Question

The lengths of air column are 10 cm and 8 cm in a faulty barometer when it reads 72 and 71 cm of Hg. The value of atmospheric pressure is:

• A. 68 cm of Hg
• B. 71.5 cm of Hg
• C. 76 cm of Hg
• D. 74 cm of Hg

Answer 1

The correct option is C 76 cm of Hg

Let the height of air column = $l$ cm and $P$ be the atmospheric pressure
In a barometer, the enclosed air column obeys Boyle's law
For the enclosed air column,
$(P-reading)\times l$ = constant
Case 1: $(P-72)\times 10$
Case 2: $(P-71)\times 8$
Since both are equal,
$(P-72)\times 10$ = $(P-71)\times 8$
2P = 720 - 568 = 152 cm of Hg
P = 76 cm of Hg