Heat of vaporization of water is expressed as △H=40.7KJ/mol. How much heat is required to vaporize 0.123g of vapor water?

0.123gx(1mol/18g/mol)=0.00683molx40.7KJ/mol=0.277981KJ

I did it.. I hope it is correct, but I am not sure !

Please check it , Thanks

Yes, that's correct, but be sure to keep your answer to a certain number of significant figures. Here, the question put 3 significant figures, so, your answer will be 0.278 kJ.