CHEMISTRY COMMUNITY

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According to the rate law, the reaction is first‑order in A and second‑order in B. If the concentration of a first‑order reactant is halved, then the reaction rate changes by a factor of 1/2. If the concentration of a second‑order reactant is tripled, then the reaction rate changes by a factor of (3...

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To find the time that will elapse as the concentration decreases from 0.0679 mol⋅L−1 to 0.0164 mol⋅L−1, you can use the integrated rate law for first‑order reactions. Don't forget to convert your answer to minutes in the end :)

Hope this helps

Can Yilgor 2D

The instantaneous rate of change of [A] at a particular moment is the slope of the tangent line at that point. The slope is the rise divided by the run. To find the slope of the orange tangent line, identify two sets of coordinates on that line. One point through which it is known that the orange li...

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I have been studying a lot lately, so I'm definitely exhausted :( I'm glad that the quarter is almost over.

Can Yilgor 2D

When the fast equilibrium step is the first step, and the slow step the second, we need a substitution equation for the intermediate, the reactant of Step 2 that was the product of Step 1. A + B <=> C + D (fast eq) D + E ==> F + G (slow) We need a substitution for [D]. Luckily, since Step 1 is fast...

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A straight line plot of "ln[reactant] vs time" means the reaction is first order.
A straight line plot of "1/[reactant] vs time" means the reaction is second order.
A straight line plot of "[reactant] vs time" means the reaction is zero order.

Can Yilgor 2D

Thank you for the clarification. Good luck to everyone :)

Can Yilgor 2D

I like to think of calculating instantaneous rate as finding the slope of the tangent line at that point. What other way could you get the slope of that tangent line? You could find the secant line that has the same slope. In other words, a time at 0 seconds and a time at 80 seconds will average ou...

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Here are the definitions of enthalpy and entropy that I have in my notes:

Enthalpy: the amount of heat released or absorbed at constant pressure (state property)
Entropy: the number of accessible states/positions that can be occupied (extensive property)

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I have been craving fried chicken lately :)

Can Yilgor 2D

The reduction reaction of the S.H.E. is 2H+(aq)+2e− => H2(g) At standard conditions, the potential of this reaction has been nominally set to 0.00 V. So, E naught for this reduction is 0.00 V. Because two electrons are being transferred in the reduction, n = 2. The reaction quotient for this reactio...

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Step - Up sessions go through step-by-step fundamentals working through prepared topics and worksheets. If you have difficulties following the lectures, I would suggest that you go to these sessions. Workshops cover more advanced material; therefore, I find it helpful to go to Step-Up sessions first...

Can Yilgor 2D

Why do we directly divide the pressure by the concentration in the Q expression? I thought we would have to convert the pressure to concentration by using the ideal gas equation.

Can Yilgor 2D

I believe it depends on the information given in the question. For example, if the question states that the reaction takes place at 298 K, it would help you save time to use the form of the equation E = E* - 0.05916 Volts/n(log Q). Otherwise, you would always get the right answer if you use the equa...

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To begin, determine the oxidation numbers of each element in each half‑reaction. For the reduction step of the first reaction, the oxidation number of chlorine changes from +7 to+3. Cl2O7⟶ClO−2 Next, balance the atoms of each half‑reaction. Balance every element other than oxygen or hydrogen first. ...

Can Yilgor 2D

Sapling doesn't seem to accept the answer with parentheses. Besides that, your stoichiometric coefficients look fine to me.

Can Yilgor 2D

Jessie Hsu 1C wrote:Apparently we calculate Q with P_H2/[H+]²!

Do you know why we calculate Q this way? Initially, I tried to convert pressure to molarity, but it didn't work.

Can Yilgor 2D

Thank you for the clarification! It took me 4 attempts to get the answer.

Can Yilgor 2D

For the first one: the phases of each species present should correspond to their phases as originally specified in the equation given in the question: MnO4- is (aq) S2- is (aq) H2O is (l) MnO2 is (s) OH- is (aq) S is (s) For the second one: First, begin by writing out the half-reactions. This is in...

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SophiaNguyen_2L wrote:Hi! So I actually solved this problem without taking out the Cl- and this made more sense to me. I sent my work below. Hope this helps!

Thank you! I was also very confused until I read your post.

Can Yilgor 2D

I agree with what others have said above. Just to provide a different perspective, when the forward reaction is favored, the concentrations of products increase, while the concentrations of reactants decrease. When the reverse reaction is favored, the concentrations of the products decrease, while t...

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I also think there shouldn't be a problem in posting CHEM 14A topics as long as what you are posting is relevant.

Can Yilgor 2D

I believe using Kelvin, in this case, is a safer option.

Can Yilgor 2D

From a thermodynamics standpoint, I believe K generally refers to the equilibrium constant and the absolute temperature in equations.

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Hello, we do not include solids and liquids in K expressions. This is because the molar concentration of a pure substance (solid or liquid) does not change throughout the reaction. We obtain the expression for K by writing the concentrations of the products in the numerator, raised to a power equal ...

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To pass, you will have to earn 50% of the total points, which corresponds to 200/400.

Hope this helps

Can Yilgor 2D

Hopefully, I will be taking CHEM 14C, LS 7B, and HIST 3C.

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Congratulations to everyone! I hope you had a relaxing weekend, and you are ready to get back to work :)

Can Yilgor 2D

I also felt very nervous before the test, but luckily, everything went just fine. I will try to remain calmer before the finals and remind myself that these tests will not be the last tests I will take :)

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Immi Lee - 1D wrote:
Joey_Okumura_1E wrote:I also got 490 J using the same process and was wondering the same thing.

My answer key says 4.90 x 10^2 and that's also what I calculated so I think you guys are fine!

Thank you for the clarification! Since I didn't see this error on the solution manual errors sheet, I was very confused.

Can Yilgor 2D

I doubt there will be review sessions before the final, but I find it helpful to go to office hours whenever I have questions.

Can Yilgor 2D

I try to watch the lectures as soon as they are released and do the homework problems by the end of each week. This quarter I decided to focus more on reviewing the homework problems before the midterm, which helped me a lot since most questions were similar.

Can Yilgor 2D

I find it helpful to look at what information is given to determine which equation to use. Whenever you can't decide which equation to use, it may help to ask yourself, "what information do I have available" and start from there.

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I have been wondering what the names of the songs Dr. Lavelle has been playing are since the beginning of the last quarter. I am glad that I finally learned the names of the songs :)

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In these types of questions, we try to obtain the desired chemical equation by manipulating the given chemical equations. For instance, since HCl (aq) is not present in our reaction, multiplying the second equation by 6 would help us eliminate HCl (aq) from the equation. Similarly, if we multiply th...

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Kw is the equilibrium constant for the autoprotolysis reaction of water and it equals 10^-14 at 25 degrees Celsius (298 K) "p" is the shorthand for "- log" (negative log base 10), so pKw would be equal to 14 at 298 K. I remember Dr. Lavelle mentioning that even if the temperature...

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I also agree that there were some difficult questions on the test. For the next midterm, I will try to start studying well ahead of time and definitely focus more on the homework problems.

Can Yilgor 2D

Studying for the midterms didn't leave me enough time to focus on the material that isn't included on the test, so I have to study more and catch up. Thank you for the positive outlook. I feel much better now that I read this post :)

Can Yilgor 2D

Generally, when an organic compound like (CH4) is involved in a combustion reaction the products are CO2 and H2O. Therefore, the enthalpy of combustion of methane can be another expression for the change in enthalpy of the reaction. I agree that looking over the answer choices can help you eliminate...

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You can simply think it this way, A process that releases heat into the surroundings is called an exothermic process. A process that absorbs heat is called an endothermic process You may ask yourself "do I need to heat up or cool down the ice to get water?", "is heat released when wat...

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I agree that autoprotolysis occurs between two identical amphiprotic molecules. I am also unsure about whether it happens between all weak bases or weak acids.

