How much energy is required to heat 200 g of water from 20°C to 80°C when cp is 4.18 J/g°C?

• A. 60.0 kJ
• B. 100.0 kJ
• C. 50.2 kJ
• D. 25.0 kJ

Answer: (C)

The energy required to raise the temperature of a substance is found using Q = m × cp × ΔT. For water, cp = 4.18 J/g°C. The mass is 200 g and the temperature changes from 20°C to 80°C, so ΔT = 60°C. Now multiply: 200 × 4.18 × 60 = 50,160 J. Converting to kilojoules gives 50.16 kJ, which rounds to 50.2 kJ. So about 50.2 kJ of energy is needed.