pH Q's on real DAT

[smiless]

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Overall i'm great at chem, however, never really grasped the concept of pH questions and how to solve them. Are there many on the real DAT?

thanks!!

 

[EKdental09]

I was wondering the same thing. I'm not very good at the calculations involving logs for pH, pKa, etc. and I was wondering if there were any of those on the actual DAT? I understand the simple ones with a base of 10 but do they really make you do some of the more complicated math?

 

[wbloo]

There are just a few things you need to know about pH.
pH is on a logarithim scale from 1 - 14 ( I think you can have a negative pH, but that is beyond the scope of the DAT) So going from a pH of 4-2 would be an increase of 10^2 in acidity since going from 4-2 is by a difference of two units. (always raise ten to the amount of units)

On this scale, 7 is neutral (Think strong base + strong acid)
Less than 7 is acidic (Think HCL = Very Strong acid - has a low pH= lower pH = more acidic, also anything with a lot of Oxygens becomes more acidic)

Greater than 7 is basic
Very basic = KOH = a high pH


pH is measured by the concentration of H+ions (dealing with acids)
pH = -log(concentration H+)
p0H is measured by the concentration of OH ions
POH= -log (concentration 0H-)
pH + pOH = 14
These formulas are only used if you have a strong acid or strong base.

If a base is given in a problem and it asks for pH, you must first solve for pOH then subtract it from 14 to get pH- I saw a lot of problems like this on achiever and top score...

For weak acids, the Handerson Hasselbach equation is used, since it doesn't completly dissociate into H+ ions.
pH = Pka + log(concentration of base/concentration of acid)

ka x kb = 1 x 10 ^-14
pka = -log ka
pkb = - log kb
ka = acid dissociation constant
kb = base dissociation constant
kw = 1 x 10 ^-14


The math may look complicated but if you change everything to scientific notation it becomes easier. Math is my weak point but I just reviewed some basic logs to save time on these types of questions.

log(1) = 0 which is why a buffer at the pka value is best:
Remember a buffer works best at a pH of pka +/- 1.
pH = pka + log (concentration of base/concentration of acid)

If the concentrations of base and acid are the same, you end up with

pH = pka + log (1)
log(1) = 0
pH = pka

which would be a great buffer.

Hope this helps.

 

[Contach]

to approximate the pH given a concentration of H+:
* VERY IMPORTANT SKILL *
-log(bx10^-n) = n - logb
where log1 = 0
and log 10 = 1
so basically I just move the decimal place once to the right.. so..
-log(bx10^-n) = n - logb = n - 0.b

hope that makes sense.

example:
-log(1x10^-7) = 7
-log(9x10^-7) = 7 - log9 =~ 7 - 0.9 =~ 6.1
-log(4x10^-7) = 7 - log4 =~ 7 - 0.4 =~ 6.6

its crude approximation, but it's the most indepth ill go.

 

[Contach]

For weak acids, the Handerson Hasselbach equation is used, since it doesn't completly dissociate into H+ ions.
pH = Pka + log(concentration of base/concentration of acid)

hrm.. I thought Henderson Hasselbach was for buffers.. not weak acids...

the only time conc of base = conc of acid is in a buffer. otherwise you will not know the conc of base if u are given an acid. you might as well do the keq. there's no point in using this for weak acids.

 

[cwalsh]

hey contach, great log explanation! helped a lot!!

 

[wbloo]

a buffer can contain a weak acid....here is what I found on wiki:

"In chemistry, the Henderson–Hasselbalch (often misspelled as Henderson–Hasselbach) equation describes the derivation of pH as a measure of acidity (using pKa, the acid dissociation constant) in biological and chemical systems. The equation is also useful for estimating the pH of a buffer solution and finding the equilibrium pH in acid-base reactions (it is widely used to calculate isoelectric point of the proteins).


Hope I cleared that up...

 

[utdent20]

has anyone done the destroyer problems on here, some of em are killers.. I mean the calculations are just not do-able in the time alotted

 

[Contach]

has anyone done the destroyer problems on here, some of em are killers.. I mean the calculations are just not do-able in the time alotted

yeah destroyer is f'ing stupid.
i am only doing the QR and bio.
Chem, and especially orgo chem is a waste of time.

 

[rooney09]

hrm.. I thought Henderson Hasselbach was for buffers.. not weak acids...

the only time conc of base = conc of acid is in a buffer. otherwise you will not know the conc of base if u are given an acid. you might as well do the keq. there's no point in using this for weak acids.

yeah I think you are right. Use that equation when you are dealing with buffers. But if its a weak acid problem then you use the ICE charts and Ka.

 

[dentalwhiz]

BUMP! Contach is saving lives out here, great explanation!