A 50 g sample of water is heated from 20°C to 80°C. Using c = 4.18 J/g°C, how much energy is required?

• A. 7,500 J
• B. 15,000 J
• C. 22,000 J
• D. 12,540 J

Answer: (D)

Calculating energy to heat a substance uses q = m × c × ΔT. Here, the mass is 50 g, the specific heat of water is 4.18 J/g°C, and the temperature change is from 20°C to 80°C, so ΔT = 60°C. Compute: 50 × 4.18 × 60 = (4.18 × 60) × 50 = 250.8 × 50 = 12,540 J. The energy required is 12,540 joules (about 12.54 kJ). This reflects how heating water needs a lot of energy per degree because of its relatively high specific heat.