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Air is trapped in a uniform closed-end horizontal tube by a 36 cm mercury column. When the tube is held vertically with the open end up, the length of the trapped air column decreases to 19 cm. By how many cm does the mercury column shift downward in the vertical position? (Atmospheric pressure = 76 cm of Hg.)

1. 5
2. 7
3. 9
4. 11

Correct answer: 9

Solution

Horizontal: P1 = 76 cmHg. Vertical (open end up, closed end at bottom): gas is compressed by atmosphere + mercury weight, so P2 = 76 + 36 = 112 cmHg. Boyle's law: 76*L1 = 112*19 = 2128, so L1 = 28 cm. The air column shrank by 28 - 19 = 9 cm, so the mercury column shifted down by 9 cm.

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