TBR Buffers and Titration Ex 5.2

[capnamerica]

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In choices C and D, when they say OH- and H3O+ respectively, is that basically the same thing as saying A- and HA?

 

[Rabolisk]

Post the question? My intuition says no, but I can't know without example.

 

[capnamerica]

Post the question? My intuition says no, but I can't know without example.

Which of the following solutions results in a buffer with a pH of 5.0, given that HA has a pKa of 4.7

A. HA with 0.5 equivalent of A-
B. A- with 1 equivalent of HA
C. HA with one-third equivalent of OH-
D. A- with one-third equivalent of H3O+

 

[IntelInside]

Which of the following solutions results in a buffer with a pH of 5.0, given that HA has a pKa of 4.7

A. HA with 0.5 equivalent of A-
B. A- with 1 equivalent of HA
C. HA with one-third equivalent of OH-
D. A- with one-third equivalent of H3O+

I am going with choice D

 

[fizzgig]

B. equal concentrations of acid and conj base - pH would be 4.7. so this is out
A. some HA with less A-, more acid than conj base, so pH would be below 4.7, so this is out.
C. HA with one third of that HA deprotonated by OH-, so you have a mix of 1/3 A- and 2/3 HA, again more acid than base, pH below 4.7, so this is out.
D. A- with one third of that A- protonated by H30+, so you have a mix of 1/3 HA and 2/3 A-, more base than acid, pH would be above the halfnhalf point of 4.7, so this is it.

someone correct me if that's wrong.

 

[Rabolisk]

B. equal concentrations of acid and conj base - pH would be 4.7. so this is out
A. some HA with less A-, more acid than conj base, so pH would be below 4.7, so this is out.
C. HA with one third of that HA deprotonated by OH-, so you have a mix of 1/3 A- and 2/3 HA, again more acid than base, pH below 4.7, so this is out.
D. A- with one third of that A- protonated by H30+, so you have a mix of 1/3 HA and 2/3 A-, more base than acid, pH would be above the halfnhalf point of 4.7, so this is it.

someone correct me if that's wrong.

I agree and to answer the question H3O+ and OH- are not the sane thing as HA and A-. The latter two react with the acid or base. For example 1 mole. Of HA and .5 mole of OH- would react to form roughly .5 moles each of HA and A-.

 

[capnamerica]

Yeah, I knew A and B were wrong, but since they used OH- and H3O+ instead of A- and HA, I didn't understand choices C and D. Are answer choices phrased like this on the MCAT?

 

[fizzgig]

i dont think it would be too far fetched for answers like C and D to be given. they are specifically testing the kind of thought process you yourself had - is an option that says A- and HA mixed the same as OH- and HA mixed? no. the OH is strong and will fully react given enough acid, yielding a new A- HA mix. i could see them asking for that process.