A rectangular box of negligible mass measures 5m by 1m by .5m. How many kgs of mass can be loaded onto the box before it sinks in the water?

V=5*1*.5 = 2.5m^3
density of water =1g/cm^3 =1000 kg/m^3

I also know that when the density of the mass is greater than the density of water then the mass will sink. How would I solve this problem? Thanks in advance for your help.