65.100 A and V
Christmas Exam 1996
233 students
Paper may be taken from room
Calculators only allowed
Answer all questions.
1. [10] Combustion of 45.62 mg toluene, a common solvent containing only carbon and hydrogen, yields 35.67 mg H2O and 152.2 mg CO2.
(a) What is the empirical formula of toluene?
(b) The molecular weight of toluene is 92.0 g/mol. What is the molecular formula of toluene?
2. [10] Balance the following REDOX reaction in acidic solution:
As2O3(s) + NO3-(aq) à H3AsO4(aq) + HNO2(aq)
Advice: don’t guess - follow the steps suggested in lectures.
3. [10] The following ions all have the same number of electrons: Ti+4, Sc+3, Ca+2, S-2. Arrange them in increasing order of size and explain your answer.
4. [10] Explain why Xenon (Xe) can form compounds even though the ground state of this atom has a full octet of electrons in the valence shell.
5. [10] Predict the shapes of the following species using the VSEPR theory:
NO2+, NO2-, XeF3+. You must name the shape. Advice: don’t guess - follow the rules.
6. [10] Calculate DHo for the following reaction:
2 NO(g) + O2(g) à N2O4(g)
using the following data:
N2O4(g) à 2 NO2(g) DHo = 57.2 kJ
NO(g) + ½ O2(g) à NO2(g) DHo = -57.0 kJ
7. [10] (a) Using the data in the table below, calculate the standard free energy change, DGo, at 25oC for the following reaction:
2 H2S(g) + SO2(g) à 3 S(s) + 2 H2O(g)
(b) Is the reaction spontaneous at 25oC? Calculate the temperature (in oC) at which DGo = 0.
Species |
DHo, kJ/mol |
So, J K-1 mol-1 |
H2S(g) |
-20.6 |
205.7 |
SO2(g) |
-296.8 |
248.1 |
S(s) |
0 |
31.8 |
H2O(g) |
-241.8 |
188.7 |
8. [10] (a) State the Pauli Exclusion Principle.
(b) State Hund’s rule.
(c) What numbers are the solution to Schrodinger’s wave equation?
(d) Write the COMPLETE electronic structure (1s22s2 etc) for Fe+3.
9. [10] For the reaction:
2 N2O(g) + O2(g) = 4 NO(g)
the standard free energy change, DGo is +137.9 kJ/mol at 25oC.
(a) Write an expression for Kp for the reaction.
(b) If pN2O = 0.1 atm and pO2 = 0.2 atm, calculate pNO at 25oC.
10. [10] For the reaction: 2 NO(g) + Br2(g) = 2 NOBr DHo = -16.2 kJ/mol
Which way would the equilibrium shift (i.e. Left or Right) if you:
(a) Added more NO(g)
(b) Removed some NOBr(g)
(c) Compressed the mixture to a higher pressure
(d) Increased the temperature
11. [10] Suppose you have a 1 mol sample of Ar(g) in a 1 L container at 25oC.
(a) Calculate the pressure according to the ideal gas law.
(b) Calculate the pressure according to the van der Waals’ equation. For Ar, the van der Waals’ constants are a = 1.35 L2 atm mol-2 and b = 0.032 L mol-1.
(c) Comment on why the pressures calculated in (a) and (b) are different.
(d) Do the calculation in part (b) using ammonia instead of Ar. (For ammonia, a = 4.17 L2 atm mol-2 and b = 0.037 L mol-1)
(e) Comment on why the calculated pressures for Ar and NH3 are so different.
12. [10] (a) What is a sigma (s) bond?
(b) What is a pi (p) bond?
(c) What is the hybridization in CH4? In NH3?
(d) How many resonance forms are there of the CO3-2 ion?
(e) How do antibonding molecular orbitals differ from bonding molecular orbitals?
Some Useful Data and Equations
P = nRT/(V - nb) - a(n/V)2
R = 0.082 L atm K-1 mol-1
R = 8.314 J K-1 mol-1
NAV = 6.023 x 1023 mol-1