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Chemistry Questons

Posted: Sat Mar 24, 2018 7:07 pm
by MCAT_Retaker
I thought in a galvonic cell, electrons are attracted to the cathode?

Re: Chemistry Questons

Posted: Sat Mar 24, 2018 8:36 pm
by jessmarah
Hummmm I also would like to know the answer to this one. I reviewed this topic earlier today & was under the impression that in the Galvanic cell the anode is negative while the cathode is positive. Electrons naturally would flow from the anode to the cathode (reference p.172 of the chem. book "the electrode where oxidation happens is known as the anode...Therefore, a surplus of electrons is generated at the anode because electrons are lost during oxidation, and they travel to the cathode" .

HOWEVER (and more importantly ) I think that what the question wanted us students to identify is the fact that a galvanic cell can "release electrons Without Any Power Input " vs The electrolytic cells which require an outside energy source ( like that of an external battery). The Electrolytic cells are Non Spontaneous (G>0) and as a result would need an external battery to create an electrical current that would force the nonspontaneous reaction to occur.

If you find out anything else please inform me or correct me if I am wrong.

Re: Chemistry Questons

Posted: Sun Mar 25, 2018 5:45 am
by MCAT_Retaker
I also have another Gen Chem question. Page 162. Question 7.
From the content review book on page 162, can you explain how they got the mass. I understand the volume. I was going to use 1 mol is equal to 22.4 L and then use molar mass?

Re: Chemistry Questons

Posted: Mon Mar 26, 2018 4:12 am
by MCAT_Retaker
Also for question 2, on page 113. I dont understand how lipid solubliy relates to polarizability. Please let me know this one too. thank you.

Re: Chemistry Questons

Posted: Mon Mar 26, 2018 1:01 pm
by NS_Tutor_Andrew
Hi MCAT_Retaker,

The first two sentences of the explanation for that question from the CR video were incorrect, and the second half of the explanation is correct. We will update that question on the CR video today. But yes, as jessmarah pointed out, there are two key points: (1) the processes in galvanic cells are spontaneous, while they are nonspontaneous in electrolytic cells, and (2) reduction (electron loss) will always happen at the cathode and oxidation will always happen at the anode. Combining these bits of info, the sign conventions for galvanic cells are fairly straightforward -- the anode is (-) b/c electrons are continually being produced, and the cathode is positive b/c electrons are being used up. Then you can simply remember that the sign conventions are flipped for electrolytic cells.

The first half of the explanation for Q7 on pg. 162 of the book, calculating the volume needed, is correct. The second half is incorrect. You can calculate the mass of RbOH needed as follows: (1) calculate # of moles from molarity & volume: 0.06 M RbOH * 0.1 L = 0.006 moles of RbOH; (2) then use the MW of RbOH (102 g/mol) to convert from moles to mass: 0.006 mol * ~100 g/mol = 0.6 g. The solution as presented uses (2)*(50 mL) instead of 100 mL, following the logic from the first step (I personally think that's a little convoluted, but I suppose it's a question of taste) but then forgets to convert 100 mL to 0.1 L, resulting in an answer that is off by a factor of 3. I personally think that this a great lesson about why it's good to break questions down into simple steps rather than trying to write clever but more complicated equations that incorporate extra steps, and needless to say, this will be added to the list of top-priority fixes for the next edition.

Thanks as always for your thoughtful questions!

Re: Chemistry Questons

Posted: Mon Mar 26, 2018 1:03 pm
by NS_Tutor_Andrew
P.S., note that the question in Q7 on pg. 159 contains the correct answer choice -- the mass is 0.6 g, or 600 mg, as given in the answer choices. The error is in the explanation only.

Re: Chemistry Questons

Posted: Tue Mar 27, 2018 4:42 am
by MCAT_Retaker
Thank you for always answering questions so promptly. Not sure if others have already noticed them too. It bothers me when I see something incorrect.

I also wish the quizes in the lessons had the correct answer clearly shown or an option to re do it. I see the explanation, but I still like to re do the question. I always have to restart the lesson video to do so. Maybe its just me too. Not sure if others dont mind.

P.S. I asked another question above, about lipids.

Re: Chemistry Questons

Posted: Wed Mar 28, 2018 10:55 am
by NS_Tutor_Andrew
Hi MCAT_Retaker,

Completely understood. Re: the quiz interface in the videos, I hear you and will make sure that your feedback is on the radar of the whole content team. The basic issue with changing the quiz interface is that it's more complicated on the back end than it might appear—but your point is very much taken.

Re: polarizability, the basic idea is that lipophilicity and hydrophobicity aren't actually the same thing, although we usually use them synonymously. For a substance to be lipophilic, it must dissolve in a lipid environment, and that means interacting with another nonpolar substance. This is different from hydrophobicity, which is driven by the *lack* of interactions. Two nonpolar substances will interact based on London dispersion forces, which are due to transient dipoles. The more polarizable a substance is, the more it will experience London dispersion forces, so the more it will interact with other such substances. This page contains one of the better reviews that I've seen in a freely accessible online source.

That said, passage-based questions like this one for which the correct answer incorporates relatively advanced knowledge often have an "escape hatch" through (a) leveraging passage info and (b) process of elimination, so be alert to that possibility. The passage info points you to thinking about lipid-solubility, so the answer choice must be related to that. A might not seem obvious at first, but B, C, and D have nothing to do with intermolecular forces, which are what drives solubility/mixing/etc., so they can be eliminated.

Hope this helps clarify this question!

Re: Chemistry Questons

Posted: Mon May 21, 2018 8:40 pm
by MCAT_Retaker
Is 20%, supposed to be 20? this is from lesson 10.

Re: Chemistry Questons

Posted: Wed May 23, 2018 9:04 am
by Administrator
Solutions can be described using a percentage as weight/weight or volume/volume or even weight/volume. Here, it look like the question is asking about a 20% weight/weight solution.