How much energy in kJ is given off when 1.00 g of steam at 100.0 degree C cools to room temperature 25.0 degree C? Solution Step 1 : steam at 100 C is converted to water at 100 C heat given off is q1 = mass of steam x dHvap q1 = 1 g x 2257 J/g q1 = 2257 J Step 2 : water at 100 C is converted to water at 25 C heat given off is q2 = mass x specific heat x temp change q2 = 1 g x 4.184 ( J/gC) x 75 C q2 = 313.8 J so total heat given off = 2257 J + 313.8 J total heat given off = 2570.8 J so 2.571 kJ of energy is given off .

1. How much energy in kJ is given off when 1.00 g of steam at 100.0 degree C cools to room
temperature 25.0 degree C?
Solution
Step 1 : steam at 100 C is converted to water at 100 C
heat given off is
q1 = mass of steam x dHvap
q1 = 1 g x 2257 J/g
q1 = 2257 J
Step 2 : water at 100 C is converted to water at 25 C
heat given off is
q2 = mass x specific heat x temp change
q2 = 1 g x 4.184 ( J/gC) x 75 C
q2 = 313.8 J
so
total heat given off = 2257 J + 313.8 J
total heat given off = 2570.8 J
so
2.571 kJ of energy is given off