A student is determining the molar mass of an unknown volatile liquid by vaporizing…
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
A student is determining the molar mass of an unknown volatile liquid by vaporizing it in a 1.000 L flask and applying the ideal-gas equation to the resulting gas sample. During preliminary trials conducted near the liquid’s normal boiling point, the measured pressure–volume product deviated appreciably from the value predicted by PV=nRT.
The student can adjust only the temperature of the water bath (which determines the gas temperature) and the pumping speed of the vacuum manifold (which determines the total pressure in the flask). No changes to flask volume or sample size are possible.
(a) Describe two specific adjustments—one involving temperature and one involving pressure—that the student could implement to reduce the deviation between the measured PV product and the value predicted by the ideal-gas equation. For each adjustment, explain in terms of particle-level interactions why it would decrease the magnitude of the deviation.
(b) The student repeats the experiment at several temperatures while holding the total pressure at 0.300atm. Explain why the percent error in the calculated molar mass increases sharply as the bath temperature approaches the normal boiling point of the liquid.
(c) Two different liquids—helium and carbon dioxide—are available as alternative calibration gases for the apparatus. Predict which gas would require more stringent experimental conditions (closer to ideal) to achieve the same percent error goal and justify your answer with reference to molecular properties.