Question

1 g of water at atmospheric pressure has volume of 1 cc and when boiled it becomes 1681 cc of steam. The heat of vaporisation of water is 540 cal/g. Then the change in its internal energy in this process is

• A. 540 cal
• B. 500 cal
• C. 1681 cal
• D. None of the above

Hint:

Most heat goes to internal energy; some does work against atmosphere.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
\(\Delta U = Q - W\), where \(W = P\Delta V\).
Step 2: Detailed Explanation:
\(Q = 540\ \mathrm{cal}\)
\(W = P\Delta V = 1.013 \times 10^5 \times (1680 \times 10^{-6}) = 170\ \mathrm{J} \approx 40.7\ \mathrm{cal}\)
\(\Delta U = 540 - 40.7 \approx 500\ \mathrm{cal}\)
Step 3: Final Answer:
Change in internal energy is 500 cal.