CHEMISTRY COMMUNITY

Chem_Mod

See pages 1 and 2 at https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14A/Chem14AB_TextBook.pdf Over 930 students submitted Sapling homework end of Week 1. For problems with anything: Attend a TA office hour or UA drop-in session and screenshare with them in zoom so they and other stude...

Chem_Mod

For this problem, you know that because of conservation of energy, the energy of the light hitting the metal = the work function (energy required to remove an electron from the metal) + kinetic energy of the electron. The problem gives you the frequency of the incident light, which you can use to fi...

Chem_Mod

The reason you can't just use the frequency it takes to eject an electron from a metal surface is because the electron itself has a velocity of 3.6 * 10 ^6 m/s. The electron thus has kinetic energy, and because of conservation of energy, you need to take this into account when trying to find the ene...

Chem_Mod

Let X = mass of NaNO3 and Y = mass of Na2SO4. Given this, we can assume that X + Y = 5.37g. We can determine the mass percentage of sodium in each of the two compounds. NaNO3 is 27.049% Na and Na2SO4 is 32.370% Na. We can now set up a separate equation since we know that 1.61g of the mixture is sodi...

Chem_Mod

The values for all constants will be provided on the exams on the constants and equations sheet.

Chem_Mod

Hello, Dr. Lavelle has emphasized that there will not be much material on this upcoming midterm and there are 102 hours of assistance outside of lecture for students to prepare for the upcoming midterm. That being said, he does not see the need to have any midterm review sessions. However, please at...

Chem_Mod

It is not known and makes little difference until one sees the questions as some will be very short, less than a minute, and other questions will take longer.

For example, a test with 20 short questions could take less time than a test with 8 long questions.

Chem_Mod

This post should make it clear:

viewtopic.php?f=17&t=64223

Chem_Mod

I already answered this in detail at:

viewtopic.php?f=157&t=64458&p=251309&sid=f933ead3632d22ad663eaa423116dd98#p251309

Chem_Mod

Test is multiple choice taken on CCLE. (I didn't say there was partial credit. Maybe a student did.) Test is multiple choice and there will be more lower point short questions. Instead of 8 long questions there will be some very short questions and some intermediate length. This way the points are s...

Chem_Mod

Each photon must have the requisite energy required to eject a electron because one electron can only interact with one photon. Two photons cannot combine their energies and emit an electron if both have a lower energy than required. Photon energy is not constant, and that is seen in the Einstein eq...

Chem_Mod

You would not use the de Broglie equation to calculate the wavelength of electromagnetic radiation (photons). You would use c = lamda*nu. The de Broglie equation is used to calculate the wavelengths of particles with rest mass. You can, however, calculate the de Broglie wavelength of the emitted ele...

Chem_Mod

The midterm will cover material up to the point specified in the announcement. This includes material starting from the beginning of the course (including Fundamentals).

Chem_Mod

For part c, the photon needs enough energy to eject the electron and cause it to move with the given velocity. Therefore, the energy required is the work function plus the kinetic energy. The work function was found in part b, and 1/2mv^2 can be calculated with the given velocity and mass of an elec...

Chem_Mod

Hello! 9.11 x 10^-31 kg is the mass of an electron. It is listed on the equation sheet on his website.

Chem_Mod

Yes, the topic of atomic orbitals will be covered in week 3.

Chem_Mod

You can use either, depending on the information given in the question.

Chem_Mod

You should follow sig fig rules in your answers. See the "Everything you want to know about Sig Fig" page on his website for further details!

Chem_Mod

For part b, the question is essentially asking you what the value of the work function is. So you will be using the equation E = work function + KE to solve the problem. The given info gives you the KE of the ejected electrons and the frequency of the radiation (just convert this to energy) and so j...

Chem_Mod

Yes, because it says you need to know how to derive the equation on the learning outcomes, you will be expected to know how to do this.

Chem_Mod

Hello,

If you attended my Week 3 Workshop anytime from 8-10 AM on October 19, 2020, then this is for you!! I am very sorry but I made a small mistake on Q9. I have updated the answer key so please re-refer it. If you have any questions you can email me :)

- Riya Shah

Chem_Mod

This is a great question and the answer requires a fairly deep dive into quantum mechanics. I won’t be going into the math (much outside the scope of this class), but I will attempt to paint a solid picture. Spectroscopy is incredibly important throughout the pure and applied sciences and you'll be ...

Chem_Mod

You can derive the Rydberg formula using En = -hR/n^2. First set up delta E = E final- E initial. After simplifying the equation you can use the Bohr frequency condition (delta E = hv) to find frequency. We can use Rydberg's equation/En = -hR/n^2 because the frequency of an individual line in a spec...

Chem_Mod

Ask questions on any topic is TA Office Hours, or UA Drop-In Sessions, or Chemistry Community. Over 30 hours per week.

Discussion Sections, Step-Up Sessions, and Workshops are structured with pre-organized content.

Chem_Mod

Hi Ashley- one question: I am on track to graduate from UCLA with a bachelor's in Neuroscience in 3 years (as in 3 years at UCLA total) and I'm wondering if med schools will look down on this. I came into college with a lot of credits because I did a few college classes in high school and I feel th...

Chem_Mod

Quantum number l describes the subshell (ex: s, p, or d subshell) while m subscript l describes all the possible orbitals. To figure out how many orbitals there are in a subshell, you would use the 3rd quantum number (m subscript l). M subscript l = [-l, l] . Since l=2, therefore m subscript l = [-2...

Chem_Mod

The electron returns to its ground state.

The more interesting question is why not?

Ans. The excited state is not stable as the positively charged nucleus electrostatically pulls the negatively charged electron back down to the ground state.

Chem_Mod

Sapling includes the Etextbook and the Student Solutions Manual. All three for the price of one. If you have access to Sapling website then you already have access to the Etextbook and the Student Solutions Manual. Please ask TA, or UA, or another student to show you: The Ebook is on the right side ...

Chem_Mod

I got something going for Monday. Enjoy. :-)

Chem_Mod

You can solve for the energy of the emitted photon using planck’s equation E=hv or E=hc/lambda. Then, for parts b and c, you can use the energy given off per photon as a conversion factor for dimensional analysis to convert from grams of sodium to energy given off for the entire sodium sample.

Chem_Mod

You can always come to the UA workshops if you have the time. Check the website for timings.

Chem_Mod

You would have to use the velocity you solved for to calculate the kinetic energy given the formula 1/2mv^2 for 1 electron. Then use dimensional analysis to go from 1 electron to one mole of electrons by Avogadro’s number.

Chem_Mod

Correct. But there will be more shorter questions so it evens out.

Chem_Mod

This is very useful to students. Thank you Julia.

Chem_Mod

Thanks

Chem_Mod

Due to remote exams, they are multiple choice.

Chem_Mod

If 50% or higher is required to pass with a C- or higher.

Then to pass the class is not curved.

Chem_Mod

Midterm 1 covers material up to the end of Week 3.

Week 4 is not included. :-)

Enjoy your weekend.

Chem_Mod

The Heisenberg uncertainty principle (also known as indeterminacy principle) basically states that you can never know both the exact position and velocity at the same time. Because of this, if you know the position of an object, you will not know the exact momentum of the object. Likewise, if you kn...

Chem_Mod

Here, you should use the de broglie wavelength equation: wavelength = h / (m * v). Rearrange the equation so that you find the speed, or v, and plug the given values in.

Chem_Mod

Essentially it is saying that when an electron goes from a higher to a lower energy level, the difference in energy is given off as a photon. This is why the equation relates delta E to (h)(frequency)!

Chem_Mod

In the actual experiment, on the screen you see a bunch of bright slits and a bunch of dark slits. The bright slits are the constructive interference, and the dark slits are the destructive interference.

Chem_Mod

You can think of it both ways. So yes, the energy of the electron does depend on the level it is on. But you can also think of it as how attracted it is to the nucleus. If n=1, the electron is more strongly attracted to the nucleus than if n=20. Therefore, the n=1 electron would have a larger, but n...

Chem_Mod

Electrons bound in an atom have quantized energy levels while free electrons can have any amount of kinetic energy.

Chem_Mod

The Lyman series corresponds to UV light and n=1. The Balmer series corresponds to visible light and n=2. The smaller the energy, the longer the wavelength. UV has a smaller wavelength than visible light so visible light has the smaller energy.

Chem_Mod

The energy emitted by the electron is actually greater when going from n=2 to n=1 because the energy difference decreases the higher up you go. Therefore, this light emitted is UV light because of its high energy.

Chem_Mod

Hello! As a previous student of the class, I would suggest practice problems as much as possible whether that be assigned textbook problems, UA worksheets, Sapling, or the Audio-Visual topics!

Chem_Mod

Make sure to read this link posted on my class website:

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

Chem_Mod

In lecture, it is mentioned that at theoretical n = infinity, the Energy value is equal to 0. Therefore, as the principal quantum numbers go from larger to smaller values, the Energy is becoming more and more negative. When thinking about transitions between principal quantum numbers, for example, n...

Chem_Mod

Light intensity can be defined as photon energy per second per unit area. The intensity of light is also designated by the amplitude of the wave model. Therefore, increasing the number of photons would increase the intensity of the light, but the wavelength and frequency would remain the same since...

Chem_Mod

This $16 webcam looks good: https://www.amazon.com/Microphone-Reduction-Widescreen-Streaming-Conferencing/dp/B08925G8F1/ref=sr_1_27?dchild=1&keywords=webcam&nav_sdd=aps&pd_rd_r=b4b81bd4-8add-47db-8203-21adf885e825&pd_rd_w=RPPWF&pd_rd_wg=CT4ul&pf_rd_p=f8813af5-8d61-4988-b701-5...

Chem_Mod

Yes, so you know that you are dealing with an electron. So in this case, you are dealing with something that has mass. We learned in lecture that objects with mass also have wavelengths and frequencies, which is why we have the equation wavelength = h / mv . Therefore, all you need to do is to subst...

Chem_Mod

We will continue to cover Quantum topics next week, so you do not need to complete all of the textbook topics for quantum this week. You only need to do the questions that cover topics learned in lecture this week.

Chem_Mod

Students will not get full credit for an assignment if they give up on a question since they did not provide the correct answer. Students do have the ability to use hints and also multiple attempts until they get it right (with no reduced score after using hints and repeat attempts). Students are al...

Chem_Mod

This information was sent via email multiple times.

Make sure class email is not in your junk/spam/blocked email.

Details are also on my class website:

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

Chem_Mod

Read this thread:

viewtopic.php?t=63961

Chem_Mod

Hopefully students are finding low cost Webcams.

Did the above links I posted help?

Chem_Mod

Why did no international students attend my Wednesday 8 to 9pm zoom session?

This is not an easy time for me.

Should I change it to:
Wed 3 to 4pm
Wed 4 to 5pm
Wed 5 to 6pm

Thanks

Chem_Mod

Many on Amazon under $25. https://www.amazon.com/s?k=webcam&rh=n%3A172282%2Cp_n_price%3A1253503011&nav_sdd=aps&pd_rd_r=b4b81bd4-8add-47db-8203-21adf885e825&pd_rd_w=RPPWF&pd_rd_wg=CT4ul&pf_rd_p=f8813af5-8d61-4988-b701-57bd7e498604&pf_rd_r=20AP63Z6XQQJZDZ35FFF&qid=16028...

Chem_Mod

Chem_Mod

Kailyn,

Yes, as I discussed in class.

PS. To receive points, posts must be in correct topic.

Chem_Mod

Hello! Begin by finding the energy using the equation E=hc/wavelength. You can then use dimensional analysis by converting the mass of sodium to moles and then to atoms and multiplying by the E value found to get final units of Joules. For part c, use a similar approach by converting 1 mol into atom...

Chem_Mod

The ultraviolet spectrum of atomic hydrogen is the Lyman series, so n(1)=1. By using Rydberg's equation you can solve for n(2).

Chem_Mod

For part c, the incoming light needs enough energy to eject the electron as well as cause it to move with the given kinetic energy. Therefore, the energy of the photon is the energy calculated in part b added to 1/2mv^2. To find the wavelength, put in this value into the equation E=hc/wavelength. Fo...

Chem_Mod

The energy of light/electromagnetic radiation uses the equation E=hc/frequency. The intensity of light, as a particle, is the number of photons present at a given time. As a wave, the intensity is the amplitude squared.

Chem_Mod

Hello! Wavelength is equal to h/p as given by the equation sheet. P is equal to mass times velocity. So you can solve both parts of this equation by plugging in the constant h, the mass of each particle respectively, and the velocity given.

Chem_Mod

Hello! First begin by converting eV to J using the conversion factor listed on the equation sheet. By combining E=h(frequency) and c=(frequency)(wavelength) you can find the equation wavelength=hc/E. By plugging in the values of h and c as constants and the number calculated previously for E, you ca...

Chem_Mod

This can be calculated using the uncertainty principle. Use the equation delta x = 1/2 (h/(m)(delta v)) where h is a given constant, m is equal to 8.00kg and delta v is equal to 5.0 m/sec.

Chem_Mod

Identifing the different regions is expected. This was covered in class.

Chem_Mod

The conversion between eV and Joules will be given on the formula sheet.

Chem_Mod

The formula for kinetic energy is 1/2mv^2 where m is the mass of the electron in kilograms, and v is velocity in m/s. The question gives you the velocity of the ejected electron, and the mass of the electron will be given on the formula sheet. Therefore, you can plug in those values to find the kine...

Chem_Mod

Graphing calculators can store text because you can write letters and save a lot of material on it. In years past, scientific calculators have only been permitted, so it's highly likely that it's the same policy, even if it's not mentioned in the syllabus. I would ask your TA just to double check if...

Chem_Mod

Hi everyone, Hope you all are having a great Week 2! If you're coming to my workshop today, please download the attached worksheet. Reminder that you do not need to do the worksheet beforehand and answers will not be posted. If you can't make it to the workshop feel free the download the worksheet a...

Chem_Mod

Are the different lengths between n=1 n=2 and so on measured in nanometers. I don't understand how visible light has larger wavelength but appears to be shorter on the model shown in lecture number 6. The scale used for the Bohr model of the hydrogen atom is not length; rather, it is energy. An ele...

Chem_Mod

Hi, you can find out when the step-up session and the other peer learning sessions are on one of the links on Dr. Lavelle's website. I attached the link for you.

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

Chem_Mod

The group of lines that show infrared light being emitted by electrons going from n=# to n=3 is called the Paschen series.

Chem_Mod

Textbook (Chemical Principles) and Sapling are excellent.

I set them up as a learning tool with no penalty for using hints, repeat attempts, etc.

Chem_Mod

Wil Chai 3H wrote:
Chem_Mod wrote::-)
Thanks for the compliment!

I enjoyed it too but you have to bring back Portishead in the future!

Definitely!
Now you're giving away the answer to what many have asked. :-)

Chem_Mod

How do you find the molar mass of sulfide in M(OH)2? And how do you know there is sulfide in M(OH)2? Hi Seraphina, it seems like you may have misinterpreted the problem. M(OH)2 is a metal hydroxide. Since you are given its molar mass and you know the molar masses of O and H from the periodic table,...

Chem_Mod

Is there a way to figure out exactly how to rearrange the equation's E = hv and c = λv for a particular problem? You want to try to isolate the variable that you are solving for/is unknown and equate it to given or known quantities. This applies generally to chemistry, math, physics, etc. Mackenzie...

Chem_Mod

Sig fig rules still apply to these types of problems. Remember to take into consideration sig figs of measured quantities and not the sig figs of constants you use in the problem.

Chem_Mod

Magnetic fields do affect electrons but in a different manner, which you will learn more about in Physics 5.

Chem_Mod

How do you know what the threshold energy is to use in the equation when solving for the energy per photon? Is it a constant or do you solve for it depending on the given values in the problem? The threshold energy would be given in the problem, but sometimes you might have to solve for it if you a...

Chem_Mod

Here is the link to my post:
viewtopic.php?f=157&t=63679&p=246754

I also created this link with the same information:

Equipment Request https://lavelle.chem.ucla.edu/wp-conten ... equest.pdf

Chem_Mod

:-)
Thanks for the compliment!

Chem_Mod

This is always the case for Hydrogen. For this class, we only deal with Hydrogen because it has one electron. We would need to use a more sophisticated model for multi-electron atoms or systems.

Chem_Mod

If the electron drops from a higher energy state (n=2, n=3, etc) to the ground state (n=1), the light emitted falls into the UV region. This is known as the Lyman series. Dropping to the ground state has the largest energy difference (delta E). Since delta E= hv, a large delta E corresponds to a hig...

Chem_Mod

When plugging numbers into formulas or equations, make sure that those values are SI units like meters, seconds, etc. If the problem gives you nm (nanometers) then you must convert this to meters before plugging in the number to the equation. Here are some common prefixes: G (giga) = 10^9 M (mega) =...

Chem_Mod

Yes, you are correct!

Chem_Mod

There are different wavelengths of light, which would mean that there are different frequencies of light. Since the Energy = (Planck's constant) (frequency), not all bursts of light will have the same energy.

Chem_Mod

Just click on your name and you see many details. :-)

Chem_Mod

As communicated, there are two midterms in discussion section 50 minutes each. See Syllabus.

Chem_Mod

An electron that becomes energized, or excited, does move to a higher energy state. This means the electron transitions from a level close to the nucleus to a level farther away from the nucleus. The hydrogen spectral lines do correspond to this. For example, in the Balmer series, you will see lines...

Chem_Mod

The way the experiment is designed requires a conducting solid, and therefore surface needs to be a metal.
If I have time in class I may go into details, but we are already doing atomic spectroscopy. In a review session I'll go into more detail.

Chem_Mod

Your TA is also your proctor and if your TA can see your work area via Zoom proctoring then all good. If it is difficult for your TA/proctor to see your work area (desktop/tabletop area) then an external webcam is a better option. Webcams are available on Amazon, etc., for $10-$20. They will be even...

Chem_Mod

Students can attend any and all resources in my classes. Try them all and see which you prefer.

Chem_Mod

Your TA is also your proctor and if your TA can see your work area via Zoom proctoring then all good. If it is difficult for your TA/proctor to see your work area (desktop/tabletop area) then an external webcam is a better option. Webcams are available on Amazon, etc., for $10-$20. They will be even...

Chem_Mod

Your TA is also your proctor and if your TA can see your work area via Zoom proctoring then all good. If it is difficult for your TA/proctor to see your work area (desktop/tabletop area) then an external webcam is a better option. Webcams are available on Amazon, etc., for $10-$20. Independent of pr...

Chem_Mod

Your TA is also your proctor and if your TA can see your work area via Zoom proctoring then all good. If it is difficult for your TA/proctor to see your work area (desktop/tabletop area) then an external webcam is a better option. Webcams are available on Amazon, etc., for $10-$20. They will be even...

Chem_Mod

As I discussed in my Monday discussion section the organic chemistry content is covered in Chem 14C and 14D (and would not be tested in Chem 14A and 14B).

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