CHEMISTRY · Multi-part · A Level · Chemistry · Amount of substance · Amount of substance
Problem
Two unknown gases, X and Y, were studied by measuring their compressibility factors over a range of pressures at 298 K. A researcher summarized the measurements as follows:
• For gas X, Z decreases below 1 at pressures up to about 20 atm, reaching a minimum of 0.90, then rises sharply above 1 at pressures above 100 atm.
• For gas Y, Z is never less than 1.00; instead, it rises gradually from 1.00 at low pressure to 1.20 at 200 atm.
Additional information about the gases is provided:
• Gas X is known to have a permanent dipole moment of 1.5 D.
• Gas Y is monoatomic with a molar mass of 84 g mol−1.
(a) Identify the pressure region in which attractive forces dominate the behavior of gas X. Justify your answer using the given Z trend.
(b) Explain why gas Y never exhibits Z<1 within the pressure range studied, connecting your reasoning to its molecular characteristics.
(c) At 298 K and 150 atm, which gas occupies the larger molar volume? Support your answer qualitatively without numeric calculation.
(d) Predict how substituting a heavier isotope for gas Y (keeping electronic structure unchanged) would affect the shape of its Z versus P curve at 298 K. Provide a brief molecular rationale.