Shown below is the structure of a peptide in its fully protonated form. The $p{K}_a$ values of all acidic hydrogens are indicated. What is the isoelectric point of this peptide?

A$.4\mathrm{.}1$

B$.5\mathrm{.}1$

C$.5\mathrm{.}8$

D$.6\mathrm{.}2$

E$.7\mathrm{.}2$

A $30\mathrm{.}0mL$ sample of $0\mathrm{.}30M$ weak acid $HA$ was titrated with $0\mathrm{.}30$MNaOH. The following data were collected during the titration at $25C$ :

$\backslash$table$[[mL$ of NaOH added,$5\mathrm{.}0,10\mathrm{.}0,15\mathrm{.}0,20\mathrm{.}0,25\mathrm{.}0,30\mathrm{.}0],[pH,3\mathrm{.}42,3\mathrm{.}81,4\mathrm{.}11,4\mathrm{.}41,4\mathrm{.}81,8\mathrm{.}64]]$

What is the $K_a$ for the weak acid $HA$ at $25C?$

A$.7\mathrm{.}76{10}^{-5}$

B$.1\mathrm{.}69{10}^{-5}$

C$.3\mathrm{.}89{10}^{-4}$

D$.2\mathrm{.}51{10}^{-9}$

E$.4\mathrm{.}56{10}^{-7}$

CHEM$112$B

Practice Exam $3$D

$4$