simple acid-base problem that i cant really figure out how to answer...

[phatj]

The pKa value of the important human metabolite lactic acid is 3.08. Sodium
hydroxide is added to a solution of lactic acid until the pH reaches 4.00. Are the
majority of lactic acid molecules dissociated or undissociated in the solution at pH
4.00? Justify your answer.

ok lactic acid is a weak acid so its not going to completely dissociate...
sodium hydroxide is obviously a strong base.

pH is 4 so its still mostly acidic..........

ok so lactic acid is weak so its conjugate base is relatively strong
naoh is strong base so its conjugate is relatively weak....

since solution is acidic at 4.00 ... uhhhh




EDIT i think this is it:
i think this is the mathematical basis for this question. I think the answer is there are more dissociated moleclues

pH - pKa = log(products/reactants)
In other words
pH - pKa = log(dissociated/undissociated)

Here we have 4 - 3.08 = log (diss/undiss)
so 0.92 = log(diss/undiss)
so 10^.92 = diss/undiss
so 8.3 = diss/undiss

so more dissociated

 

[TechBoyJK]

oh.... wrong acid...

 

[TheoPetro]

not touchen that w/ a 10000000ft, PhD in chem, pole. I just took my last chem class EVER and you have brought back memories that were repressed for a reason....heres a bump tho

 

[phatj]

chem is fun though! my brain isnt working tonight... i took gen chem 2 years ago and cant remember this stuff

 

[TheoPetro]

Originally posted by: phatj
chem is fun though! my brain isnt working tonight... i took gen chem 2 years ago and cant remember this stuff

dont start with me...

 

[ruffilb]

>_>

Never taken chem in my life, but I will next year, probably.

 

[stankonia3000]

are you making this too hard? pKa is the pH at which a particular subsituent on a compound is 50% ionized via loss of a proton, right? lactic acid can lose the proton on the carboxylic acid group, creating the ionic form...at pH 4.0, the majority of lactic acid molecules should be protonated...is that the same as "undissociated"??

its been a long time since chemistry so the above may be completely wrong

 

[StevenYoo]

Originally posted by: stankonia3000
are you making this too hard? pKa is the pH at which a particular subsituent on a compound is 50% ionized via loss of a proton, right? lactic acid can lose the proton on the carboxylic acid group, creating the ionic form...at pH 4.0, the majority of lactic acid molecules should be protonated...is that the same as "undissociated"??

the pKa is the tendency for an acid to lose it's proton.

the lower the pKa (the more negative), the stronger the acid.

the higher the pKa, the weaker the acid.

at higher pH's, there is a higher tendency for an acid to lose its proton.

Since the pH is above lactic acid's pKa, there should be more unprotonated molecules.

 

[StevenYoo]

here check this out

 

[phatj]

i think this is the mathematical basis for this question. I think the answer is there are more dissociated moleclues

pH - pKa = log(products/reactants)
In other words
pH - pKa = log(dissociated/undissociated)

Here we have 4 - 3.08 = log (diss/undiss)
so 0.92 = log(diss/undiss)
so 10^.92 = diss/undiss
so 8.3 = diss/undiss

so more dissociated

 

[StevenYoo]

Originally posted by: phatj
i think this is the mathematical basis for this question. I think the answer is there are more dissociated moleclues

pH - pKa = log(products/reactants)
In other words
pH - pKa = log(dissociated/undissociated)

Here we have 4 - 3.08 = log (diss/undiss)
so 0.92 = log(diss/undiss)
so 10^.92 = diss/undiss
so 8.3 = diss/undiss

so more dissociated

great jooooeeeerb

 

[lokiju]

Originally posted by: TechBoyJK
oh.... wrong acid...

:laugh: