CHEMISTRY COMMUNITY

Faaizah Arshad 1H

The answer key has a mistake; it should include a negative sign. It makes sense for the answer for this to be negative because when the [H30+] > 1 M, then the pH will be below 0. You are correct!

Faaizah Arshad 1H

Super acids are when the concentration of hydronium are above 1M and the concentration of hydroxide is smaller than 10^-14 M. Super bases are when the concentration of hydroxide is above 1M, so the concentration of hydronium is below 10^-14 M.

Faaizah Arshad 1H

An exothermic reaction is one in which heat is produced. If you think of "heat" as a product, then adding heat to an exothermic reaction would cause there to be more products than reactants so the reaction would shift to the left to produce more reactants.

Faaizah Arshad 1H

Volume doesn't change when adding an inert gas because you are not changing the amount of physical space there is of the container. You are simply adding gas that is unreactive to the container. There are more gas molecules that will collide (so the pressure has increased) but the space of the conta...

Faaizah Arshad 1H

If you add more reactant to a reaction that is already equilibrium, and you therefore perturb the system away from equilibrium, then according to LeChatelier's principle, the reaction will respond by producing products to minimizing the change and help return to the equilibrium state.

Faaizah Arshad 1H

For this question, the y-axis is shown in kPa. On Dr. Lavelle's constants and equations sheet, there is a conversion that says 101.3 kPa = 1 atm. Very similarly, 100 kPa = 1 bar. The textbook uses the second conversion to change kPa into bar.

Faaizah Arshad 1H

It's best to not have fractions as coefficients in your balanced chemical reaction. To turn the fractions into whole numbers, you can multiple the entire reaction by a common denominator.

Faaizah Arshad 1H

Changing pressure or volume does not change K. They can change the concentration of the substances in the reaction, but by the leChatelier's principle, the reaction will shift to minimize that change and return to the equilibrium concentrations of reactants and products. Changing temperature does ch...

Faaizah Arshad 1H

What are the bond angles for T shaped?

Faaizah Arshad 1H

Hi, are compounds containing silicon amphoteric (like SiO2)?
It seems like there wasn't a clear list of amphoteric compounds given, so I'm not sure which metalloids to identify as amphoteric.

Thanks

Faaizah Arshad 1H

Yes the ligand must follow the lone pair spacer spacer lone pair model in order to be a chelating ligand because that is the only formation that allows the ligand to "wrap around" a transitional metal cation

Faaizah Arshad 1H

You are correct that the second Chlorine is outside the brackets so it is not a ligand and is not part of the coordination sphere. However, you must take it into account when you are determining the oxidation number of the transition metal because the oxidation number of the transition metal will de...

Faaizah Arshad 1H

At least in the case of p orbitals, the px, py, and pz axes are 90 degrees from each other so that is why the orbitals will be at 90 degrees.

Faaizah Arshad 1H

That's a great mneumonic,. Heads up that HClO3 is also a strong acid; you left that out in the list.

Faaizah Arshad 1H

I'm not sure if you are asking in regards to a particular coordination compound (if you are, it would be helpful if you could write the entire formula), but in ethylenediamine, there are 2 NH2 and they are separated by two spacers so the entire molecule is bidentate because each N has a lone pair th...

Faaizah Arshad 1H

Why is formic acid stronger than acetic acid?
The textbook answer key says this: The Ch3 group in acetic acid has electron donating properties which means that it is less electron withdrawing than the H attached to the carbon in formic acid.

I'm struggling to understand this explanation. Thanks!

Faaizah Arshad 1H

Here's a list of the strong acids: HCl, HNO3, HI, HBr, H2SO4, HClO3, HClO4
Strong bases usually come from Group 1 and Group 2 hydroxides, like LiOH or Ba(OH)2

Faaizah Arshad 1H

Here is the link to the Final Review sessions: https://lavelle.chem.ucla.edu/wp-conten ... ns_14A.pdf

You can also locate it on the right of Professor Lavelle's Chem 14A website.

