Buffer problem

[Dochopeful13]

Can someone please explain what I’m missing here. The question asks what is the ph of a solution that contains 10 times the concentration of acetic acid as sodium acetate. The pka of acetic acid is 4.8. Wouldn’t the ratio be 10/1 but the problem shows 1/10.

However, the equation is a/ha.

Thank you.

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[BerkReviewTeach]

You have ten times more weak acid than conjugate base, so you know the pH will be (log 10) lower than the pKa. The HH equation is pH = pKa + log [A-]/[HA], which is why they list the ratio as 1-to-10. As long as you know that the log of 10 is 1, then you know that the pH differs from pKa by 1.0. Again, because you have more acid than base, the pH must be lower than the pKa. The pH in this case should be 3.8, based on their numbers.

Be able to use the Henderson-Hasselbalch equation for buffers and the Bennett equation for a weak acid [the shortcut form is pH = (pKa + pHi if it fully dissociated)/2]. The first equation is really common and found in every chemistry book. The second one was created back in 1989 by the guy who went on to start Hyperlearning MCAT in 1990 and BR in 1992. The derivation and examples of how to use it can be found in the books he wrote.

 

[jhmmd]

Can someone please explain what I’m missing here. The question asks what is the ph of a solution that contains 10 times the concentration of acetic acid as sodium acetate. The pka of acetic acid is 4.8. Wouldn’t the ratio be 10/1 but the problem shows 1/10.

However, the equation is a/ha.

Thank you.

You can assume that it's roughly 6. Any questions? See pH estimation tips thread.