An open-tube mercury manometer is used to measure the pressure in a gas tank. When the atmospheric pressure is…

Analyze the question: An open-tube manometer is connected to a gas tank. The mercury in the open arm is 25 cm higher than in the arm connected to the tank. We need the absolute pressure of the gas.

Concept: If the mercury in the open side is higher, the gas pressure is greater than atmospheric pressure, because the gas pushes the mercury up the open arm.

P(absolute) = P(atm) + ρgh

Solution:

h = 25 cm = 0.25 m

ρgh = 13600 × 9.8 × 0.25 = 13600 × 2.45 = 33,320 Pa

P(absolute) = 101,325 + 33,320 = 134,645 Pa

Check against the other options:

• A (108,986) would need a gauge pressure of about 7,661 Pa, which is not what we get.
• C (122,364) would need about 21,039 Pa.
• B (165,238) would need about 63,913 Pa.

Only D equals 101,325 + 33,320.

Tip: For manometer questions, first decide which side the mercury is higher on:

• Open side higher → gas pressure is above atmospheric: P = P_atm + ρgh
• Open side lower → gas pressure is below atmospheric: P = P_atm − ρgh

Also remember to convert cm to m before calculating.