Faaizah Arshad 1H

A monodentate ligand is a lewis base that has one lone pair that it can donate, such as NH3. A bidentate or polydentate must follow the "lone pair, spacer, spacer, lone pair" rule. An example of a bidentate ligand is NH2CH2CH2NH2 because the two nitrogens have lone pairs that they can dona...

Faaizah Arshad 1H

Since shape can affect polarity and polarity can determine which intermolecular interactions a molecule can engage in, we can use shape and polar/nonpolar properties to predict IMFs. For example, in the case of water, since the molecular geometry is bent, the dipoles do not cancel, so water can exhi...

Faaizah Arshad 1H

The textbook puts 2 as the subscript for Zn(OH)2 and Zn(NO2)2. You can determine this by asking yourself what the ground state electron configuration of Zn. Since Zn has an e- configuration of [Ar]3d10 4s2, it will lose the two 4s electrons to reach a stable state. This means that Zn will become Zn2...

Faaizah Arshad 1H

In the textbook, anionic ligands were named as fluorido, chlorido, or bromido with the -ido at the end. However, in lecture and in the textbook answer key, the ide is completely dropped and replaced with o like chloro. When do you use -ido and when do you use -o?

Thanks!

Faaizah Arshad 1H

Yes, the ligands are in alphabetical order here. In the chemical formula, aqua came before oxalato.

Faaizah Arshad 1H

Yes, double bonds count as only one region of electron density.

Faaizah Arshad 1H

A delocalized pi bond is present in a molecule that has double bonds and also has resonance. For example, CO3 2- would have delocalized pi bonds because the multiple bond character is spread over three oxygen atoms.

Faaizah Arshad 1H

Dr. Lavelle briefly mentioned that hybrid orbitals are made from linear combinations, but he did not go into the details during lecture. The assigned homework problems do not discuss this either, so I would assume that we do not need to know it.

Faaizah Arshad 1H

Equatorial atoms are in the same plane in the middle of a molecule. Trigonal bipyramidal has 3 atoms in the equatorial plane. Axial atoms go up and down. They form 180 degrees in the trigonal bipyramidal geometry.

Faaizah Arshad 1H

Pi bonds form when p orbitals align side-by-side, so they are parallel to each other. If you rotate the bond, the p orbitals won't be parallel, so the bond will break.

Faaizah Arshad 1H

I would suggest attending his office hours which are on Monday from 3-4pm PST and Wednesday from 5-6pm PST. Also, I believe his email is: lavelle@chem.ucla.edu

Faaizah Arshad 1H

I don't think so because the syllabus does not say anything about the class being curved. Here's the syllabus, and the grade breakdown is on page 2.

https://lavelle.chem.ucla.edu/wp-conten ... SYLL_1.pdf

Faaizah Arshad 1H

CO2 is not a polar molecule, but it's bonds are polar. It has a linear structure so it's dipoles cancel, which makes it a nonpolar molecule.

Faaizah Arshad 1H

A molecule can hydrogen bond with itself if it has one partially positive hydrogen that is bonded to an O, N, or F, and if the molecule has a lone pair on a partially negative O, N, or F. A molecule can hydrogen bond with another molecule if one of the molecules has a partially positive hydrogen tha...

Faaizah Arshad 1H

One of the homework problems said to predict which is the strongest C—X bond: CF4, CCl4, or CBr4. The answer is CF4 because it is the shortest of the three bonds. However, the answer key further says: "Note that electronegativity and polarity arguments would predict the C—F bond to be the weake...

Faaizah Arshad 1H

Hi, I came across this question while practicing and am not sure how to approach it or what would be the correct answer.

What is the order of the following cations from least to most polarizing power?
Cs+ , Ba2+, Li+, Ca2+

Thank you!

Faaizah Arshad 1H

I am still confused about this. For K and Ca, is the 4s subshell always lower energy, so 4s fills first? For Sc and all transition metals, is the 3d subshell always lower in energy, so 3d fills first?