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Less than 10^-4 is safe territory for approximation!

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For that problem, I first started out with writing out a standard chemical equation for a base reacting with water since we know that an amine is usually a base. With that, we get: B (aq) + H_{2}O (l) \rightleftharpoons HB^{+} (aq) + OH^- (aq)) Since we know the ...

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I am not sure if there is an exact cutoff, and I believe we don't have to memorize anything numerical. I don't think we will be asked to comment on the strength of an acid or a base by just looking at a single pKa or pKb value. I assume we could be asked to compare the relative strengths of acids an...

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If ΔH is positive, the reaction is endothermic. (heat is required for the reaction to occur) If ΔH is negative, the reaction is exothermic. (heat is given off) In these types of questions, I try to keep in mind Le Chatelier's principle, which basically states that chemical systems respond to the cha...

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As Dr. Lavelle mentioned in today's lecture, it's very rare for reactants and products that have different structures to be equally stable. I believe we will learn more about this when we discuss thermodynamics.

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I calculated the Kc from the equilibrium concentrations initially given. Kc = [NO]^2/[N2][O2] = (0.600)^2 / (0.100)(0.100) = 36 Then you create the ICE table where the initial concentrations are 0.100, 0.100, and 0.900 for N2, O2, and NO respectively. The change in concentrations would be +x, +x, -...

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I guess if more NO is added, the equilibrium shifts to the left, so it could be reasonable to use the reverse reaction instead of the forward reaction. I assume it would depend on the specific question to determine whether to use the forward or reverse reaction.

Can Yilgor 2D

This is what it says about posting an image on the FAQ page of Chemistry Community: Yes, images can be shown in your posts. If the administrator has allowed attachments, you may be able to upload the image to the board. Otherwise, you must link to an image stored on a publicly accessible web server,...

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Hello, I understand that while writing Kp, we divide the partial pressure values by the standard pressure (1 bar). Overall this doesn't affect the expression since values in the denominators cancel out. I was wondering about the reasoning behind dividing the partial pressures by the standard pressur...

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Mauricio Maravilla 3C wrote:Does anyone know if 14B covers radioactivity?

We are going to use the same textbook, so if "radioactivity" is a topic in the textbook, I would assume we are going to cover it. I am just assuming though :)

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I believe this class does not have a curve. The final grades are calculated out of 500 points.

Can Yilgor 2D

The timing was a problem for me too. The questions that require calculations took most of my time, and I felt rushed. Hopefully, we all did well enough to get a decent grade.

Can Yilgor 2D

I believe we will see our final grades by Saturday on either CCLE or MyUCLA!

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There are a lot of exceptions in chemistry, so I recommend thinking about the definitions depending on the reaction you are given.

Bronsted acid: proton donor
Bronsted base: proton acceptor
Lewis acid: electron pair acceptor
Lewis base electron pair donor

Hope this helps

Can Yilgor 2D

I believe we need to know some common examples of acids like - HNO3, Nitric acid - H2SO4, Sulphuric acid - H2CO3, Carbonic acid, etc. I assume we are mainly responsible for what we have discussed in lectures. We will probably be provided the names of the acids if we haven't seen them before. I doubt...

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Good luck everyone :)

Can Yilgor 2D

Hi! Strong bases are bases that are completely ionized in water. As you mentioned, group 1 (alkali metal) and group 2 (alkaline earth metal) oxides and hydroxides are considered as strong bases. The list of strong bases may be different in different resources, so I assume we are mainly responsible f...

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If you know the [H+] then you could use the equation: [H+] * [OH-] = 1.0 * 10^-4 to find [OH-] (this comes from Kw or the equilibrium constant for water at 25 degrees) To find pH and pOH, you could take -log[H+] or -log[OH-] (remember that "p" means taking the -log in chemistry!) Hope this...

Can Yilgor 2D

I review my notes to be sure that I know everything that I need to know for the finals. Besides that, listening to music is a nice way to deal with stress.

