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kphilxo
A sample of water releases 4.50 x 103 J of heat energy and its temperature drops from 80.0ºC to 68.0ºC. What is the mass of the water?
I looked up the specific heat capacity of water and I found it was 4.186Joules per gram per degree Kelvin. You should be able to figure it out from this constant and a quick look at this page: http://en.wikipedia.org/wiki/Specific_heat_capacity#Extensive_and_intensive_quantities