How much TOTAL energy did the water take to boil away completely?

Question

Mr. G is going to make some tea. He fills his kettle with 400 g of hot tap water at 49 degrees C, turns on the stove, and then leaves it on the stove for all of class. When he returns, he finds that the teapot is empty - all the water has boiled away. How much TOTAL energy did the water take to boil away completely?

Helpful notes: Water boils at 100 degrees C. The specific heat of water is 4.19 J/g degree C. The latent heat of water melting is 334 J/g, and the latent heat of water boiling is 2260 J/g.

Answer 1

Step 1: Find the heat to raise temperature of 400 g water from 49ºC to 100ºC:

q = mC∆T

q = heat = ?

m = mass = 400 g

C = specific heat of water = 4.19 J/gº

∆T = change in temperature = 100 - 49 = 51º

q = (400 g)(4.19 J/gº)(51º) = 85,476 J

Step 2: Find the heat to evaporate 400 g water at 100ºC

q = m∆Hvap

q = heat

m = mass = 400 g

∆Hvap = heat of vaporization = 2260 J/g

q = (400 g)(2260 J/g) = 904,000 J

Add J from steps 1 and 2 to get ...

85,476 J + 904,000 J = 989,476 J

Converting to kJ = 989 kJ (Answer)