Question

EXAMPLE 6-2 Using a Manometer to Measure Gas Pressure When the manometer in Figure 6-5(c) is filled with liquid mercury (d = 13.6 g/cm"), the barometric pressure is 748.2 mmHg, and the difference in mercury levels is 8.6 mmHg. What is the gas pressure Pgas? Analyze We must first establish which is greater: the barometric pressure or the gas pressure. In Figure 6-5(c), the baro- metric pressure forces liquid mercury down the tube toward the gas sample. The barometric pressure is greater than the gas pressure. Thus, AP = Pgas – Pbar. < 0. Solve The gas pressure is less than the barometric pressure. Therefore, we subtract 8.6 mmHg from the barometric pressure to obtain the gas pressure. Pgas = P bar. + AP = 748.2 mmHg – 8.6 mmHg = 739.6 mmHg Assess Because all pressures are expressed in millimeters of mercury, the pressure difference (AP) is numerically equal to the difference in mercury levels. Thus, the density of mercury does not enter into the calculation. PRACTICE EXAMPLE A: Suppose that the mercury level in Example 6-2 is 7.8 mm higher in the arm open to the atmosphere than in the closed arm. What would be the value of Pgas?

Answer 1

almosphere open to Pahm = 70 17.8 mm IT 3 7.8 + 748.2 Dorota 756 mm Hg at same lever pressure is same