For a phase transition, we assume a constant temperature (and also, pressure). So, the heat flow
$bb(q_P)$
$= bb(mDeltaH)$ , if$DeltaH$ is in$"J/g"$ , or
$= nDeltaH = bb(m/(M_m)DeltaH)$ , if$DeltaH$ is in$"J/mol"$ ,where:
$m$ is the mass of the ice in$"g"$ .$n$ is the$bb("mol")$ s of the ice.$M_m$ is the molar mass of the ice in$"g/mol"$ .$DeltaH$ is the enthalpy of fusion, which is about$"6.02 kJ/mol"$ for ice at$0^@ "C"$ and$"1 atm"$ .
So, simply solve for the heat flow required to melt
$color(blue)(q_P) = (300 cancel"g" xx cancel"1 mol"/(18.015 cancel"g"))("6.02 kJ"/cancel"mol")$
$=$ $color(blue)("100. kJ")$