Friday, March 31, 2017

homework - How to calculate the concentration of all relevant species in a buffer of a given pH?



You prepare 1.0 L of a 0.25 M acetic acid solution with a final pH of 6.0. What are the molar concentrations of all relevant acetic acid species ([HA] and [AX]) given that the Ka for acetic acid is 1.74105 M?



I am getting confused with this problem.



Since the pH is given, I know what the [HX+] is. So now when I try to do the ICE table


HAHX+AXInitial0.2500Changex+x+xEnd0.25x+x+x


And from here I begin to assume [HX+]=[AX], which I am not sure about. Then I set x=106.0=1106 so I get [AX]=1106 M and [HA]=0.24999 M which I think is incorrect, and to even further ensure my that it's incorrect, when I attempt to check the Ka value with this, it does not match.


My second approach: [HA]=0.25 M


In this I determine the pKa from the Ka which turns out to be 4.759, which indicates that there should be more [AX] than [HA].


I now use the Henderson–Hasselbalch equation: 6.0=4.759+log([AX][HA])17.40=[AX][HA]17.40=[AX]0.25 M[AX]=4.35 M


I feel more confident about my second answer.


Can someone please help me out with this particular problem and perhaps tell me procedure I should use as well as what the correct answer should come out to be and why?




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