Quantcast

TBR Gen Chem Titrations Q33

BeMo Free Repeat™ On Us + MCAT 520 Challenge™
This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

ThinkDoc

New Member
10+ Year Member
Joined
Dec 9, 2009
Messages
6
Reaction score
1
The passage gives the titration curves for a conjugate pair. The question then asks you to compare the pH at the starting point for the acid with the pH at the equivalence point for the conjugate base.

The initial concentrations of the acid and the conjugate base are .10 M in 25 ml, and you bring the volume to 50 ml by adding 25 of the titrant.

The answer key says that concentrations of the two point differs by a factor of 2, therefore the pH should differ by the log of the square root of 2. How come?
 

LoLCareerGoals

Full Member
5+ Year Member
Joined
Mar 18, 2012
Messages
271
Reaction score
2
Can you post a full question for those of us without this book? It is probably due to the coefs of the reaction (2 proton acid), hard to tell without actual reaction.
 

Tarasaurusrex

Full Member
Joined
Jun 24, 2012
Messages
40
Reaction score
6
M1V1 = M2V2

At equivalence point

M1 = 0.1M
V1 = 0.025L
V2 = 0.05L

So M2 = 0.05M

x, -x, -x

-x^2 = (M1/M2) = -2

-x = (2)^(1/2)

x = log(2^(1/2))
 

V5RED

PGY-2 Family Medicine
Lifetime Donor
10+ Year Member
Joined
Jan 13, 2011
Messages
1,171
Reaction score
431
It is based on the quick formula they use in the BR books for determining pH at equivalence points.

For the -log[HA]/2 part, your [HA] for the less concentrated form is half as much as for the concentrated form. This means a difference of -log[2]/2 (ie one concentration is double of the other) this is equivalent to -log(2^1/2) because you can convert a scalar multiplied by a logarithm into a logarithm using said scalar as an exponent.

Basically they got really fancy with their answer and didn't bother to explain how they got there.
 
This thread is more than 9 years old.

Your message may be considered spam for the following reasons:

  1. Your new thread title is very short, and likely is unhelpful.
  2. Your reply is very short and likely does not add anything to the thread.
  3. Your reply is very long and likely does not add anything to the thread.
  4. It is very likely that it does not need any further discussion and thus bumping it serves no purpose.
  5. Your message is mostly quotes or spoilers.
  6. Your reply has occurred very quickly after a previous reply and likely does not add anything to the thread.
  7. This thread is locked.

Similar threads

Top