CHEMISTRY COMMUNITY

Charlotte Chen 3B

What are the grade boundaries?

Charlotte Chen 3B

I think hybrid is more realistic but for smaller classes def in person

Charlotte Chen 3B

Planning on taking Chem 14BL+CL but I'm still on the waitlist for both, does anyone know if they accept everyone off the waitlist usually?

Charlotte Chen 3B

Do we still have to post this week if we already have 50 posts from previous weeks?

Charlotte Chen 3B

Hi! So we know that the slowest step determine the rate of the overall reaction = k[I-][HClO], but because that rate includes HClO, which is an intermediate step, you have to make an alternate expression for HClO from the first elementary step given. The forward rate for the first elementary step i...

Charlotte Chen 3B

Vapors tend to consist of a mixture of 2 or more different phases at room temperature (liquid and gas), and gases contain one thermodynamic state at room temperature. I would treat them as equivalent for this class though

Charlotte Chen 3B

In the context of acids and bases, single arrows usually indicate the reaction dissociates completely which is why the reaction goes to completely, but double arrows tend to indicate the reaction does not dissociate completely which is why it's reversible.

Charlotte Chen 3B

Can we still solve using x even if it's past the cut off?

Charlotte Chen 3B

I think K is a more generalized expression that could either be referring to Kc or Kp depending on the context of the question

Charlotte Chen 3B

Chemical equilibrium

Charlotte Chen 3B

I balanced it the same way as well, I would just make sure to use the formulas/symbols they give you in the drop down box to plug in, and make sure to avoid using unnecessary parentheses

Charlotte Chen 3B

Sara_Lim_2C wrote:How did you get that Q is pressure divided by [H+]? I'm not sure how that gives Q.

I think we treat the pressure of H2 as partial pressure or concentration since it's a gas, so we can plug this into the expression for Q and solve for it

Charlotte Chen 3B

You don't include pure solids or liquids in the reaction quotient because their concentrations stay constant throughout the reaction

Charlotte Chen 3B

The different phases in the cell are separated by a vertical line, and the phases are listed in the order in which they are in contact. The salt bridge is represented by a double vertical line

Charlotte Chen 3B

You would calculate Q by doing PH2/[H+]^2, and then you would sub this into the Nernst equation, using n=2 because there are 2 moles of electrons transferred, F=96485 and R=8.3145, and the rest of the values given in the problem

Charlotte Chen 3B

Thank you all. I have gotten it right now. I was wondering why the line diagram wants the chemicals in a different order than they are in in their respective reactions? Is there a convention for this? The different phases in the cell are separated by a vertical line, and the phases are listed in th...

Charlotte Chen 3B

Audrey Han 3L wrote:Are we allowed to use R=8.314 in this problem?

Yes, you use R=8.314 to sub into the Nernst equation

Charlotte Chen 3B

Using the link that the question provided, you need to calculate the Enthalpies of formation for each product and then add all of these values up. Example: If the enthalpy of formation for H2O is -241.82 then remember to multiply it by 10 for this particular reaction. The sum of enthalpies for the ...

Charlotte Chen 3B

Because we were given the pressure in torr, we need to multiply by 1atm/760torr. We can then use this value to calculate work which would be the change in volume multiplied by the pressure in atm. The units come out to be L.atm. The units for work must be in joules, so you must multiply by (8.314mJ...

Charlotte Chen 3B

4C.13 An ice cube of mass 50.0 g at 0.0°C is added to a glass containing 400.0 g of water at 45.0°C. What is the final temperature of the system (see Tables 4A.2 and 4C.1)? Assume that no heat is lost to the surroundings. For this question, I know the heat lost by the ice cube = - heat gained by wat...

Charlotte Chen 3B

4B.5 An ideal gas in a cylinder was placed in a heater and gained 5.50 kJ of energy as heat. If the cylinder increased in volume from 345 mL to 1846 mL against an atmospheric pressure of 750. Torr during this process, what is the change in internal energy of the gas in the cylinder? For this questio...

Charlotte Chen 3B

Yes that makes sense, thank you!

Charlotte Chen 3B

A constant-volume calorimeter was calibrated by carrying out a reaction known to release 3.50 kJ of heat in 0.200 L of solution in the calorimeter (q=−3.50kJ), resulting in a temperature rise of 7.32°C. In a subsequent experiment, 100.0 mL of 0.200 M HBr(aq) and 100.0 mL of 0.200M KOH(aq)were mixed ...

