(no subject)
drememagik: tell me how to do pka and ph problems
VegetaAV: we only had one problem like that, we can';t use calculators . . .
drememagik: an easy one that i dont understand: if a histidine residue in an amino acid sequence has a pka value of 6.0 for its side chain and the pH of the environment was 5.5, what % of that side chain will be protonated?
VegetaAV: well, the pka means that 50-50 are ionized and not
VegetaAV: so it should be close to that
drememagik: my options are 24, 32, 58, 76 or need more info
VegetaAV: just a guess would be 58
drememagik: and the answer is 76
VegetaAV: but use ph=pka +log ([acid]/[base])
VegetaAV: reverse acid and base
VegetaAV: base on top
VegetaAV: and acid would be on bottom (protonated)
VegetaAV: and you can use it to find a ratio
VegetaAV: that should work
drememagik: hmmm..
VegetaAV: did that work?
drememagik: im checkin
drememagik: so im gettin that [acid]/[base] = .32
VegetaAV: it's base over acid
VegetaAV: i typed it wrong the first time
drememagik: ok well then thats .32
VegetaAV: yeah
VegetaAV: and the base has a neg charge while the acid is neutral
VegetaAV: it's defined [A-]/[HA]
VegetaAV: so the protonated would be the bottom
VegetaAV: and just flip the ratio over
drememagik: meaning?
VegetaAV: meaning . . .
VegetaAV: crap
VegetaAV: one sec
VegetaAV: you don't get any concentrations?
drememagik: nope
drememagik: all i get are those 2 numbers
VegetaAV: then i'm stuck
VegetaAV: :-(
VegetaAV: we didn't have anything like that
drememagik: aww and i thought you almost had it
VegetaAV: all conceptual
VegetaAV: me too
drememagik: ok one more... tho this one is much more difficult
drememagik: so you have 1 L of 25mM lactic acid (with pka 3.73)
drememagik: and the pH is 4.21
drememagik: you add 1mL of 1.0 M soln of KOH
drememagik: whats the new pH?
VegetaAV: i've never done one of those
drememagik: man, thats not fair that my biochem class is harder than yours!
VegetaAV: the other one i did for hw, but weeks ago
VegetaAV: well, we didn't do almost anything of pka and stuff
drememagik: ive got no clue how to do it, and the prof never taught it, and its not in the book
drememagik: stupid prof
VegetaAV: we had to do some crazy graphs (without a calculator) on Michaelis-Menses reactivities
drememagik: but it sure as hell is on the practice exam
drememagik: yeah i think i could do those
drememagik: man, you were supposed to blow me away with your knowledge of biochem
VegetaAV: i'm supposed to blow you away with my knowledge of camus
VegetaAV: this whole pre-med thing gets to me
VegetaAV: i've been sick of it for 2 years
drememagik: too bad im not interested in camus right now... biochem is my priority since its going to kick my asssss
VegetaAV: i guess try to work backwards . . .
VegetaAV: thats what i do sometimes
drememagik: i have no idea how to get there tho
drememagik: but i guess ill try that since i have no other options besides just guessin on the exam
VegetaAV: yep
drememagik: at least its on a curve, and everyone else is lost