Question

Air is trapped in the space (labeled A) above the mercury level in the tube of a barometer, which measures 745 mm Hg. If the atmospheric pressure is 760 mm Hg, then the pressure of the trapped air is

• A. 15 mm Hg
• B. 760 mm Hg
• C. between 745 and 760 mm Hg
• D. 745 mm Hg

Answer 1

Answer: (A)

15 mm Hg

The pressure in the tube in level with the surface of water outside the tube must be same.

Hence $1atm=P_{Hg}+P_{airtrap}$

$⟹760=745+P_{airtrap}$

$⟹P_{airtrap}=15mmHg$