Faaizah Arshad 1H

I am also very confused on this topic. When is 3d a higher energy level than 4s, and when is 4s a higher energy level than 3d? I'd appreciate if someone could respond to this with a comprehensive answer.

Thank you!

Faaizah Arshad 1H

Ground state electron configurations are when electrons follow the configuration that corresponds to the lowest total energy and that follows the building up principle (Hund's rule & Pauli Exclusion principle). An excited state is when an electron is in a higher energy state than predicted by th...

Faaizah Arshad 1H

No, not all molecules are involved in dipole-induced dipole interactions. These interactions only occur when a polar molecule that has a permanent dipole (due to partial charges) comes near a nonpolar molecule that does not have a dipole moment. The polar molecule will cause the electron densities o...

Faaizah Arshad 1H

Here are the exceptions to the octet rule:

Less than an octet:
H, He, Be, Li, radicals

Expanded octet:
P, S, Cl, and atoms in period 3 or below that have a d orbital

Faaizah Arshad 1H

Dipole dipole interactions occur between two polar molecules that have permanent dipoes. When the polar molecules come near each other, they will orient so that the partial positive charge of one molecule aligns with the partial negative charge of the other molecule. This will increase attraction be...

Faaizah Arshad 1H

Ga is in the Group 13 column which is everything under Boron. Gaining 5 electrons in order to achieve the octet is a lot. It's unlikely that Ga will find 5 atoms to bond with. So, it will just lose 3 electrons and become a cation, just like Boron would. It will lose the 3 electrons from the 4p and 4...

Faaizah Arshad 1H

I think Lone stands for both lone pairs and unbonded electrons.

Faaizah Arshad 1H

Midterm 2 material covers only up to Week 6 Wednesday (11/11/2020).

Week 6 Friday was the start of a new section called Molecular Shapes & Structure.

Faaizah Arshad 1H

Yes, 1D is the start of atomic orbitals and quantum numbers, so that will be on the second midterm.

Faaizah Arshad 1H

When we say that electrons are delocalized, we mean that the electrons for a multiple bond in a resonance structure are equally likely to be anywhere on the resonance structure. The delocalized electrons are not associated with a particular atom or covalent bond. If they were, then you would have si...

Faaizah Arshad 1H

In homework problem 2A.1, we're asked to give the number of valence electrons including d electrons for elements. Why is the number of valence electrons for Sb 5 electrons instead of 15 electrons? I get that there are 5 electrons due to 5s2 and 5p3. But, why are we not counting the electrons in 4d10...

Faaizah Arshad 1H

I think the main takeaway is to make sure that the formal charges are as close to zero as possible. Sometimes, you'll draw a Lewis structure that has the correct number of total valence electrons, but it's not the best structure because the formal charges could be closer to zero. Apart from that, it...

Faaizah Arshad 1H

This connects to the idea of electron deficient atoms that Professor Lavelle discussed in lecture. Boron and Aluminum tend to form molecules in which they have six valence electrons and not the full octet. So, if you drew AlCl3 (for example) then you'd noticed that Al forms three bonds (one with eac...

Faaizah Arshad 1H

Effective nuclear charge refers to how much positive nuclear charge a valence electron experiences in an atom. Shielding refers to the blocking of positive charge from the nucleus to a valence atom. As the number of shells in an atom increases, the amount of shielding increases as well since the cor...

Faaizah Arshad 1H

Double bonds have more electrons that are strongly attracted to the nuclei of the atoms that are bonding, so they pull the atoms closer, which means that the length of the bond between the two atoms is shorter. In single bonds, fewer electrons are shared between two atoms, so there is less attractio...

Faaizah Arshad 1H

The number tells you what shell the electron is in. It is the principal quantum number (n).

Faaizah Arshad 1H

A shell is all orbitals that have the same value of n. For example, the n = 2 shell. A subshell is all orbitals with the same value of both n and l. For example, the n=2 shell can have l=0,1. A subshell could be 2s, or a subshell could be 2p. Orbitals have the same value of n, l, ml. For example, th...

