A mass of 5kg of ammonia at its boiling point is vaporized when 6.5 MJ of heat is supplied to it. Calculate the specific latent heat of vaporization of ammonia?

##6.5/5 = 1.3 MJ kg^-1##

The "Latent heat of vaporization" for a substance(in this case ##NH_3##) is the heat that is required to cause a change in phase of a given mass(not specified)

We are told that ##6.5 MJ## caused this change at the boiling point. Which tells that ALL the heat gained = the heat needed to change in phase. Hence, ##6.5 MJ## IS the latent heat for vaporization of ##NH_3##

But the question asks for "specific" latent heat, so we divide by the mass, ## (6.5MJ)/(5kg) = 1.3 MJ kg^-1##