Final answer:
The specific heat of water is 4.184 J/g°C. To heat 800 g of water from 21°C to 85°C, we can use the equation: Energy = specific heat capacity x mass x temperature change. The energy required is 210 kJ.
Step-by-step explanation:
The specific heat of water is 4.184 J/g °C, so to heat 1 g of water by 1 °C requires 4.184 J. We note that since 4.184 J is required to heat 1 g of water by 1 °C, we will need 800 times as much to heat 800 g of water by 1 °C. Finally, we observe that since 4.184 J are required to heat 1 g of water by 1 °C, we will need 64 times as much to heat it by 64 °C (that is, from 21 °C to 85 °C).
This can be summarized using the equation:
Energy = specific heat capacity x mass x temperature change
Energy = 4.184 J/g °C x 800 g x 64 °C = 210,000 J (= 210 kJ)
Because the temperature increased, the water absorbed heat and q is positive.