Charlotte Chen 3B

Hi. Thanks for all the help. How do I correctly put the scientific notation into Sapling? I keep getting a message that says "we could not recognise your answer, make sure it's in valid form." You should get a prompt when you click into the answer box that gives you the option to input &q...

Charlotte Chen 3B

I solved it this way:
Q = (PCH4)2/(PC2H4)(PH2)
∆G = ∆Gº + RT ln Q
Q = (0.600)2/(0.500)(0.350) = 2.057

You can then plug this into ΔG = ΔG∘ + RTlnQ!

Charlotte Chen 3B

Since we know
ΔG°rxn = Σ G°f (products) - Σ ΔG°f (reactants), you can disregard temperature to solve for ΔG°rxn:

ΔG°rxn = [ 2(-65.3 kJ/mol) + 0] - [ 1(+209 kJ/mol + 4(0) ]
= -130.6 – 209
= -339.6

Charlotte Chen 3B

Given the information A+BC⟶2D⟶DΔH∘ΔH∘=−720.7 kJΔS∘=287.0 J/K=546.0 kJΔS∘=−185.0 J/K calculate ΔG∘ at 298 K for the reaction A+B⟶2C For this question, I calculated the overall ΔHº to be -1812kJ and the overall ΔSº to be -0.083kJ, and I subbed this into the equation ΔGº=ΔHº-TΔSº to get -1787kJ, but th...

Charlotte Chen 3B

Hi! So the way I like to think about it is that this reaction has 4 steps. Step 1 is heating the reactants to 100 degrees Celsius,. So first, we have to convert our celsius to Kelvin by using the conversion 298.15K + degrees celsius given. Next, we assume 1.00 mol, and use the equation ∆S = n*Cp*ln...

Charlotte Chen 3B

Heat capacity is the amount of energy required to raise the temperature of an object by one degree, in this case the object is the calorimeter. So overall this question asks about the heat capacity of the calorimeter itself instead of the specific/molar heat capacity of certain substances in the rea...

Charlotte Chen 3B

In general, when would we use the molar heat capacity at constant volume (3/2 R) value given for atoms in the table? Would it be when the gases are monatomic?

Charlotte Chen 3B

After finding q, how do we find ∆u? I know we use the equation ∆u= q+w but I'm not sure how to find work done with the variables given?

Charlotte Chen 3B

Hello, Regarding sapling 15, I keep getting -1.08, but it is wrong. I used the PV=nRT and w=-Pdelta T equation but am still getting the wrong answer. Any help would be appreciated. Thank you! I also used the w= -∆nRT equation and it worked for me, the way you would solve it using the values given i...

Charlotte Chen 3B

The first path is a reversible expansion so you would use w = -nRTln(V2/V1). However you first need to find moles so you would use the ideal gas law to find it (n = PV/RT). Then once you solved for moles you input it in -nRTln(V2/V1). For Path B because it is at a constant pressure, irreversible re...

Charlotte Chen 3B

i just redid this problem after the comments here said to use the inverse Kp, but that kept giving me the wrong answer even though ever other part of my problem was set up similarly to everyone's in the comments. i only got the correct answer once i used 391 instead of 1/391, can anyone explain why...

Charlotte Chen 3B

There's a link Dr. Lavelle shared previously with a table of the strong acids and bases to know:

https://sites.google.com/site/chempendi ... cids-bases

I think for the purposes of this course and as stated on the table, you can assume any other acids/bases not on the table are weak!

Charlotte Chen 3B

Just to add on to the other answers, sometimes you won't directly get given all the variables e.g. you may be given mass instead of mols, but as long as there is sufficient information provided you should be able to calculate the necessary values to complete the expression (in this case you would di...

Charlotte Chen 3B

Does anyone know if Lavelle is also strict on significant figures? I just want to make sure that I do not loose easy points on the midterm, so if anyone who has taken him previously could say, I would appreciate it! Also, the midterms are multiple choice so usually you should be able to select the ...

Charlotte Chen 3B

Just as an add-on to that, there are 4 different methods of heat transfer that we commonly see - Conduction: transfer of heat through a solid (any heat inside an object is transferred to its surface), Convection: transfer of heat from a surface by a fluid (such as air creating 'convection currents' ...

