Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Nugget on December 10, 2018, 12:25:58 AM
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Hi guys! I need help with a certain question:
Compound X
pKa = 9.45
Solubility = 61.7 mg/L
MW = 259.34
Calculate solubility at pH 4.2 in mg/L
I got some wierd answer with like 10 million so i dont think im correct :'(
Assuming Basic due to pKa:
pH = pKa + Log(s0/s-s0)
4.2 = 9.45 + Log(61.7/s-61.7)
-5.25 = Log(61.7/s-61.7)
10^-5.25 = 61.7/s-61.7
(61.7 รท 10^-5.25) + 61.7 = s
s = 10972045.66 mg/L (2d.p.)
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Your result is "correct" - that is, there is no error in the math, but there are two erroneous assumptions that the concentration of non-protonated form will be always 61.7 mg/L and that protonated form is always fully soluble. These assumptions are reasonably correct as long as the concentrations of both forms don't differ too much, but they don't hold forever. What your result is telling you is that at pH 4.2 you are way too far from pKa for these assumptions to hold.
No way to find the answer from the data given.