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Subject Test Chemistry

Question 16 of 17

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SAT Subject Test

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 figure shows an open-end manometer with a gas in the bulb portion and liquid mercury in part of the tube portion. The tube of the manometer emerges horizontally out from the right side of the bulb and after a short distance makes a 90 degree bend down toward the bottom of the figure, and then curves up and becomes vertical again on the right side of the figure, forming a U-shape. The end of the tube on the right is open and extends higher than the left side of the tube. Liquid mercury almost fills the right side of the tube, but only goes up a short distance in the left side of the tube. The difference between the height of the liquid mercury in the left side of the tube and the right side of the tube is labeled h.

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

II only

Correct Answer: 

III only

Correct Answer: 

I and II only

Correct Answer: 

I and III only

Correct Answer: 

I, II, and III

Correct Answer: 

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).

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