Question

calculate the heat required to melt 5 kg of ice at 0 degree celsius

Answer 1

One gram of ice required 80 cal of heat to convert to one gram of water at 0 degree
It would take  80*5*1000 =400000 cal to melt ice at 0 degree

Answer 2

Heat required at 0 deg C, to melt ice at the same temperature to become water -- is called the latent heat of fusion.  It is equal to  79.8 Cal / gm of ice  or 333.55 Joules / gm.

       Heat required = 5 * 1000 * 79.8 = 399 kilo Calories
                     or  5 * 1000 * 333.55 =  1, 667.75 Kilo Joules
                                   = 1.66775 mega Joules.