Faaizah Arshad 1H

Two electrons in the same orbital will be paired because they must have opposite spins. One will have an up spin and the other a down spin. Electrons in different orbitals can be parallel because they can have the same spin. Electrons in the same orbital cannot be parallel because that would oppose ...

Faaizah Arshad 1H

This is an interesting question! I researched into it and found that a photon is the smallest possible particle of LIGHT, whereas a quanta is the smallest particle of ANY atomic/subatomic substance (electrons and neutron). So, a photon is a quanta, but not all quanta are photons. http://www.quantump...

Faaizah Arshad 1H

Two electrons must be in opposite spins in one orbital. By knowing the spins and the number of electrons in an orbital (whether the orbital has paired or unpaired electrons), we can determine if there is a magnetic field.

Faaizah Arshad 1H

The 2px state has a yz nodal plane, the 2py state has an xz nodal plane, and the 2pz state has an xy nodal plane. Nodal planes are areas where the probability of finding an electron is zero.

Faaizah Arshad 1H

I don't think we need to memorize how d-orbitals look. s and p orbitals are the main ones that we've discussed in lecture, and the homework problems do not require us to draw any orbitals. It is more helpful to understand what n, l, ml, ms are, and how they tell you the state of an electron.

Faaizah Arshad 1H

I suppose that we can think of it that way. If the problem provided a value for frequency or wavelength, then you would base your significant figures on that.

Faaizah Arshad 1H

For this question, you want to use deBroglie's equation. Convert the given wavelength into meters and plug it in as wavelength in deBroglie's equation. Use the electron's mass (9.11 x 10^-31 kg). Planck's constant is 6.626 x 10^-34 J-s. Then, solve for velocity. Once you solve for velocity, you can ...

Faaizah Arshad 1H

For this question, my calculations showed that the wavelengths of a proton and neutron are the same up to 3 significant figures, which makes sense because protons and neutrons are very close in mass.

Faaizah Arshad 1H

Does increasing amplitude of a wave mean increasing the frequency and decreasing the wavelength? Or is amplitude unrelated to frequency and wavelength?

Thanks!

Faaizah Arshad 1H

It would be helpful if you could rewrite the exact question here or post a screenshot. The Sapling homework is shuffled, so we don't know which problem you're referring to :)

Faaizah Arshad 1H

We have not learned this in lecture, and it will not be covered on the first midterm. Dr. Lavelle did say that the first 19 Sapling homework problems are content that we will be tested on; however, the system shuffled the questions in Sapling. I would suggest taking a look at section 1D in the textb...

Faaizah Arshad 1H

Hi! We will not be submitting our written exam work. Yes you should print the constants and equations sheet for the exam, once Dr. Lavelle finishes updating it!

Faaizah Arshad 1H

From what I've been told, because we have not learned nomenclature and naming compounds, the formula of unfamiliar compounds should be given to us.

Faaizah Arshad 1H

This is not one of the homework problems listed on Professor Lavelle's syllabus, which means that it does not directly correlate to the content we've learned. I would not worry about solving it. We may learn more about J.J. Thomson's cathode rays experiment in future chemistry classes! :)

Faaizah Arshad 1H

I use the Rydberg's constant given in Hz to find the frequency, and then convert the frequency to wavelength using c = lambda x nu

Faaizah Arshad 1H

I think the green walls are the nodal planes, which are regions around the nucleus where the probability of finding an electron is zero. They are located between the lobes. s orbitals do not have nodal planes because they are spheric symmetric, so the probability of finding an electron does not depe...

Faaizah Arshad 1H

Can someone please explain this problem that was in the Heisenberg Indeterminacy Equation Post-Module Assessment? Two students are 'studying' together when Bob says "I do love you Alice" and Alice responds "But how can I be sure?" Would using Heisenberg's uncertainty principle he...

Faaizah Arshad 1H

This means +/- 7.5 because the uncertainty is 15 Angstroms total. If it was +/- 15, the question would have said that the position is known within 30 Angstroms.