Can Yilgor 2D

The structure of the final exam will be similar to those of midterms. I believe later this week, we will receive all the information we need.

Can Yilgor 2D

I have been trying to meet the deadlines and finish the quarter strong. It has become quite exhausting recently, so I think we are close to a well-deserved winter break!

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I am not sure whether they would give us a chart for the final exam or not. I believe we will mostly encounter ligands that we have seen in lectures or discussion sections. Please see the link below in case you would like to learn more about some common ligands. Hope this helps. https://sites.google...

Can Yilgor 2D

I also thought about the same thing. I guess it's just uncommon for coordination compounds to have a see-saw arrangement.

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I see people mentioning that planar shapes would never be ideal for 6 bond, because the electrons want to be as spread out from each other as possible. What number of bonds would be fine being in a planar shape? Common examples of planar shapes are trigonal planar and square planar. Keep in mind th...

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I agree that memorizing the VSEPR chart will help you remember which atoms are on the same plane. However, with any given structure, the key point is to imagine it in 3D. There are some nice simulations online if you would like to check out.

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Dr. Lavelle answered this question previously on Chemistry Community. Please see the link below for his answer.

Hope this helps

viewtopic.php?t=32258#:~:text=Localized%20electrons%20exhibit%20normal%20behavior,atom%2C%20it%20visits%20two%20atoms.

Can Yilgor 2D

I guess it has something to do with the structure of the guanine nitrogenous base, but I'm not certain and I don't want to confuse anyone. You could see the below link which describes the interaction of cisplatin with DNA, but I think it's hard to understand and well beyond the scope of our course. ...

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So I believe the general idea is that if there's an even number of carbons, the hydrogens will be in the same plane. From my understanding, it's because in H2CCH2, the pi bond between C=C prevents the two carbon atoms from rotating, so the hydrogens are in the same plane. But with H2CCH2, the middl...

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Grading scales change from class to class, but I think it's reasonable to aim for 90 % and above to get an A. I believe estimating grades can be stressful, so just try to stay motivated and you'll be fine :)

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I believe ion-ion interactions lead to an ionic bond. They result from two atoms with differences in electronegativity and opposite charges bonding to one another. Thank you for the clarification. Since we are talking about interactions between atoms, can I still consider ion-ion interactions as ex...

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The midterm went well, but I have doubts about some of my answers. Do you know if we are going to be able to review our answers after the results are released? I don't think we are going to have the chance to do that since it says "review not permitted" on CCLE. Don't you think it would be...

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Do Ion - Ion interactions lead to the formation of ionic bonds? I don't think that this interaction involves the transfer of an electron from a cation to an anion. I would be glad if someone could provide a detailed explanation of the "Ion - Ion interactions" that we have covered in the in...

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I know that we need to place the atom with the lowest ionization in the center of our Lewis structures. What is the logic behind this? I would appreciate it if someone could clarify it for me.

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Can I think of the Aufbau Principle as a combination of the Pauli Exclusion Principle and Hund's Rule? My understanding is that we consider the Pauli Exclusion Principle and Hund's Rule while drawing electron configuration models according to the Aufbau Principle. I would be glad if someone could co...

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I find reviewing the lectures a few days before the test very helpful in refreshing my memory. I generally go over the homework questions multiple times and try to attend Dr.Lavelle's discussion sections.

Can Yilgor 2D

Breaking Bad and Black Mirror are among my favorites. I started watching Grey's Anatomy, but I am not sure if I will be able to finish all 16 seasons.

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I am still having trouble understanding the difference between the terms "ample" and "overwhelmingly" bond character. I know that real structures are a blend of resonance structures, but I am unsure about how to connect this idea to the bond character. Any kind of clarification w...

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While drawing Lewis structures, we try to minimize the formal charges to obtain a more stable structure. Sometimes, drawing a double bond instead of having a lone pair helps us reduce the formal charge. Also, we need to consider the total number of valence electrons while drawing Lewis structures. H...

