Does the pressure of air initially in the flask introduce an error? Explain your

Question

Does the pressure of air initially in the flask introduce an error? Explain your answer. Is it necessary to consider the pressure of water vapor initially in the flask? Explain your answer. Briefly discuss whether each of the following "mistakes" would produce a high result, a low result, or make no difference in the value obtained for the gas constant, R. The water level in the flask dropped below the tip of tube. Loss of some potassium chlorate from the test tube. Incomplete decomposition of the potassium chlorate. The correction for the vapor pressure of water was omitted in the calculation. Failure to use the MnO_2 catalyst. Why is it necessary to equalize the pressure of the receiving beaker and the flask before measuring the delivered volume of water?

Explanation / Answer

2)

Yes, it necessary to consider the pressure of water vapor initially in the flask. Vapor pressure is the gas pressure (force/area) exerted by water vapor molecules alone. The higher the concentration of water vapor molecules (number density), the higher the vapor pressure. The average air pressure at sea level is about 1013 millibars (mb). If the total air pressure is 1013 mb and water vapor makes up 1% of the air molecules, then the vapor pressure is 1% of 1013 mb or 10.13 mb. Water vapor is a trace gas in the atmosphere of Earth -- the maximum vapor pressure is never more than about 40 mb. For now the important concept is that vapor pressure is one way to keep track of the amount of the gas water vapor. For our purposes, the higher the vapor pressure, the greater the amount of water vapors in the air.

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