Final answer:
The U-tube manometer connected to a closed tank is used to measure the pressure inside the tank by observing the height difference of the liquid (mercury) in the tube.
Step-by-step explanation:
The purpose of a U-tube manometer when connected to a closed tank is to measure the pressure inside the tank. A manometer works by using a nonvolatile liquid, usually mercury, within the U-shaped tube. When one end is connected to the tank and the other end is either closed or open to the atmosphere, the difference in the liquid levels in the two arms of the tube indicates the pressure. If one arm is closed (a closed-ended manometer), the height difference corresponds to the absolute pressure of the gas. For an open-ended manometer, the liquid level difference reflects the difference in pressure between the gas in the container and atmospheric pressure.