Answer
$50 \ mg$
Work Step by Step
We are given the density and the volume ($V=0.50\ mL,\ \rho=1.0\ g/mL$), so the total mass is:
$m=0.50\times1.0 = 0.50\ g$
Since the solution contains 10% by mass of the compound:
$m_c=0.50\times0.10=0.050\ g$
$0.050\ g \times \dfrac{1\ mg}{1\times10^{-3}\ g}=50 \ mg$
