Calculate the amount of heat (in kj) required to convert 74.6 g of water to steam at 100°c

You just need the energy of turning of water in to steam. Water is a little funny because once water gets to 100 degrees C, it stops changing in temperature whilst it all turns to steam. The amount of energy required to turn 1 gram of water into steam is 2257 Joules. We call this the latent heat of vaporisation. So to turn 74.6 grams of water to steam, you would need:

2257J/gram x 74.6g = 168,372.2 J = 168.4 kJ

(latent heat of vaporisation: 2257 J/gram)