Answer
$1.58489\cdot10^{-4}\leq H^+\leq 1.58489\cdot10^{-3}$.
Work Step by Step
We know that $pH$ is given by:
$pH=-\log[H^+]$
Thus, we can solve for the hydrogen ion concentration as:
$H^+=10^{-pH}$.
Hence, here the range of the hydrogen ion concentration is:
$10^{-3.8}\approx1.58489\cdot10^{-4}$
and
$10^{-2.8}\approx1.58489\cdot10^{-3}$
Thus
$1.58489\cdot10^{-4}\leq H^+\leq 1.58489\cdot10^{-3}$.
