# Chem teacher has questions, will even pay for help

I’ll actually send you money via PayPal or Venmo if you like, for good insight on these questions. I got AP Chemistry materials from a long time teacher who even grades the AP test, but I don’t want to bother him with asking for tutoring. So I have some questions on the semester final he gave me that I’m stuck on.

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The Ka for HF is 7 E-4, and Ksp for PbF2 is 4 E-8. What is the approximate Kc for the reaction? 2HF + Pb+2 <-- --> PbF2 + 2H- . All are aqueous except PbF2 is solid.

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We have a reversible reaction: 2SO2 + O2 <-- --> 2SO3. All are gases.
All are put in a container so that each has a partial pressure of 1.0 atm. At equilibrium, the partial pressure of the product is 1.33 atm. What is Kp of this process?

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What is the maximum amount of KCl that can be added to a 50.0 ml sample of 0.200 M Pb(NO3)2 solution before a precipitate forms? Ksp of PbCl2 is 1.3 E-5.

From the SDMB registration agreement:

• Please do not ask other members to do your homework or provide input for your article, paper, or other off-board project. We may waive this rule if you obtain prior permission from a moderator or administrator.*

In case this matters, I’ve posted this in the marketplace.
http://boards.straightdope.com/sdmb/showthread.php?p=20646927#post20646927

Moderator Note

The way we’ve interpreted this over the years is basically we’ll help you if you’re stuck, but don’t ask us to do your work for you. And this is pretty much what the OP is asking for - help because they are stuck. There’s no rule problem here.

ETA: Also, let’s just stick to the facts in GQ. If you want to take up Cardinal’s offer for financial compensation, you may discuss that in the marketplace thread.

(As for the actual chemistry questions, it’s been too many decades since I took chemistry, so unfortunately I’m of no help there)

Cardinal has been a member since 1999. I don’t think he’s currently taking AP Chem. He’s teaching it.

OP, you want to go here:

https://apcommunity.collegeboard.org/web/apchem

The AP on-line communities are great. They love new teachers, they love giving advice.

And don’t be intimidated because someone is a reader. It’s just a matter of putting in the time and applying. In fact, you should apply as soon as you reach your third year.

Thanks for the link the AP message boards. I’ve put the questions there, too, but I still would actually pay someone for help sooner than later.

They must be teaching new stuff in AP Chem, or I just have a terrible memory for it. It only was…umm…15-20 years ago (OH MY GOD HAS IT BEEN HALF MY LIFE?!)… I don’t remember anything involving the concepts of a K_something for a reaction or anything other than a K_a for acids and a K_b for bases, and those would be stated as pK_a anyway (much nicer numbers).

This one’s easy, if you can assume the ideal gas law, which would be pretty normal for this sort of thing.

If the ideal gas law is used as an approximation, the partial pressure is proportional to the number of each molecule present.

So, the changes in partial pressures will be proportional to the changes in the number of molecules added or subtracted - the increase of .33 atm on the right will be accompanied by a decrease of .33 atm SO[sub]2[/sub] and .165 atm O[sub]2[/sub].

So, K[sub]p[/sub] = 1.33[sup]2[/sup]/ (.67[sup]2[/sup] * .835)

The first one I’m blanking on the chemistry equivalent of the chain rule… but the third one at the core is trivial, just figure out the maximum concentration of Cl[sup]-[/sup] given the known Pb[sup]2+[/sup] concentration and grind out a whole bunch of unit conversions.

Oh, coming back to this, I didn’t include units on the second problem - the number I gave is in units of 1/atm, which is obviously not going to be acceptable, so more unit conversions will be needed there.

For anyone who cares, here are the answers I got on the AP Community board, which I recognize now as correct. I knew I would feel a little dumb when I saw the answers.

https://apcommunity.collegeboard.org/group/apchem/discussion-boards/-/message_boards/message/126873571?p_p_auth=gdgQ6wez

So we can help teachers with their homework. Nice twist.

(ETA: Yeah, I had a college chem teacher once who could have used some help. He had a habit of putting questions on exams that the “knew” the wrong answer for.)

You can’t see it unless you have a college board professional account.

Were they nice? Generally the communities are pretty cool. (I teach/have taught AP Lang, Lit, and Econ, and am a reader for Lang).

The other teacher who helped put in pictures, so copy and paste doesn’t work. I made a Gdrive folder in case anyone wants to read his answers:

AP Chem help - Google Drive It was very helpful. I even sent him Amazon money.