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I think we are encouraged to attend the discussion sessions since we go over useful supplementary materials, but I don't think we earn points for just attending, so I am pretty sure you will not be penalized for just missing a discussion section :)

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Bond length is defined as the distance between the centers of two covalently bonded atoms. The length of the bond is determined by the number of bonded electrons (the bond order). The higher the bond order, the stronger the pull between the two atoms and the shorter the bond length. Generally, the l...

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Although electron affinity values generally increase across a period, since nitrogen has a half-filled p subshell it is more stable than carbon. Therefore, carbon has a greater affinity for an electron than nitrogen to become more stable, which is an exception to the general trend.

Can Yilgor 2D

Yes, I believe we are expected to know since we have covered the periodic trends in lectures.

Can Yilgor 2D

I wouldn't worry about a topic that isn't on the outline, but it would also be useful to read through the chapter in the textbook just to make sure you know the basics.

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A coordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom. A covalent bond is formed by two atoms sharing a pair of electrons. The atoms are held together because the electron pair is attracted by both of th...

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A wave function- with quantum numbers n, l, ml- that is a solution of the Schrodinger's Wave Equation is called an orbital. To fully describe the SHAPE and ORIENTATION of an orbital we use quantum numbers. The Angular Momentum Quantum Number (l) describes the SHAPE of the orbitals, which differ for ...

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Principal Quantum Number (n) determines ENERGY and SIZE of an orbital. Angular Momentum Quantum Number (l) describes the SHAPE of an orbital. Magnetic Quantum Number (ml) labels different orbitals of a subshell. It gives the ORIENTATION of the angular momentum. If we take 4s orbital as an example, n...

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Since we could earn maximum of 5 points per week, in order to get the full participation points, you should have at least 20 posts by the end of week 4.

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The ground state of a quantum mechanics system refers to its lowest energy state (most stable electron arrangement). An excited state is any state with energy greater than the ground state. For example, an oxygen at the ground state has the electron configuration 1s^2 2s^2 2p^4. This is the most sta...

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Although your final answer could be wrong, following the correct procedure can help you earn partial credit. Let's say you found the correct answer, but forgot to convert it into the units specified in the question; since you have applied the correct method you could still earn some points.

Can Yilgor 2D

I don't think that we will go over each problem one by one, but I am pretty sure TA's can answer questions related to specific problems that were on the midterm.

Can Yilgor 2D

Yes, we earn our participation points for CHEM 14A by posting on Chemistry Community, but I am not sure if it is the same format for CHEM 14B.

Can Yilgor 2D

If we post less than five times in a week, can we make up for it by posting more the following week?

Can Yilgor 2D

There was one question about constructing a net ionic equation in the assigned textbook problems, so I guess it might also come up in the midterm.

Can Yilgor 2D

I believe Peer Learning Sessions and office hours are happening as usual. However, Dr.Lavelle's discussion sessions on Monday and Wednesday are cancelled for this week.

Can Yilgor 2D

I believe we are allowed to have the constant/equation sheet on midterm. As Dr.Lavelle said yesterday, he is going to send an email with all the instructions and details related to the midterm, so check your inbox for that :)

Hope this helps

Can Yilgor 2D

I believe that the equations are similar. The difference is that Einstein's equation uses a specific speed (c=speed of light), while the kinetic energy formula uses the object's speed. In fact, the two equations are derived from the same equation for total energy. Please see the link below for furth...

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In our discussion section, my TA wasn't sure about using our phones to access Zoom during assessments. However, he told us that he will ask Dr.Lavelle about it. If we were to use our phones, it would be like as you said. We would be using "Respondus" on our computers while showing our work...

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According to the "Aufbau (building up) principle", electrons fill sub-levels from the lowest energy level upwards. Since the 4s sub-level is lower in energy than the 3d sub-level, it is filled first. In other words, electrons may go into the fourth main energy level before the third main e...

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You could find the schedule and the zoom links for step up sessions on CHEM 14A website under the section "Peer Learning Sessions and Step Up Program".

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