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A 4-litre container holds 3 L of liquid water and O2 gas above it. The total gas pressure (O2 + water vapour) is 720 mm Hg. The container is connected to an initially empty 3-litre container at the same temperature. Find the final partial pressure of O2(g). [Vapour pressure of water at 27 deg C = 20 mm Hg]

1. 175 mm of Hg
2. 350 mm of Hg
3. 200 mm of Hg
4. 800 mm of Hg

Correct answer: 350 mm of Hg

Solution

Initial P(O2) = 720 - 20 = 700 mm Hg in volume 1 L. After expansion to 4 L: P2 = 700 * 1/4 = 175 mm Hg. Water vapour pressure remains 20 mm Hg. Wait — re-check: total volume available to O2 changes from 1 L to 1 + 3 = 4 L, giving P(O2) = 700/4 = 175 mm Hg. But option says 350. If empty container is 3 L and gas expands from 1 L to 1+3=4 L: 700*(1/4)=175. Answer is 175 mm Hg.

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