Faaizah Arshad 1H

An electron is confined within the radius of an atom because it can occupy different quantized energy levels and orbitals that make up the radius.Therefore, this size is the uncertainty of the electron's position.

Faaizah Arshad 1H

It would be great if we were given a direct conversion from miles to meter. However, even if that is not given, you can use prior knowledge to set up conversion factors: 1 mile x (5280 feet / 1 mile) x ( 12 inches / 1 foot) x (2.54 cm / 1 inch) x (1 meter / 100 cm) = meter It's longer, but it will s...

Faaizah Arshad 1H

I believe that the only constants and equations that will be given to us on the exam are the ones in the "Constants and Equations" PDF posted on Professor Lavelle's website. I've attached it here: https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14A/constants_equations.pdf Any...

Faaizah Arshad 1H

To respond to your second question about what region of the EM spectrum the wavelength corresponds to: Once you calculate the answer, you'll find that the wavelength is 605.8 nm. It's important to remember that visible light ranges approximately 400 nm to 700 nm. Since 605.8 nm lies within that rang...

Faaizah Arshad 1H

Step 1) I like to first determine what variable I need to solve for. We are looking for the uncertainty in the speed of the electron, which is delta v. Step 2) We are told that the uncertainty in the position of the electron is 1% of the hydrogen atom radius (0.05nm). 0.01 x 0.05 nm = 0.0005 nm Conv...

Faaizah Arshad 1H

Hi Natallie, I found this challenging the first time I did it as well. Here's a step by step method to solve it: Step 1) When you read this problem the first time, you should be able to identify that we are discussing atomic spectroscopy in which a hydrogen atom is emitting a certain wavelength of l...

Faaizah Arshad 1H

Delta x would be 10 meters. I like to remember that "delta" means change. Since the change in your position can be +5 meters or -5 meters, the overall change is 10 meters.

Faaizah Arshad 1H

In today's lecture, Professor Lavelle shows us the different colors of wavelengths that are emitted by the machine. Why does he say that the electrons are returning to ground state? If we are looking at the visible region, and therefore the Balmer series, shouldn't the electrons be returning to the ...

Faaizah Arshad 1H

Does atomic spectroscopy support the idea that electrons have wavelike properties or particle like properties, or neither?

Thank you!

Faaizah Arshad 1H

I understand that if only absorbed wavelengths of light are detected, then the experiment is called atomic absorption spectroscopy, while if emitted wavelengths of light are detected, then the experiment is called atomic emission spectroscopy. However, isn't the only way to detect the wavelength tha...

Faaizah Arshad 1H

Yes, the 6.626x10^-34 is the value of h, which is also known as Planck's constant.
We do not need to memorize constants or equations. They will be provided as a formula sheet on the exam.

Faaizah Arshad 1H

Yes, I think that's a great analogy. If the bridge is not long enough (the incoming photon does not have a high enough frequency), then the electron cannot walk from n=1 to n=2. If the bridge is long enough (the photon has a high enough frequency) and matches the exact difference from one energy lev...

Faaizah Arshad 1H

Normally the question specifies if it wants to wavelength in nanometers. If it doesn't specify, then I assume meters should be fine. Otherwise, it doesn't hurt to write it in both meters and nanometers :)

Faaizah Arshad 1H

Start by recalling the equation of the photoelectric effect: Kinetic energy = hv - work function We're given the wavelength. I would convert the wavelength (nm) into meters, and then convert that into the frequency. 194 nm (10^-9 m / 1 nm) = 1.94 x 10^-7 m 2.998 x 10^8 m/s / 1.94 x 10^-7 m = 1.55 x ...

Faaizah Arshad 1H

Do both constructive interference and destructive interference result in a diffraction pattern? Or is it only destructive interference that results in diffraction.

Thank you!

Faaizah Arshad 1H

A. What is the kinetic energy of the ejected electron. Step 1: Kinetic energy expression: 1/2mv^2 Step 2: Think about what information is given to you or what information you know that you can plug into the kinetic energy expression. We are told v = 6.61 x 10^5 m/s We know from prior calculations th...

