How much thermal energy is needed to change 80 g of water initially at 60°C to steam? [c = 4200 Jkg

JAMB Physics 2023 208 views
Source: JAMB · 2023

How much thermal energy is needed to change 80 g of water initially at 60°C to steam?

[c = 4200 Jkg⁻¹K⁻¹, l = 2.26 x 10⁶ Jkg⁻¹]

  1. 194 kJ ✓
  2. 246 kJ
  3. 126 kJ
  4. 224 kJ
Explanation

Two steps are needed. First heat the water from 60°C to 100°C: 0.08 × 4200 × 40 = 13 440 J.

Then turn it into steam: 0.08 × 2.26 × 10⁶ = 180 800 J.

Total = 13 440 + 180 800 ≈ 194 240 J = 194 kJ.

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