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OK, Number 34 on the regional test has me stumped. I got it right, but I'm not sure if my logic was correct or if I was just lucky. Can somebody walk me through it?

34. Solid AgNO3 is slowly added to a solution containint 0.0010 M each of NaCl and NaBr. What concentration of Br- would remain in solution when AgCl begins to precipitate? The solubility product constant for AgCl is 1.8*10^-10 and that of AgBr is 3.3*10^-13.

Correct answer is 1.8*10^-6

Thanks a lot!!!

And does anyone know how to do #39? That's the only one I skipped on the test (other than the faulty physics question, that is). I had no idea how to do it.

I didn't take it in competition, but, after looking at the test, this is how my scores probably would have gone:

Biology: [unparseable or potentially dangerous latex formula]
Chemistry: [unparseable or potentially dangerous latex formula]
Physics: [unparseable or potentially dangerous latex formula]

You're given two [unparseable or potentially dangerous latex formula] values at two different temperatures and asked to find the [unparseable or potentially dangerous latex formula] of the self-ionization of [unparseable or potentially dangerous latex formula]

Kw for the self-ionization of water is 1.13E-15 at 273 K and becomes 9.61E-14 at 333 K. Calculate [unparseable or potentially dangerous latex formula] for the self ionization of water.

You need two equations:

Delta G = Delta H -T Delta S

and dG = -RT ln(K)

so,

dH-TdS = -RTln(K)

and

ln(K) = -dH/R(1/T) + dS/R

That gets the equation into the form of y=mx+b

You can solve for dH given the two Kw and temps.

Ah, I guess I didn't know the second equation. Thanks. :)

omg, bradp learned [unparseable or potentially dangerous latex formula]

WOOHOO

i am so proud of you, my little boy is growing up :)

That is cool. Where can I find how to LATEX??

[unparseable or potentially dangerous latex formula]

Interesting!

nevermiind