Question

You dissolve 2mg of aspartic acid (133 g/mole) into 13 mL of water, and adjust the pH to 1. How many moles of NaOH do you need to convert 100% of this amino acid into the zwitterion?

Answer 1

Aspartic acid pka1=1.88,pka3=3.65

Zwitter ionic form at pH between 1.88 and 3.65

HOOCCH2-CH(⁺NH3)-COO^-

SO pH has to be changed from pH 1 to 1.88 for converting it to zwitter ionic form[when pH>pka then the species is deprotonated]

At pH=1   [pH=-log[H3O+]

[H3O+]=10^-pH=10^-1=0.1 M

At pH=1.88

[H3O+]=10^-1.88=0.0132 M

Change in [H3O+] required=0.1-0.0132=0.0868 M or mol/L

Volume of solution=13ml=0.013 L

Moles of [H3O+] that needs to be neutralized to bring about the required change in pH(from 1 to 1.88)=0.0868 mol/L*0.013M=0.00113 moles [H3O+]

So moles of NaOH to be added=0.00113 moles(answer)