Faaizah Arshad 1H

You shouldn't have to use the molar mass of sodium. You should use the mass of an electron, because the problem tells us that we are ejecting an electron of a certain velocity. This means that we are discussing the kinetic energy of an electron, not of the sodium atom. The mass of an electron = 9.11...

Faaizah Arshad 1H

The m value in the kinetic energy expression is going to be the mass of an electron, which is 9.11 x 10^-31 kg because the problem tells us that we are ejecting an electron of a certain velocity. You should not have to do any additional conversions with the velocity because its units are already in ...

Faaizah Arshad 1H

To solve Heisenbeg uncertainty problems, we will use the following equation: Delta p x Delta x >= h / 4 pi To find Delta p: The problem tells you that 1.7 x 10^-15 m is the electron's uncertainty in position, which is delta x. Our calculation then looks like this: Delta p x (1.7 x 10^-15) >= (6.626 ...

Faaizah Arshad 1H

The photoelectric effect suggests that you can eject an electron from a metal surface if photons that strike the metal have an energy that is equal to or greater than the work function of the electron. Any excess energy is emitted as kinetic energy of the electron. A. What is the kinetic energy of t...

Faaizah Arshad 1H

One thing that is common between the lines of a series is their basic principal quantum number.

There are two series :
Lyman series: initial principal quantum number is n = 1
Balmer series: initial principal quantum number is n = 2

Faaizah Arshad 1H

First, you should find the energy per photon of the light emitted. You can do this by converting the given wavelength (1850nm) into frequency (v) and then using E = hv to find the energy. Now that you've found the energy per photon, a good way to calculate how many photons are generated is to set up...

Faaizah Arshad 1H

The first thing I did to solve this problem was determine what the formula of magnesium sulfate heptahydrate is. Magnesium sulfate is MgSO4 Hepta hydrate is "hepta" (7) and "hydrate" (H2O) : 7 H2O Therefore, the formula for magnesium heptahydrate is MgSO4 7H20 a) In order to solv...

Faaizah Arshad 1H

Here's how I solved this problem: 1. Find the moles of Cl- ions that result from NaCl. I used stoichiometry to make sure my units canceled. First convert grams of NaCl to moles of NaCl by dividing by molar mass of NaCl. Then convert moles of NaCl into moles of Cl- by knowing that in 1 mole of NaCl t...

Faaizah Arshad 1H

Hi Aaina, Magnesium sulfate heptahydrate would be MgSO4 7H20. When I'm unsure about naming compounds or molecules, I like to break their name apart and think of the prefixes or suffixes. Here, heptahydrate has the prefix "hepta" which means 7. Hydrate means water, so we're speaking of H20 ...

Faaizah Arshad 1H

I don't think it's mandatory to include units in every step of solving the problem, although they can help you track your calculations and cancel units. However, if you end up getting the final answer incorrect, you may gain extra partial credit if you also show units while solving the problem. The ...

Faaizah Arshad 1H

Hi Brendan, Yes this is correct. 5000 has 1 significant figure. The trailing zeros do not count here. 5000.0 has 5 significant figures. The zeros here do count because there is non zero number at the beginning and then a decimal, so you must count all the zeros after the nonzero number and to the ri...

Faaizah Arshad 1H

Because Molarity is defined as the number of moles of solute dissolved in one liters of solution, you must use Liters in your calculation. Therefore, the units for Molarity are mol/L. If you'd like to later provide your answer in mL then you can convert from Liters to mL. However, the actual calcula...

Faaizah Arshad 1H

I believe that since 7 is an integer, it should not affect the significant figures in your calculation. Integers and exact numbers do not affect sf. You would most likely be given an inexact measurement that could be used as the least number of significant figure to determine your answer. Take a loo...

Faaizah Arshad 1H

In Professor Lavelle's recent email, he specified that all the peer learning sessions start Monday, so I believe that is Week 1!

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