Acid/Base question

[Trisphorin]

I have a couple of questions related to acid/base and i would really appreciate it if someone can help me with.

1. does pH + pOH always adds up to 14? because in berkeley review, they said that it only add up to 14 at 25 degrees. In questions that doesn't specify temperatures, the explanantion always used pH + pOH=14 to solve the problem.

2. pKa vs. pH
From my understanding, pH is a concentration of H+ and pKa is how readily an acid dissociates. pH is affected by both the pKa and the concentration of the acid so a weak acid of higher concentration can have a lower pH than a strong acid with lower concentration. is that correct?

Thank you,

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[GreenRabbit]

1. They are being nitpicky with the first statement. pH + pOH = pKw. At room temperature, pKw = 14. You don't need to know how pKw varies, and unless the question states otherwise, you can assume pH + pOH = 14.

2. You're correct.

 

[Der Kaiser]

I have a couple of questions related to acid/base and i would really appreciate it if someone can help me with.

1. does pH + pOH always adds up to 14? because in berkeley review, they said that it only add up to 14 at 25 degrees. In questions that doesn't specify temperatures, the explanantion always used pH + pOH=14 to solve the problem.

2. pKa vs. pH
From my understanding, pH is a concentration of H+ and pKa is how readily an acid dissociates. pH is affected by both the pKa and the concentration of the acid so a weak acid of higher concentration can have a lower pH than a strong acid with lower concentration. is that correct?

Thank you,

In regard to your first question I too would be interested in the answer, but for the purposes of the MCAT I would wager that pH + pOH always adds up to 14. This is how it was for me in General Chemistry II.

A heavily diluted strong acid could be weaker in pH than a highly concentrated weak acid. The pKa is found by taking the -log of the Ka, and yes it tells us the extent of dissociation. So a weak acid would have a lower pH as long as it was at a conc. that allowed for more total H+ ions in solution.

 

[BerkReviewTeach]

1. They are being nitpicky with the first statement. pH + pOH = pKw. At room temperature, pKw = 14. You don't need to know how pKw varies, and unless the question states otherwise, you can assume pH + pOH = 14.

Be careful how quickly you dismiss the exception here, because there is one case in particular that has MCAT applicability. If you consider biological systems at 36-37 degreesC, then the 14 for water rule no longer applies. You are right in the approach of assuming pH = pOH = 14 unless told otherwise, but the trick is knowing that being told otherwise could be as simple as looking at a buffer system in the body. It's highly unlikely they'd ask you about a pH of solution for a titraion done at 42 degreesC by a left handed chemist 0.17 miles above sea level with a humidity of 56.2%. But in bio systems it's a possibility that they may ask a question as to why neutral water is not pH = 7.0 under in vivo conditions.

Excellent perspective Trisphorin on the pH vs. pKa relationship.

 

[justhanging]

I have a couple of questions related to acid/base and i would really appreciate it if someone can help me with.

1. does pH + pOH always adds up to 14? because in berkeley review, they said that it only add up to 14 at 25 degrees. In questions that doesn't specify temperatures, the explanantion always used pH + pOH=14 to solve the problem.

2. pKa vs. pH
From my understanding, pH is a concentration of H+ and pKa is how readily an acid dissociates. pH is affected by both the pKa and the concentration of the acid so a weak acid of higher concentration can have a lower pH than a strong acid with lower concentration. is that correct?

Thank you,

If you know that Keq changes with temperature then you know that Kw changes with temp and pkw changes with temp and finally pOH + pH will change with temp. A good question to ask yourself is what makes a neutral solution?

a) pH = 7
b) [OH] = 1.0E-7
c) pH = pOH
d) pH + pOH = 14