Get Delta U to heat water from 25 C to 100 C and Delta U to vaporize the water at 100 C.

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Delta U1 = ( m ) ( Cv ) ( T100 - T25 )

Delta U1 = ( 100 g ) ( 4.18 J / g - C ) ( 100 C - 25 C ) = 31350 J

Delta U2 = ( m ) ( u sub fg at 100C )

Delta U2 = ( 100 g ) ( 2257 J/g ) = 225700 J

Q = Delta U1 + Delta U2

Q = 31350 J + 225700 J = 257050 J = 257.05 kJ <----------------

Supposing you mean bringing the water to 100°C, instead of bringing to 100°C and boiling it all away:

(4.18 J/g·°C) x (100 g) x (100 - 25)°C = 31350 J = 3.14 kJ