Answer
$MgCl_2$ is soluble in water because most compound that contain $Cl^-$ are soluble, and the ones with $Mg^{2+}$ are not an exception.
Work Step by Step
According to the Table 9.8 (Page 294), an ionic solid, like $MgCl_2$ is soluble if it contains $Cl^-$ and does not contain $Ag^+$, $Pb^{2+}$ or $Hg{_2}^{2+}$.
