Multiple Choice
What is the pH of a 0.050 M solution of HNO2 (Ka = 4.6 × 10^-4)?
17. Acid and Base Equilibrium
pH of Weak Acids
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At what pH will alanine exist predominantly in its zwitterionic form, where it is overall uncharged?
A
pH 9
B
pH 6
C
pH 12
D
pH 1
Verified step by step guidance1
Understand that alanine is an amino acid with both an amino group and a carboxyl group. In its zwitterionic form, the amino group is protonated (NH₃⁺) and the carboxyl group is deprotonated (COO⁻), resulting in an overall neutral charge.
Identify the relevant pKa values for alanine. Typically, the pKa for the carboxyl group is around 2, and the pKa for the amino group is around 9.7. These values indicate the pH at which each group is half protonated and half deprotonated.
Recognize that the zwitterionic form of alanine is most stable at a pH that is between the pKa values of the carboxyl and amino groups. This is because at this pH, the carboxyl group is deprotonated and the amino group is protonated.
Calculate the isoelectric point (pI) of alanine, which is the pH at which the molecule exists predominantly in its zwitterionic form. The pI is the average of the pKa values of the carboxyl and amino groups: \( \text{pI} = \frac{\text{pKa}_{\text{COOH}} + \text{pKa}_{\text{NH}_3^+}}{2} \).
Determine that the pH at which alanine exists predominantly in its zwitterionic form is approximately 6, which is the average of the typical pKa values for the carboxyl and amino groups.
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