While conducting an experiment to isolate and amino acid, you direct your lab to
ID: 714556 • Letter: W
Question
While conducting an experiment to isolate and amino acid, you direct your lab to make separate solutions of pH 2.1, pH 8.5, and pH 11.9. The amino acid in the reaction mixture is alanine, CH3CH(NH3+)CO2H (pKa values are alpha-carboxyl group = 1.8, alpha-amino group = 10.8). In this reaction, the inital energy (delta G) was 3.5kJ/mole. After an effector was inserted into the reaction, the energy was determined to be 3.0 kJ/mole.
As given in the reaction mixture, the chemical formula of alanine shows that it would not exist:
a) In an acid solution and would have a plus one charge.
b) In a basic solution and would have a negative one charge.
c) In an acid solution and would have a negative one charge.
d) In a basic solution and would have a plus one charge.
Explanation / Answer
Generally, if pH < 7, the solution is said to be acidic and if the pH > 7, the solution is said to be basic. The solutions being made are solutions of pH 2.1(acidic), 8.5(basic) and 11.9(basic). We are given the pKa values of the carboxyl and amino group in the acid at which they deprotonate.
At pH < 1.8(acidic), both carboxyl group and the amino group will be protonated thus having a net charge of +1.
At pH = 2.1(acidic), the carboxyl group will deprotonate (as the pKa value of carboxyl group is 1.8) and will have a charge of -1 whereas the amino group will still be protonated and have a charge of +1, thus net charge will be zero.
At pH = 8.5(basic), the carboxyl group will deprotonate (as the pKa value of carboxyl group is 1.8) and will have a charge of -1 whereas the amino group will still be protonated and have a charge of +1, thus net charge will be zero.
At pH = 11.9(basic), the carboxyl group will deprotonate having a charge of -1 and the amino group will also be deprotonated having a charge of 0, thus net charge will be -1.
Thus, the chemical formula of alanine shows that it would not exist in an acid solution and would have a negative one charge and also, will not exist in a basic solution and have a plus one charge.
The answers are (c) and (d).
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