Given that CH3COOH has a pKa = 4.8 and CH3+NH3 has a pKa = 10.7, answer the foll

Question

Given that CH3COOH has a pKa = 4.8 and CH3+NH3 has a pKa = 10.7, answer the following:

Part A:

What pH would you make the water layer in order to cause both compounds to dissolve in it?

What pH would you make the water layer in order to cause the acid to dissolve in the water layer and the amine to dissolve in the ether layer?

pH>

What pH would you make the water layer in order to cause the acid to dissolve in the ether layer and the amine to dissolve in the water layer?

pH<

Water should have pH 6.8 - 10.7. Water should have pH 4.8 - 8.7. Water should have pH 6.8 - 8.7. Water should have pH 4.8 - 10.7.

Explanation / Answer

In order to have a effective dissolution, either of the forms : charged or neutral has to be atleast 100 times more concentrated than the other form. It can be calculated from Hinderson hasselbalch equation that in order to obtain a 100: 1 ratio of acidic form:basic form, the pH must be two pH units lower than the pKa of the compound; and in order to obtain a 1 00: 1 ratio of basic form: acidic form, the pH must be two pH units greater than the pKa of the compound.

If both compounds are to dissolve in water, they both must be charged. Therefore, the carboxylic acid
must be in its basic form, and the aniine must be in its acidic form. To accomplish this, the pH will have to
be at least two pH units greater than the pKa of the carboxylic acid and at least two pH units less than the
pKa of the ammonium ion. In other words, it must be between pH 6.8 and pH 8.7.

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For the carboxylic acid to dissolve in water, it must be charged (in its basic form), so the pH will have to be
greater than 6.8. For the amine to dissolve in ether, it will have to be neutral (in its basic form), so the pH
will have to be greater than 12.7 to have essentially all of it in the neutral fortn. Therefore, the pH of the
water layer must be greater than 12.7.

______________________

To dissolve in ether, the carboxylic acid will have to be neutral, so the pH will have to be less than 2.8 to
have essentially all the carboxylic acid in the acidic (neutral) form. To dissolve in water, the amine will
have to be charged, so the pH will have to be less than 8.7 to have essentially all the amine in the acidic
form. Therefore, the pH of the water layer must be less than 2.8

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