9. The normal boiling point of methanol is 64.7C and the molar enthalpy of vapor

Question

9. The normal boiling point of methanol is 64.7C and the molar enthalpy of vaporization if 71.8kJ/mol. The value of S when 1.95 mol of CH3OH(l) vaporizes at 64.7C is

   ________ J/K

   [A] 414 [B] 0.414 [C] 2.16 [D] 4.73 [E] 2.16

10. The normal boiling point of water is 100.0 °C and its molar enthalpy of vaporization is 40.67 kJ/mol. What is the change in entropy in the system in J/K when 22.1 grams of steam at 1 atm condenses to a liquid at the normal boiling point?

   [A] -88.8 [B] -40.7 [C] 88.8 [D] 373 [E] -134

11. The normal boiling point of C2Cl3F3 is 47.6 °C and its molar enthalpy of vaporization is 27.49 kJ/mol. What is the change in entropy in the system in J/K when 20.2 grams of C2Cl3F3 vaporizes to a gas at the normal boiling point?

[A] 27.5 [B] -4.19 [C] 4.19 [D] 9.23 [E] -9.23

   12. The thermodynamic quantity that expresses the extent of randomness in a system is __

   [A] entropy [B] enthalpy [C] bond energy [D] heat flow [E] internal energy

Explanation / Answer

(9) Delta S = (n x q vap ) / T = 1.95 x 71.8 / 337.7 = 0.414 kJ/K = 414 J / K

(10) No.of moles o f vapour = 22.1 / 18 = 1.23 mol

Delta S = (n x q con ) / T = 1.23 x (- 40.67) / 373 = - 0.134 kJ/K = - 134 J/K

(11) No.of moles = 20.2 / 187.5 = 0.108 mol

Delta S = (n x q vap ) / T = 0.108 x 27.49 / 320.6 = 0.00926 kJ/K = 9.26 J/K

(12) Entropy is the measure of randomness of system

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