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Equilibrium constants (in atm) at 0 deg C are: SrCl2.6H2O(s) <-> SrCl2.2H2O(s) + 4H2O(g), Kp = 5 x 10⁻¹²; Na2HPO4.12H2O(s) <-> Na2HPO4.7H2O(s) + 5H2O(g), Kp = 2.43 x 10⁻¹³; Na2SO4.10H2O(s) <-> Na2SO4(s) + 10H2O(g), Kp = 1.024 x 10⁻²⁷. Among SrCl2.2H2O(s), Na2HPO4.7H2O(s) and Na2SO4(s), which is the most effective drying agent at 0 deg C?

1. SrCl2.2H2O, because it maintains the lowest equilibrium water vapour pressure (about 1.5 x 10⁻³ atm)
2. Na2SO4, because its Kp value is numerically the smallest
3. Na2HPO4.7H2O, because it absorbs the most water molecules
4. All three are equally effective at 0 deg C

Correct answer: SrCl2.2H2O, because it maintains the lowest equilibrium water vapour pressure (about 1.5 x 10⁻³ atm)

Solution

Computing P_H2O = Kp^(1/n): SrCl2 gives about 1.5 x 10⁻³ atm, Na2SO4 about 2.0 x 10⁻³ atm, Na2HPO4 about 3.0 x 10⁻³ atm; the lowest belongs to SrCl2.2H2O, so it is the most effective drying agent.

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