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How much heat is required to convert 200 g of water at 100 °C completely into steam at 100 °C? (Specific latent heat of vaporisation of water = 540 cal/g)
A16000 cal
B108000 cal
C20000 cal
D2700 cal
Answer & Solution
Correct answer: B. 108000 cal
1. The water is already at the boiling point, so only latent heat is involved.
2. Heat for vaporisation: $Q = m \times L_{vap}$.
3. Substitute: $Q = 200\ \text{g} \times 540\ \text{cal/g}$.
4. $Q = 108000\ \text{cal}$. Using the fusion value 80 cal/g instead gives the wrong 16000 cal, so that option is rejected.
_Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 5 "Heat", p.72_
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