exam quwtion exchange Chemical Knowledge versus Conventional "Wisdom" Margaret J. Steffel The Ohio State University, Marian Campus Marion, OH 43302
The question presented below is intended for general chemistry students. T o the students who think logically, i t is a straightforward application of the general gas equation. However, many students seem to mistrust their own logical conclusions, and, therefore, they miss the last part of it, comparison of the densities of moist and dry air (same temperature and pressure). I t is common to describe humid air as "heavy", a subjective description based on the physical discomfort we feel when the relative humidity is high. Many students interpret this as meaning "dense" and give the wrong answer to this part of the question even if they answered all previous parts correctly. Further, the prejudice generated by this common terminology is so strong that some students do not believe the obvious chemical and mathematical logic even after the question is discussed and after they look up densities of moist air in a handbook! The students who are most apt to answer correctly are ones who have some knowledge of aviation; because lift is affected by the density of air, the moisture content of air is a factor in some flight-related calculations. (luestlon a) Write the general gas equation with the amount of gas expressed in moles. Rewrite the general gas equation with the amount of gas expressed in grams. Rearrange the second equation to express the density of a gas. Name the factors that determine the density of a gas.
JOHN J.
edited by: ALEXANDER
University d Cincinnati Cincinnati. Ohio 45221
h) Rearrange the second equation to show how the molar mass of a gas is calculated from experimental data. Clean, dry air has a density of 1.169 gA a t 21% and 740 tom. Use these data to calculate the average molar mass of the gases in air. Name the predominant components of air and state their molar masses. c) Compare qualitatively the average molar mass of the gases in moist air to the average molar mass of the gases in dry air. Compare qualitatively the densities of moist and dry air a t the same temperature and pressure. Acceptable Solutlon a)
PV = nRT
Gas density is determined by a gas's temperature (relatedinversely), pressure (related directly),and molar mass (related directly).
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""
= 28.9 glmol
N2(18%),28 glmol; 02(21%),32 glmol; (Ar (09%).40 glmol) c)
Moist air contains H20,which has a molar mass of 18 glmol, as well as Nz, 02,etc. The average molar mass of the components of air is lower when water vapor is present. Therefore, if moist air and dry air have the same temperature and pressure, moist air will be less dense than dry air.
Volume 63 Number 4
April 1986
317
