The equilibrium constant for the dissolution in water of a nonionic compound, su
ID: 1010194 • Letter: T
Question
The equilibrium constant for the dissolution in water of a nonionic compound, such as diethyl ether (CH_3CH_2OCH_2CH_3) can be written as: Ether(I) doubleheadarrow ether (aq) K = [ether (aq)] y_ether At low ionic strength, y = 1 for neutral compounds. At high ionic strength, most neutral compounds can be salted out of aqueous solution. That is, when a high concentration (typically greater than 1M) of a salt such as NaCl is added to an aqueous solution, neutral molecules become less soluble. Does the activity coefficient, y_ether increase or decrease at high ionic strength. Explain (in terms of K). Why does the buffer capacity reach a maximum when pH - pK_a?Explanation / Answer
9. The activity coefficient value for ether at high NaCl concentration would be very low such that the value would decrease at high ionic strength. Which makes the K,
K = [ether(aq)] x activity coefficient
value too small to be extracted in aqueous layer.
Thus, the neutral compound in ether would preferentially be extrcated in ether layer and can be easily separated by salting out the layer.
10. Maximum buffer capacity is when the buffer resists change in pH by addition of any base or acid to the maximum strength.
According to Hendersen-Hasselbalck equation, when dissociated form [A-] = undissociated form [HA],
pH = pKa + log([A-]/[HA])
pH = pKa
We have equal amounts of undissociated and dissociated forms in solution. This is the flattest part of the curve and thus any addition of acid H+ or base OH- would not change the pH of the buffer. Thus this is the part where buffer has maximum buffer capacity.
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