Alanine has $p{K}_{a1}=2.3$ and $p{K}_{a2}=9.7$. At pH 9.70 the average net charge per molecule (ignoring the cationic form, which is negligible) is

At pH = $p{K}_{a2}$ the $\mathrm{NH}{}_3{}^{+}$ group is exactly half deprotonated, so half the molecules are zwitterions (charge 0) and half are anions (charge $-1$). Average charge = $0.5(0)+0.5(-1)=-0.50$. The cation fraction is about $10^{-7.4}$ and is ignored.