Chemistry Homework: Manometer Calculations?

Question

I had some difficulty paying attention in chemistry today; the first question on the homework is:
"A closed manometer connected to a container of nitrogen has a height difference of 720mm in the two arms. What is the pressure of the nitrogen in the atmospheres?" how do you solve this? I remember she mentioned there was a difference between an open and closed manometer and she had two drawings. Thank you so much for any help.

Answer 1

There are two types of manometers. A closed manometer (sealed at one end) has a vacuum in the closed end and is open to the sample to be measured on the other end. In this case, the vacuum does not exert a pressure (P=0), so the pressure is equal to the difference in heights on the two arms. So in this case, 720 mmHg. You need to convert this to atm units using 1 atm = 760 mmHg.

If the manometer was open, it means that instead of a vacuum where the closed end would be, the arm is open to the atmosphere. In this case, you have to account for the pressure of the atmosphere. If the mercury is higher in the arm closest to the sample, it means the pressure in the sample is less than the atmosphere (because it's not pushing back as much) so the sample pressure is
P(sample) = P(atm) - difference in height
If the mercury is higher in the atmosphere arm, the sample pressure is greater
P(sample) = P(atm) + difference in height

Answer 2

720 / 760 atm