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Manometer reading: open-tube manometer measures gas pressure. Mercury height difference = 20 cm with gas side LOWER. Gas pressure:
AP_atm + 20 cm Hg
BP_atm - 20 cm Hg (gas pressure is below atmospheric)
C20 cm Hg
DAlways 1 atm
Answer & Solution
Correct answer: A. P_atm + 20 cm Hg
If gas-side mercury is LOWER than open side, gas pressure > P_atm. So P_gas = P_atm + Δh (Hg). With 20 cm difference: P_gas = 76 + 20 = 96 cm Hg. Wait — re-reading: if gas side is lower, push of gas is pushing mercury down on its side and up on open side. Actually 'lower' means gas mercury is pushed below open one. So gas P > atm. Answer A correct. (If gas above: gas side higher → gas P < atm.)
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