What mass of water would release 16700J of energy when freezing?
Your tool of choice here will be the enthalpy of fusion,
For a given substance, the enthalpy of fusion tells you how much heat is either needed to melt
Water has an enthalpy of fusion equal to
#DeltaH_"fus" = "333.55 J"#
This tells you that when
In your case, you know that
Use the enthalpy change of fusion as a conversion factor to determine how many grams of liquid water would give off this much heat when freezing
#"16,700" color(red)(cancel(color(black)("J"))) * overbrace(("1 g H"_ 2"O")/(333.55 color(red)(cancel(color(black)("J")))))^(color(blue)(= DeltaH_"fus")) = color(green)(|bar(ul(color(white)(a/a)color(black)("50.1 g H"_2"O")color(white)(a/a)|)))#
The answer is rounded to three sig figs, the number of sig figs you have for the heat given off.