PChem Question

[josh81883]

Any pchem experts out there? I need a 2nd opinion on a problem.

If I make a soln. of 0.2 M NaH2PO4 and 0.2 M Na2HPO4, what is the concentration of PO4 -3? pKa's for phosphoric acid are 2.23, 7.31, and 12.74.

Here's what I was thinking, I just need a 2nd opinion...

H2PO4 and HPO4 should make a buffer soln:

so pH = pKa(H2PO4) + log ([HPO4]/[H2PO4])
pH = pKa(H2PO4) + 0
pH = 7.31

so [H+] = 4.9 * 10^-8 M

to get [PO4 ]

The Ka for the pKa of 12.74 is 1.82 * 10^-13

Ka = [H+][PO4]/[HPO4]

if [PO4] = x;

x = (Ka * [HPO4]) / [H+]
x = ((1.82 * 10^-13)(0.2 - x)) / (4.9 * 10^-8)
x = ((1.82 * 10^-13)(0.2)) / (4.9 * 10^-8)
x = 7.24 * 10^-7 M

That's what I first though, but I just want a second opinion. Do I need to take into accound any H2PO4 being converted to H3PO4 or anything like that?

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[dalynn]

What you're doing looks right based on analytical, etc., but our pchem I class is entirely energy/Schrodinger eqns/MOs right now, so I'm not sure if there's a different way to approach that from a pchem perspective.
gluck!!!

I have hours of pchem to do this weekend (big quiz Monday, which I consistently seem to get C's on no matter how much I study)...our prof was actually considering buying us all bumper stickers that say "I survived P.Chem" at the conclusion of the course. Are we ever going to need to intricately understand the hydrogen atom for med school ?

Sometimes it's fun to complain.

 

[Medikit]

What you need to do is take the complex conjugate so that you are only looking at the real components. Make sure your function is normalized and then apply your Hamiltonian. Remember that you can only simultaneously measure properties with operaters that commute.

 

[liverotcod]

42?

 

[mshheaddoc]

42?

you know about the worshpping of 42?

 

[ShyRem]

42 is THE answer


I'm sorry... what was the question?

 

[liverotcod]

What you need to do is take the complex conjugate so that you are only looking at the real components. Make sure your function is normalized and then apply your Hamiltonian. Remember that you can only simultaneously measure properties with operaters that commute.

Medikit, that's just fabulous. "...apply your Hamiltonian." Damn.

As for 42, I'm still in mourning. I guess he knows the question now.