$解:(1)??V_水=\frac {m_水}{ρ_水}=\frac {200\ \mathrm {g}}{1.0\ \mathrm {g/cm}^3}=200\ \mathrm {cm}^3????$
$V_石=V_容-V_水=300\ \mathrm {cm}^3-200\ \mathrm {cm}^3=100\ \mathrm {cm}^3$
$(2)??m_石=ρ_石V_石=2.5\ \mathrm {g/cm}^3×100\ \mathrm {cm}^3=250\ \mathrm {g}??$