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JAMB Physics 1979 Question 21

Question 21

Gas Laws

An air column 10 cm in length is trapped into the sealed end of a capillary tube by a 15 cm column of mercury with the tube held vertically as shown in Fig. 5. On inverting the tube, the air column becomes 15 cm long. The atmospheric pressure during the experiment is

Diagram for this question
  1. A.90 cm
  2. B.76 cm
  3. C.75 cm
  4. D.60 cm
  5. E.50 cm
Show the answer and explanation

Answer: C (75 cm)

Explanation

With the open end up the air is at (H + 15) cmHg; inverted it is at (H − 15). Boyle's law: (H + 15) × 10 = (H − 15) × 15, so 5H = 375 and H = 75 cmHg.