
Figure (a) above shows a capillary tube
with a thread of mercury 2cm long. The length of the air trapped in the tube is 10cm. Find the length of the trapped air if the tube is inverted as shown in figure (b). [Atmospheric pressure = 76cmHg]
Answer:
In case (a), the pressure of the gas trapped in the capillary is equal to the atmospheric pressure + the pressure caused by the mercury thread.
P1 = 76 + 2 = 78cmHg
V1 = 10cm
Incase (b), the pressure of the gas trapped in the capillary is equal to the atmospheric pressure - the pressure caused by the mercury thread.
P2 = 76 - 2 = 74cmHg
V2 = ?
By using Boyle's Lwa's formula
P1V1 = P2V2
(78)(10) = (74)V2
V2 = 10.54cm
The length of the trapped air in figure (b) is 10.54cm