n, not its quantum number l (which indicates if the orbital is s,p,d,f). Basically...

Julia Mazzucato 4D

Yes, you can see the effect of the interference in the diffraction pattern, and depending on how in-phase/out of phase the waves are, you will see different patterns. If you have two waves undergoing constructive interference, the amplitude of the resulting wave gets larger, so the light will be bri...

Julia Mazzucato 4D

To answer 905289082: The energy of the light is related to the frequency of it, not the intensity per se. Amplitude is intensity (the brighter the light, the higher the amplitude), but the amplitude of the light has no bearing on the equation of the energy. The wavelength of the light has a direct e...

Julia Mazzucato 4D

It'll probably be useful, especially with problems from photoelectric effect/atomic spectra lessons. Here's a breakdown by wavelength from smallest wavelengths (highest frequency) to largest wavelength: Gamma rays, <0.01 nm X-rays, 0.01 nm - 10 nm Ultraviolet, 10 nm - 400 nm Visible Light, 400 nm - ...

Julia Mazzucato 4D

In this course, Dr. Lavelle won't expect us to know when to use each model, but we should be able to look at an experiment that was covered in class and know which model is appropriate to describe the behavior of light. So if a basic photoelectric effect experiment setup is described in a problem, w...

Julia Mazzucato 4D

In this course we should always be able to assume that it's under a vaccuum. If there are any problems not under a vaccuum, you should be given the appropriate velocity so you're still able to calculate what you need. The relationship between wavelength and frequency is the same with a non-c velocit...

Julia Mazzucato 4D

Hi, in lecture yesterday Dr. Lavelle was explaining the equation E=-hr/n^2 and how the further away from the nucleus an electron gets, the lower the absolute value of its energy will get. My mind kind of associated this with Coulomb's law and gravitational force which dictates that the farther away ...

Julia Mazzucato 4D

My Answers G.3) a) heterogenous, they will separate on their own and you can take the oil from the top. b) heterogenous, use a sifter to separate larger salt crystals from fine chalk dust. c) homogenous, boil the water to evaporate it from the salt. G.6) a) 0.08161 M, b) 0.04080 M G.12) a) 16.72 mL...

Julia Mazzucato 4D

Hi guys, I wanted to create a thread for some good practice problems from the TB (not part of the HW questions) to study for the test in week 2. Feel free to share any good practice probs you find so we can all study together! Molarity/Dilution Practice Problems G.3) Identify the following mixtures ...

Julia Mazzucato 4D

My answers

2) 43.67% yield
6) 3 moles of N2H4 will not react, you will get approx 36 moles HF, 9 moles N2, and 6 moles Cl2
8) Al₂O₃ + 3C + 3Cl₂ > 2AlCl₃ + 3CO, 125 kg of AlCl₃ produced

Julia Mazzucato 4D

Hi guys, I wanted to create a thread for some good practice problems from the TB (not part of the HW questions) to study for the test in week 2. Feel free to share any good practice probs you find so we can all study together! Limiting Reagent Calculation Problems M.2) The metal vanadium can be extr...

Julia Mazzucato 4D

Answers

F.6) 79.12% C, 9.786% H, 11.09% O
F.28) 76.09%
F.10) C5H14N2
F.16) C17H18F3NO
F.18) C47H51NO14 molecular formula=empirical formula
F.21) mass comp 62.4% C, 8.31% H, 8.93% N, 20.4% O, EF= C8H13NO2, MF= C48H78N6O12

Julia Mazzucato 4D

Hi guys, I wanted to create a thread for some good practice problems from the TB (not part of the HW questions) to study for the test in week 2. Feel free to share any good practice probs you find so we can all study together! Empirical and Molecular Formula derivation + Mass Composition practice Ma...

Julia Mazzucato 4D

L.39 A 1.50-g sample of metallic tin was placed in a 26.45-g crucible and heated until all the tin had reacted with the oxygen in air to form an oxide. The crucible and product together were found to weigh 28.35 g. (a) What is the empirical formula of the oxide? (b) Write the name of the oxide. This...

Julia Mazzucato 4D

Also, calculate significant figures using the values given to you in the problem. Scientific constants would not be used to determine the significant figures of your final answer. So you wouldn't use the SF in Avogadro's number or Planck's constant to calculate the SF in your answer. Always stick to...

Julia Mazzucato 4D

Are formula units used to describe ionic compound because they easily disassociate in water/aqueous solutions?

Julia Mazzucato 4D

I first calculated the total molar mass of the compound CuCL 2 4H 2 O, which came out to be 206.51 grams/mol. Dividing the given mass by this molar mass gave me 0.042 moles of the compound. Since the molar ratio of the compound to the oxygen is 1:4, you know you have 0.167 moles of Oxygen in the com...

Julia Mazzucato 4D

Thank you! Conversely, if anyone has any refresher tips for deriving the name of the compound from the molecular formula that would be much appreciated. For exmaple, being given NH₄NO₃ and having to figure out the proper suffixes/prefixes.

Julia Mazzucato 4D

Hi! I'm having trouble with question E.9 part a) from the textbook on page F45: Epsom salts consist of magnesium sulfate heptahydrate. Write its formula. Figuring out the correct molecular formula for a given name of a compound was unfortunately not my strong suit in HS chem as my teacher kind of gl...

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