In an open manometer, the mercury is higher by 180 mm on the side connected to a sample of He gas. If the air pressure is 102 kPa, what is the pressure of the He gas?
A. 24 kPa
B. 78 kPa
C. 102 kPa
D. 136 kPa
To solve for the pressure of a gas in an open manometer, decide if the gas pressure is greater or less than the atmospheric pressure. Convert the height difference in mmHg to the appropriate unit to match atmosphere, then add or subtract as needed. Because the Hg level is higher on the side connected to the He, the pressure must be less than that of atm. Convert the pressure difference in mmHg to kPa, which gives 24 kPa, then subtract from 102, to give 78 kPa of He.