Tutorial Answers 3

1. The molar mass of dinitrogen pentoxide is given by [2 x 14.01] + [5 x 16.00] =108.02 g mol^-1 (Note the units).
2. The sample weight is given by M = 44.02 x 4.00x10²⁰ /6.022 x 10²³ = 29.2 mg .
3. The number of oxygen atoms in 1mM of ozone is given by 3 x 6.022x10²⁰ atoms.
4. Added molarity is (5.00/58.5)x10 = 0.85. Hence finally 2.35M .
5. (i) is correct.
6. (ii) is correct.
7. • a) The C=N is the key feature. In valence bond terms C has 8e (4 from its own valence shell, 1 from phenyl, 2 from N since there are 2 bonds, and 1 from H). The N also has 8e (5 from its own valence shell, 2 from C, and 1 from O). Having made a C=N-O linkage this leaves 2e over as an electron pair, so both C and N in C=N are trigonal. It follows that the isomeric oximes are both planar. Since there is restricted rotation about the C=N double bond, the phenyl can be on the same or opposite sides as the OH, so there are two stereoisomers. The terms diastereoisomers and E and Z have not been discussed.
   The MO picture of the bonding is that both C and N are sp² hybridised, the C=N double bond comprising one s- and one p- bond. The sideways overlap involved in p- bonds is the feature which restricts the rotation.
   • b) The electron count is - 6e from S, 6e from O, and 1 each from Me and Ph groups. This gives 2 S-C bonds, 1 S=O double bond, and 2 lp's on O. This implies that S has a lp, and that bonding at S is approximately Td. A chiral molecule cannot have a mirror plane symmetry, so the fact that the system is chiral implies it is not planar in any case. The enantiomers are :

8. Of the linearly connected structures, three C joined together mean the O is either on the end (allyl alcohol or propanal) or attached to the central C, ie. propanone. If only 2 C are linked, then we have methyl vinyl ether. Rings must now be considered. The largest involves 3C and O (oxetane). The 3- membered rings can be all- carbon (cyclopropanol) or 2C + O (the epoxide). NB CH₃CH=CHOH is not an answer, since it is the enol of propanal, which cannot be isolated. Names of all of these compounds will not be known to students (or possibly to the lecturer).

   Structures are: