CHEMISTRY COMMUNITY

Samantha Pedersen 2K

I like to think about this question in terms of Ka first. As we know, the expression for Ka is [products] / [reactants]. Stronger acids are ones that almost fully dissociate to form their conjugate bases and H3O+, which increases the equilibrium concentrations of the products and results in a relati...

Samantha Pedersen 2K

My habit is putting the concentrations of each species into the ICE box. I like to use concentrations rather than moles so that I don't have to remember to do an extra step at the end of converting moles into concentrations.

Samantha Pedersen 2K

I believe this is because the problem gives us the pH which already takes into account the fact that there are 2 moles of OH- in the solution. We use the pH to calculate the molar concentration of hydroxide ions in the diluted solution for i of Part A, then we use the resulting value to calculate th...

Samantha Pedersen 2K

I had this same question! Al3+ takes the form of Al(H2O)6 3+, and someone replied to my post letting me know that we can find the Ka value for Al(H2O)6 3+ in Table 6D.1 in Topic 6D of the textbook!

Samantha Pedersen 2K

First, look at where the curves flatten as this indicates that the reaction has reached equilibrium, and therefore will allow us to find the equilibrium Pa, Pb, and Pc values. Also, notice that the y axis is labeled in units of kPa. The equilibrium pressure for A is approximately 18 kPa, the equilib...

Samantha Pedersen 2K

Problem 6E.3 from the textbook says, "Calculate the pH of each of the following solutions of diprotic acids at 25 °C, ignoring second deprotonations only when the approximation is justified." For each part of the question, the solutions manual says, "Because Ka2 << Ka1, the second ion...

Samantha Pedersen 2K

Part B of problem 6D.15 asks us to calculate the pH of 0.055 M AlCl3 (aq), and we have to use Table 6C.1 and Table 6C.2 to determine the Ka value to use in the calculation. However, I don't see a Ka value or a Kb value of the conjugate base that could be used to determine the Ka value in either of t...

Samantha Pedersen 2K

I believe 1.0 x 10^-14 is an experimentally determined value for water at 25 degrees Celsius that is now a universally accepted value. I hope this helps!

Samantha Pedersen 2K

Dr. Lavelle mentioned in lecture today that we won't be expected to solve cubic equations in Chem 14B, so I think that means we won't get any problems in Chem 14B where we have a cubic equation with K > 10^-3. If K < 10^-3 and you have a quadratic function, then you can still make the assumption tha...

Samantha Pedersen 2K

If you look at Table 5G.2, you'll see that the reaction 2BrCl(g)+H2(g)⇌Br2(g)+2HCl(g) isn't listed. However, we can get to 2BrCl(g)+H2(g)⇌Br2(g)+2HCl(g) as an overall reaction by "adding" the reactions 2BrCl(g)⇌Br2(g)+Cl2(g) and H2(g)+Cl2(g)⇌2HCl(g) together, and these two reactions are li...

Samantha Pedersen 2K

If you go to the Chem 14B website and click on the "Constants and Equations" document, you can see that the constants and equations sheet for Chem 14B is different from the one we used in Chem 14A. There isn't anything about Le Chatelier's principle on the Chem 14B constants and equations ...

Samantha Pedersen 2K

One of the learning objectives from Outline 1 says, "Use equilibrium constants to predict solubility." I'm guessing that if the equilibrium constant is large, then the compound is very soluble. If the equilibrium constant is small, then the compound isn't very soluble. Is this correct? Is ...

Samantha Pedersen 2K

This problem works out to have a cubic equation, and K is less than 10^-4. Dr. Lavelle mentioned in one of the Audio-Visual Focus Topic videos that this means we can assume 0.522-2x is just 0.522 and that 0.633-x is just 0.633. Oh, okay, I must've missed that! Do we assume that (that being 1.522-2x...

Samantha Pedersen 2K

This problem works out to have a cubic equation, and K is less than 10^-4. Dr. Lavelle mentioned in one of the Audio-Visual Focus Topic videos that this means we can assume 0.522-2x is just 0.522 and that 0.633-x is just 0.633. When you make this assumption, the equation becomes 5.66 x 10^-10 = 4x^2...

Samantha Pedersen 2K

I would definitely recommend doing all of the problems from the textbook that are listed in the outlines on Dr. Lavelle's website. I also like to star the ones that I got incorrect on my first try so I pay special attention to them when reviewing textbook problems before exams. I also highly recomme...

Samantha Pedersen 2K

For this problem, you actually don't need to set up or solve any calculations. Notice that the value of K is smaller at 700. Kelvin than at 600. Kelvin. This means that there must be a smaller number in the numerator (where the equilibrium concentrations of the products go) of the equilibrium consta...

Samantha Pedersen 2K

Problem 5.61 from the textbook says, "The overall photosynthesis reaction is 6CO2 (g) + 6H2O (l) ---> C6H12O6 (aq) + 6O2 (g), and \Delta H = +2802 kJ. Suppose that the reaction is at equilibrium. State the effect that each of the following changes will have on the equilibrium composition: tends...

Samantha Pedersen 2K

Some of the textbook problems for Outline 1 ask us to refer to Table 5G.2 to find the equilibrium constant for a certain reaction and use it to solve the problem. For each reaction, there is a K value and a Kc value listed in Table 5G.2 and I've noticed that the two values are sometimes different. H...

Samantha Pedersen 2K

Problem 6.21 from the textbook says, "The two strands of the nucleic acid DNA are held together by hydrogen bonding between four organic bases. The structure of one of these bases, thymine, is shown below. c) Mark with an asterisk any structure that can show amphiprotic behavior in aqueous solu...

Samantha Pedersen 2K

In lecture today, I thought I heard Dr. Lavelle say that (SO4)2- could pull a proton off of water to make a solution basic. I thought that Na2SO4 would be a neutral salt because Na+ comes from a strong base (NaOH) and (SO4)2- comes from a strong acid (H2SO4). I also found a few sources on the Intern...

Samantha Pedersen 2K

Hi! I've been using this PDF from Dr. Lavelle's website (https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14A/NamingCoordinationCompounds.pdf) and this website (http://www.chemistry.wustl.edu/~edudev/LabTutorials/naming_coord_comp.html) for common ligands that we need to memorize. I hop...

Samantha Pedersen 2K

One of the learning objectives on Outline 6 says, "Explain why solutions of weak acids have higher pH values than solutions of strong acids at the same concentration." It intuitively makes sense to me that solutions of weak acids have higher pH values than solutions of strong acids at the ...

Samantha Pedersen 2K

The textbook gives an easy mathematical way to find the oxidation state of transition metals in coordination compounds. I'll use Khoa's example of [Ag(NH3)2]+. 1. Multiply the quantity of each atom or ligand by its charge. You can represent the charge of the transition metal as X, because this is wh...

Samantha Pedersen 2K

Salts do not have to be either a weak acid or a weak base, some salts can be neutral. For example, Dr. Lavelle said that Na+ and Cl- both do not affect the pH. Therefore, NaCl is a neutral salt. Salts that act as weak acids include ones where nitrogen has 4 bonds and one of them is a hydrogen, ones ...

Samantha Pedersen 2K

Polydentate ligands must have at least 2 lone pairs available to be donated and typically 1-2 spacer atoms between the atoms with the lone pairs to be donated. The atoms with the lone pairs and the spacer atoms are also usually connected by sigma/single bonds so that the ligand can rotate to orient ...

Samantha Pedersen 2K

According to the Registrar's website, professors have until December 28 at 11:59 PM to submit final grades. I'm not sure how long it'll take for this class specifically though!

https://www.registrar.ucla.edu/Faculty- ... -Deadlines

Samantha Pedersen 2K

Are unhybridized orbitals always located perpendicular to the hybridized orbitals in a molecule? If so, why is this the case? Thank you!

Samantha Pedersen 2K

Yes, you should put a Greek prefix before each ligand of which there is more than one. I believe you're correct that the ligands would be written as pentaamminedichloro in this case!

Samantha Pedersen 2K

One of the learning objectives on Outline 5 says, "Identify the shape and draw structures of coordination compounds." In lecture today, I believe Dr. Lavelle mentioned we wouldn't be asked to draw something like myoglobin or hemoglobin. What types of coordination compounds should we know h...

Samantha Pedersen 2K

In addition to having multiple lone pair donating sites, I believe a polydentate ligand must also have bonds that can rotate (single bonds/sigma bonds) so that the atoms with the lone pairs can orient themselves to be on the same side to bind to the transition metal. I hope this helps!

Samantha Pedersen 2K

Remember that electrons always want to be in the lowest energy state. Spin pairing in the hybridized orbital causes electron-electron repulsion which is a high energy, unfavorable state. Putting the last electron in the unhybridized orbital by itself is a lower energy state that is more favorable. I...

Samantha Pedersen 2K

Problem 9C.3 Part D from the textbook asks us to write the formula for sodium bisoxalato(diaqua)ferrate (III). I have two questions about this. First, why is bisoxalato written before diaqua? This doesn't seem to follow the alphabetical rule. Second, the textbook gives the answer as Na [Fe (OH2)2 (C...

Samantha Pedersen 2K

In addition to redoing the textbook problems and reviewing UA worksheets, I'm planning to make flashcards from my notes to test myself on the conceptual information. Testing myself helps me identify the topics I'm not as confident about so I know to study those more. I'm also planning to go through ...

Samantha Pedersen 2K

One thing I find really helpful when determining bond angles is to consider the number of regions of electron density. For example, consider a molecule with 3 bonding pairs and 1 lone pair. This has four regions of electron density, so the "parent structure" is tetrahedral. Tetrahedral mol...

Samantha Pedersen 2K

One of the learning objectives on Outline 3 says, "Explain why lone pairs are more likely to be found in certain locations around a central atom and how and why they affect the bond angles in a molecule, cation, or anion." I know Dr. Lavelle said that in a seesaw shape molecule, the lone p...

Samantha Pedersen 2K

Hello, will we be expected to recognize T-shaped molecular geometry as one of the VSEPR shapes? Thank you!

Samantha Pedersen 2K

Hello, I was just in Dr. Lavelle's office hours, and he said that the Midterm 2 results should be released tomorrow!

Samantha Pedersen 2K

In lecture today, Dr. Lavelle emphasized that the electrons in the hybridized 2sp^2 orbitals and unhybridized 2p orbital for ethene will all be unpaired. However, one of the four hybridized sp^3 orbitals for ammonia contained two paired electrons. Will we only have paired electrons in hybridized orb...

Samantha Pedersen 2K

My understanding is that the energy difference between the hybridized 2sp^2 orbitals and the unhybridized 2p orbital is lower than the energy difference between the separate 2s and 2p orbitals. The electrostatic repulsion between the two electrons originally in the same orbital is now greater than t...

Samantha Pedersen 2K

Also, someone mentioned above that Lavelle is dropping 10 points from the first midterm to make it out of 100 rather than out of 110, does anyone have more information to confirm/deny this? I think someone said in the GroupMe that they went to his office hour one time and he said that was false. Ho...

Samantha Pedersen 2K

Since the formula for electron affinity is (energy of neutral atom) - (energy of anion), is it correct to say that a positive value for electron affinity means that energy is released when the electron is added to the atom? And a negative value for electron affinity means that energy must be added t...

Samantha Pedersen 2K

I think some other factors that affect the energy of an electron in a multi-electron atom are its principal quantum number (n) and its angular momentum quantum number (l). This is because electrons with a higher value of n are farther away from the nucleus and therefore have higher energies. The val...

Samantha Pedersen 2K

Anna Yakura 3E wrote:When/where did we learn this exception? Just wondering bc this is the first time I heard of it oops

The first place I saw it was Problem 1.13 from Outline 2 of the textbook, and the solutions manual does a good job of explaining it. I hope this helps!

Samantha Pedersen 2K

Effective nuclear charge is the net positive charge experienced by an electron. Electrons in outer shells experience a lower effective nuclear charge because the inner electrons are shielding them from the positive attraction of the nucleus. There are also periodic table trends for effective nuclear...

Samantha Pedersen 2K

The other answer does a great job of explaining the process! I just wanted to add that the electron configuration for P^(3-) would actually be [Ne] 3s^2 3p^6 because the 3- charge indicates that there is a gain of 3 electrons. Therefore, we add 3 electrons to the outermost subshell of the regular el...

Samantha Pedersen 2K

The other answers are correct that you should always try to minimize formal charge as much as you can. I just wanted to add that you also need to make sure the central atom can hold an expanded octet when you do this! In this case, sulfur can hold an expanded octet because it is in Period 3 and ther...

Samantha Pedersen 2K

I was reviewing this question: "Explain the order of electron filling in atoms of K, Ca, and Sc." My understanding is that the 4S subshell is lower in energy than 3D when it is empty, so it fills up first in K and Ca. But would the 4S or 3D subshell fill first in scandium, and why? Thank y...

Samantha Pedersen 2K

I came across the following question: "Answer the following questions related to the ground state electron configuration of titanium. What is the highest energy subshell?" I know the electron configuration for titanium is [Ar] 3d^2 4s^2, so I thought that the highest energy subshell would ...

Samantha Pedersen 2K

When molecules collide, one atom repels the electrons away from another, creating a partial positive charge on the second atom. The partial positive charge on that atom creates a partial negative charge on the atom that it is bonded to (because the electrons are repelled away from the first atom tow...

Samantha Pedersen 2K

Ionization energy is the energy required to remove an electron from an atom in the gas phase whereas electron affinity is the amount of energy released when an electron is added to an atom in the gas phase. You can also think of ionization energy as how difficult it is to remove an electron from an ...

Samantha Pedersen 2K

Remember that copper is one of the exceptions that doesn't follow the typical pattern for electron configurations, so the electron configuration for Cu (not the ion form) is [Ar]3d^10 4s^1. The 4s subshell is higher energy than the 3d subshell in this case, and electrons are always removed from the ...

Samantha Pedersen 2K

Yes, the structure that has the most atoms with a formal charge of 0 has a lower energy than structures with fewer atoms that have formal charges of 0. This is because it takes energy to separate different charges (like +1 and -1), so structures that don't have a lot of different charges don't requi...

Samantha Pedersen 2K

Once you have determined the value of L (the second quantum number), the allowed values of ml (the third quantum number) are L, L-1,... all the way down to -L. For example, if L is equal to 1, then ml can be equal to any of 1, 0, or -1. Then, we arbitrarily decide that ml=1 represents the Px orbital...

Samantha Pedersen 2K

In addition to doing textbook problems and attending Workshop sessions, I found it helpful to create a Quizlet to test myself on the conceptual information that Dr. Lavelle discusses during lecture. Best of luck on midterm 2!

Samantha Pedersen 2K

I highly recommend watching Lenox Hill on Netflix, especially for all my pre-med friends out there!

Samantha Pedersen 2K

Sulfur can have an expanded octet because it is in Row 3 of the periodic table, meaning that the quantum number n is equal to 3. When n=3, l can be 0, 1, or 2 according to the rules for quantum numbers. This means that elements in Row 3 have the S, P, and D subshells available to them to hold electr...

Samantha Pedersen 2K

What I did to memorize the prefixes for SI Units was make a Quizlet set including the prefixes and the values they represent. Then, I tested myself on the Quizlet using the Learn and Write options until I felt confident.

Samantha Pedersen 2K

My understanding is that delocalized electrons are shared equally among multiple atoms rather than confined to a single atom. This means that the charges and energy of the electrons are spread over a wider area than they would be if the electron was confined to a single atom. Therefore, the energy a...

Samantha Pedersen 2K

What is the maximum number of electrons that sulfur can have in bonding? I know that elements in row 2 can have 8 electrons because they have the S and P orbitals available to them (4 orbitals total with 2 electrons each), so I would guess that elements in row 3 like sulfur can have up to 18 electro...

Samantha Pedersen 2K

The overall charge of the nitrate molecule is -1 which indicates that there is an additional electron. Therefore, Dr. Lavelle added one additional electron to the total number of valence electrons from one nitrogen atom and three oxygen atoms to get 24 electrons.

Samantha Pedersen 2K

The Organic Chemistry Tutor on YouTube also has a lot of helpful videos on quantum numbers (and many other 14A topics)! Here is a link to one of the videos on quantum numbers: https://youtu.be/wnLcJH-Heyo

I hope this helps!

Samantha Pedersen 2K

Ionic bonds are formed when electrons are transferred from a metal atom to a nonmetal atom whereas covalent bonds are formed when electrons are shared between atoms. Substances with ionic bonds form crystals whereas substances with covalent bonds form molecules. Finally, substances with ionic bonds ...

Samantha Pedersen 2K

The uncertainty in position uses the same units as the units for position itself. The position of the electron is measured in meters, so the uncertainty in position is measured in meters too. I hope this helps!

Samantha Pedersen 2K

While studying the textbook, I would make sure to focus on the conceptual information related to what we've discussed in lecture in addition to solving the textbook problems. My guess is that the midterm will have conceptual questions in addition to the calculation-based questions.

Samantha Pedersen 2K

I believe the other answers are correct that we don't need to do any math using the Schrodinger Wave Function Equation for our midterm. I just wanted to add a little bit about the conceptual information behind the Schrodinger Wave Function Equation. The wave function is denoted by \psi , and the wav...

Samantha Pedersen 2K

I understand that the gaps between energy levels get smaller as you move farther away from the nucleus. Why exactly is this the case? I'm guessing it might have to do with electrostatic potential energy and the effective nuclear charge, but I'm hoping someone can clarify this for me. Thank you!

Samantha Pedersen 2K

In lecture today, Dr. Lavelle mentioned that isoelectronic atoms and ions have very different properties. I'm wondering why this is the case. If they have the same electron configurations and thus the same number of valence electrons, why do they behave differently?

Samantha Pedersen 2K

I agree with everyone that practice problems are your best friend! I also wanted to add that I went back through my lecture notes to make a review sheet of conceptual information since the midterm will most likely have a few conceptual problems in addition to the calculation-based problems.

Samantha Pedersen 2K

Orbitals can also be thought of as the valid solutions/wave functions from Schrodinger's Wave Function Equation. Orbitals describe where electrons are most likely to be found in an atom.

Samantha Pedersen 2K

c = 3.00 x 10^8 m.s^-1 is the speed of light. We often use it in the equation c = \lambda \nu where \lambda is the wavelength and \nu is the frequency. For example, if you are given the frequency of a wave and you want to calculate the wavelength, you can divide the speed of light (3.00 x 10^8) by t...

Samantha Pedersen 2K

In addition to the great resources mentioned above, you can review the pre-assessment and post-assessment of the Audio-Visual Focus Topic on Limiting Reactant Calculations for more practice. The Workshops and Step-Up Sessions are also a great way to do additional practice problems that are represent...

Samantha Pedersen 2K

When waves interact while they are out-of-phase, they do not form a complete circle and they create an unstable state. In-between energy levels (such as 1.4 or 2.6) are represented by these incomplete circles and thus are unstable states. Whole number energy levels (such as 1, 2, 3, etc.) are repres...

Samantha Pedersen 2K

In lecture today, Dr. Lavelle mentioned that the mass of the object dominates over the velocity in determining what the wavelength of the object is. Why is it the case that the mass plays a larger role than the velocity in determining the wavelength of the object? Are there any cases where the veloc...

Samantha Pedersen 2K

Dr. Lavelle mentioned in lecture today that we will consider anything shorter than 10^-15 meters undetectable. I hope this helps!

Samantha Pedersen 2K

One key difference is that the photoelectric effect causes electrons to be completely ejected from the atom they were part of (so that they are no longer interacting with the nucleus), while atomic spectra deals with electrons making transitions between energy levels while still interacting with the...

Samantha Pedersen 2K

It looks like you may have calculated the molar mass of Na2CO3⋅10H2O incorrectly. The molar mass of Na is 22.99 g/mol, the molar mass of C is 12.01 g/mol, the molar mass of oxygen is 15.99 g/mol, and the molar mass of water is 18.02 g/mol. When you multiply these numbers by the number of their respe...

Samantha Pedersen 2K

The photoelectric effect is the phenomenon where light of certain frequencies projected onto a metal causes electrons to be ejected from the metal. The observation of this effect led to the theory that light has not only wavelike properties, but particle-like/photon-like properties too.

Samantha Pedersen 2K

We would actually use the sum of the absolute value of the positive and negative forms of the number. For example, if the problem says the uncertainty in position is +/- 5 meters, then you would plug in 10 meters for the uncertainty in position. This is because the position can be anywhere within 5 ...

Samantha Pedersen 2K

If the position determined is more precise, then the momentum determined will be less precise. If the momentum determined is more precise, then the position determined will be less precise. This is due to the Heisenberg Uncertainty Principle: the product of the uncertainty in position and the uncert...

Samantha Pedersen 2K

The photoelectric effect is the phenomenon where light of certain frequencies projected onto a metal causes electrons to be ejected from the metal. As for the equation, every metal has a certain energy threshold that each photon must meet in order to eject an electron. This energy threshold is denot...

Samantha Pedersen 2K

I know Dr. Lavelle mentioned that black body radiation is not on our syllabus, but the concept piqued my interest so I found some articles to learn more about the topic. Here they are for anyone else who might be interested: https://astronomy.swin.edu.au/cosmos/b/Blackbody+Radiation http://www.sun.o...

Samantha Pedersen 2K

Someone asked this question during a UA Workshop, and the UA mentioned that we wouldn't know how to find the molecular formulas for those structures without taking an organic chemistry class. Hopefully Dr. Lavelle or a TA can confirm this, but I think the formula would be given to us on a Chem 14A e...

Samantha Pedersen 2K

The entire solution weighs 510. g, and only 5.45% of that is the solute (or KNO3). You can use that fact to determine how many grams of KNO3 you'll need, then you can subtract the number of grams of KNO3 from the total mass of the solution to determine how many grams of water you'll need. I hope tha...

Samantha Pedersen 2K

What are the frequencies and wavelengths of the visible light spectrum? I did some searching and found that the wavelengths are between 380 nm and 740 nm while the frequencies are 4.0 x 10^14 Hz and 7.90 x 10^14 Hz, but I wanted to make sure those are correct. Also, what are the frequencies of each ...

Samantha Pedersen 2K

I believe the correct answer would be that a bowling ball does not have wavelike properties. In the video, Dr. Lavelle mentioned that the smallest wavelength we can detect is about 10^-15 meters. A bowling ball has a relatively large mass, so when we plug its mass into the De Broglie equation along ...

Samantha Pedersen 2K

The given information is "Light hits a sodium metal surface and the velocity of the ejected electron is 6.61 x 10^5 m.s-1. The work function for sodium is 150.6 kJ.mol-1" Question 29) How much energy is required to remove an electron from one sodium atom? I tried converting the work functi...

Samantha Pedersen 2K

I began solving this problem by converting the given mL of 2-butanone into grams of 2-butanone using the conversion factor d = 0.81 g/mL (given under the drawing of 2-butanone). Then, I converted the grams of 2-butanone into moles of 2-butanone using the molar mass of 2-butanone. To continue the pro...

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