Tbr gen Chem buffers #33

[akimhaneul]

For this problem I understand that it is less concentrated at point D because of greater volume but I don't understand how I can get the exact difference in ph.

TBR said the difference is log(2)^0.5. But I don't understand how they got this number.

Thank you!


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[betterfuture]

I am going to say the answer is C? If it is, then I don't think you need to calculate the 'exact' number. At least you understand it shouldn't be 1 pH unit less than d. Should be roughly similar.

 

[BerkReviewTeach]

When you titrate, you both convert the reactant into a product and you dilute the solution. At point a, you have 0.10 M HA, before any titration has begun. At point d, you have 0.05 M HA, the product formed from the complete protonation of A- by HCl in the solution which is now 50 mL (resulting from the original 25 mL in the flask and the 25 mL of titrant added).

If you recall the Berkeley Review Weak Acid equation, then you know that pH = 1/2(pKa + pHif it fully dissociated). The pKa is the same, but pHif it fully dissociated is 1.0 for 0.10 M acid and 1.3 for 0.05 M acid. That is a difference of 0.3, which when multiplied by 1/2 leads to a difference of 0.15 between the two pHs. That is the route to the exact answer.

But as betterfuture points out, you simply need to know that points a and d are really close to another, so the difference is less than 1.0.

 

[betterfuture]

Thank you for confirming that. I was beginning to doubt myself for a second.

 

[BerkReviewTeach]

Thank you for confirming that. I was beginning to doubt myself for a second.

You are way too smart and clear thinking to need to doubt yourself! Self-doubt is going to happen from time to time, but hopefully you squelch it when it creeps in.

 

[betterfuture]

I'll let my practice scores speak for me. No, but really thank you for the kind words. I am nearing test day and self-doubt is no doubt creeping in.