Acid-Base and Buffer Calculations

May 2013
12
0
Hello :)

I honestly have NO idea on how to go about this calculation, it appears simple but I'm struggling, can somebody kindly go through the steps with me as I REALLY want to learn this!!

What is the Ratio of [Base]/[Acid] in the following Buffer system?

Information:

Target pH = 9.89
AB Pair = NH4Cl/NH3
pKa = 9.25
[NH3] > [NH4Cl]
[NH4Cl] = 0.10 M
5.35g NH4Cl Required

I have no idea on where to go from this information so could somebody please show me? I'm not that skilled in mathematics so please explain it properly, thank you :)
 
Feb 2013
154
4
Michigan
pH = pKa + logR with R=ratio of base/acid
9.89 = 9.25 + logR
0.64 = logR
10^0.64 = R
R = 4.365 = base/acid (the answer you wanted)
also,
R= [NH3]/0.10 since NH4+ is the acid.
[NH3]/0.10 = 4.365
[NH3]=0.4365
So, the concentrations in the buffer mixture are 0.4365moles/L NH3 (base) and
0.100 moles/L NH4+ or NH4Cl
 
May 2013
12
0
Thank you very much! Greatly appreciated! :)

Would you happen to know how to work this one out? I'm not too sure if this is the same question just worded differently but I tried the approach you used in the previous example and I couldn't get far at all :(

I've attached the questions to this topic reply, if anybody could show me how to do this one too that would be awesome as I love chemistry and would love to be able to do this!

Thank you kindly!
 

Attachments

Feb 2013
154
4
Michigan
In a strong acid solution (pH=1), the aspirin would be in its molecular undissociated acid form. No ions, no charge. The ratio of acetylsalicylic acid to the acetylsalicylate anion (the conjugate base) would be very large (approaching infinity). At a pH of 5, which is larger (more basic) than a solution of an acid with a pKa of 3.5, would probably have a large concentration of the anion (basic) form and a negligible concentration of the acid. The ratio would be close to zero. These are qualitative conclusions without specific concentrations based on the information given.
 
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