Each of the questions or incomplete statements below is followed by five suggested answers or completions. Select the one that is best in each case.
The bulb of the open-end manometer shown above contains a gas. True statements about this system include which of the following?
|I.||Only atmospheric pressure is exerted on the exposed mercury surface in the right side of the tube.|
|II.||The gas pressure is greater than atmospheric pressure.|
|III.||The difference in the height, h, of mercury levels is equal to the pressure of the gas.|
Select an Answer
I and II only
I and III only
I, II, and III
This is a laboratory-oriented question pertaining to the measurement of gas pressures. It demands higher-level analytical skills that involve drawing conclusions from results obtained in an experiment. To answer this question correctly, you must first understand that in an open type of manometer the air exerts pressure on the column of liquid in the open side of the U-tube, and the gas being studied exerts pressure on the other side of the U-tube. It is clear, then, that statement I is true since the data given show that the manometer is open-ended and its right side is exposed to the atmosphere. Statement II is also true because the level of liquid mercury is higher in the right side, which is exposed to the atmosphere, than in the left side, which is exposed to the gas. Thus the gas pressure is greater than atmospheric pressure. Statement III is not correct because the pressure of the gas in the bulb, expressed in millimeters of mercury, is equal to the difference in height, h, of the two mercury levels, plus the atmospheric pressure. Thus only statements I and II are true, and the correct answer is choice (C).