Charlotte Chen 3B

For question 5I.13, part c, I'm not sure how to tell Cl2 is more thermodynamically stable than Fl2 based on the size of Kc - I just thought a larger Kc value means products are favored than reactants, but I'm not sure how it relates to this problem? Screen Shot 2021-01-28 at 10.16.09 PM.png Screen S...

Charlotte Chen 3B

I would be mindful of negative signs in particular when plugging in values into the quadratic formula, because sometimes if your value is already negative it can be easy to forget there is another negative sign in front of it that will cancel it out. When you say error do you mean two negative roots...

Charlotte Chen 3B

Just to add on, for groups 1 and 2 metal hydroxides, the metal hydroxide will fully dissociate into ions as soon as the solid dissolves, but the group 2 hydroxides are only slightly water–soluble, so group 1 hydroxides are strong bases, but calcium hydroxide or magnesium hydroxide is rarely referred...

Charlotte Chen 3B

Since Ka=[H+]{A-]/[HA], you can plug in the values given so 0.00459=x^2/0.14-x , after you multiply it out you get x^2+0.00459x-6.426*10^-4, by plugging it into the quadratic equation you get roots x=0.02315823211... or x= -0.02774823211... so you eliminate the negative x value, and use the remainin...

Charlotte Chen 3B

You can use the pH to calculate [OH-] (which is equal to [BH+]), by first finding pOH through 14-pH and then doing 10^-pOH, so in this case it would be 10^-4.751. Then you can sub this into the Kb expression: 7.055*10-5=[10^-4.751]^2/(x-10^-4.751) and solve for x, after which you can sub in 10^-4.75...

Charlotte Chen 3B

Since Ka=[H+]{A-]/[HA], you can plug in the values given so 0.00214=x^2/0.121-x , then you can solve for x using the quadratic equation and once you've solved for x, you can then divide it by the original concentration 0.121M and multiply it by 100 to get the percent ionization

Charlotte Chen 3B

I have the same issue with CCLE saying I got 80% on the assignment when I got 100% on Sapling. I'm just worried because I don't know which questions it has counted as 'incorrect' as Sapling isn't giving me any indication - I was wondering if you were able to resolve this issue, and if you know why i...

Charlotte Chen 3B

You would calculate Q by just subbing in the values they gave for each of the compounds to the expression, similar to how you would calculate Kc so in this case [3.42]^2/[5.52]^3[1.14], and then compare it to the Kc value for the next part of the question

Charlotte Chen 3B

I also sometimes get confused about which side of the equation to -x and +x if the reaction is reversible, is there any way to determine it from the way the question is phrased?

Charlotte Chen 3B

I would first start with making an ICE chart and solving for the initial molarity of 2SO3 by dividing the given moles by the given volume. Then you can start filling out your ice chart. The question gives you the final moles and by dividing that by the 3 L, you get the change in molarity/x. With th...

Charlotte Chen 3B

I'm also confused about the last part of this question, and I'm especially having a lot of trouble solving for x using the quadratic equation. My numbers are a bit different, so at equilibrium [N2O4] = 0.32 + x, and [NO2] = 2.89 - 2x. My K value is 11.16. So what I have so far: K = [2.89-2x]^2/0.32...

Charlotte Chen 3B

Hey! I actually had a similar question to you, for me my x values were 0.235 and 5.78, I subbed in x=0.235 and got the right answer but I'm not sure how to know which value to eliminate. I guessed x=0.235 because it's a smaller value, but if anyone knows how in general to determine the right value I...

Charlotte Chen 3B

Also, can someone explain how the relationship Q>P means products are favored (I understand it's because the numerator is higher) but I am confused on how this tells us the direction of the reaction and what it really means regarding the stability of the reaction. If someone could explain the corre...

Charlotte Chen 3B

The way I understand it, pure solids/liquids are not included in the equilibrium expression because their concentrations remain the same through the reaction - this is calculated by their density/molar mass. For a pure solid, it has no concentration because it isn't dissolved in any solvent, so incr...

Charlotte Chen 3B

Sorry if this has already been addressed somewhere, but are the midterms/finals only during lecture times, or can they be taken at alternate times if you are in a different timezone? I took 14A over summer so I'm not sure how it works. Thank you!

Charlotte Chen 3B

Massimo_Capozza_1G wrote:Does anyone else have a problem when logging into Chemistry Community where you have to re-register each time you sign-in?

I haven't had that issue so far, maybe you could click the "Remember me" option when you log in?

Charlotte Chen 3B

Are any of the step up sessions recorded?

Search found 55 matches