NOTE: No Chem 1E03-01 students will have a formal tutorial during the week of November 1. You should try the problems on your own. Solutions will be posted.

1.a) Draw resonance forms for

SO₃^2- 26 valence electrons
 
 

CO₃^2- 24 valence electrons
 
 

NO₃^- 24 valence electrons
 
 

ClO₃^- 26 valence electrons
 
 

ClO₂^- 20 valence electrons
 
 

b) What are the bond orders for the central-atom-to-oxygen bonds in these species?

For SO₃^2-, the bond order is 4/3.

For CO₃^2-, the bond order is 4/3.

For NO₃^-, the bond order is 4/3.

For ClO₃^-, the bond order is 5/3.

For ClO₂^-, the bond order is 3/2.

c) What is the average formal charge on the oxygen atoms in each of these species?

The average formal charge on oxygen in SO₃^2- is -2/3.

The average formal charge on oxygen in CO₃^2- is -2/3.

The average formal charge on oxygen in NO₃^- is -1/3.

The average formal charge on oxygen in ClO₃^- is -1/3.

The average formal charge on oxygen in ClO₂^-, is -1/2.

d) Choose all of the species of part a that violate the octet rule.

The species that violate the octet rule are SO₃^2-,ClO₃^-, and ClO₂^-

e) Give the AX_nE_m classes and predict the shapes of all the anions.

SO₃^2- - AX₃E - triangular pyramidal

CO₃^2- - AX₃ - triangular planar

NO₃^- - AX₃ - triangular planar

ClO₃^- - AX₃E - triangular pyramidal

ClO₂^- - AX₂E₂ - bent

2. Oxalic acid, (COOH)₂, has two ionizable hydrogens and contains a carbon-carbon single bond. When one mole of oxalic acid reacts with one mole of calcium hydroxide,Ca(OH)₂, the salt CaC₂O₄ is formed.

a) Draw adequate Lewis structures to represent the bonding in the oxalate ion, C₂O₄^2-.

There are 34 valence electrons.
 
 

b) What is the carbon-oxygen bond order in oxalate ion?

Bond order = (1 + 2 + 1 + 2)/4 = 1.5

c) What is the formal charge on each oxygen atom in oxalate ion?

Formal charge on oxygen = (-1 + 0 + (-1) + 0)/4 = -1/2

3. N₂O has a linear, unsymmetrical structure that may be thought of as a hybrid of two resonance forms. Draw the two resonance forms of N₂O.

There are 16 valence electrons. (Also remember that N and O obey the octet rule.)
 
 

4. A solid non-metallic element (A) from Group 5A was burned in excess oxygen to give a white solid (B) (Rxn 1). The white solid (B) dissolves in water to give a solution of a substance (C) (Rxn 2); (C) turns blue litmus red. To this solution of (C) was added an excess of aqueous KOH to produce an aqueous solution of (D) (Rxn 3). Write balanced equations for Rxns 1, 2, and 3; identify A, B, C, D.

Reaction 1: P₄(s) + 5O₂(g) --> P₄O₁₀(s). Thus A is P₄, and B is P₄O₁₀

Reaction 2:P₄O₁₀(s) + 6H₂O(l) --> 4H₃PO₄(aq). Thus C is H₃PO₄.

Reaction 3:H₃PO₄(aq) + 3KOH(aq) --> K₃PO₄(aq) + 3H₂O(l). Thus D is K₃PO₄.

5. Phosphoric acid, H₃PO₄, contains three oxygen-hydrogen single bonds. When one mole of phosphoric acid reacts with one mole of sodium hydroxide, the salt sodium dihydrogen phosphate, NaH₂PO₄, results.

a) Draw adequate Lewis structures for H₃PO₄ and H₂PO₄^-.

H₃PO₄ has 32 valence electrons.
 
 

H₂PO₄^- has 32 valence electrons.
 
 

b) Calculate bond orders for all the phosphorus-oxygen bonds of H₃PO₄ and H₂PO₄^-.

In H₃PO₄, there is one P-O bond of bond order 2, and three P-O bonds of bond order 1.

In H₂PO₄^-, there are two P-O bonds of bond order 1.5, and two P-O bonds of bond order 1.

c) Calculate the formal charges on each oxygen atom in H₃PO₄ and H₂PO₄^-.

In H₃PO₄, all oxygen atoms have a formal charge of zero.

In H₂PO₄^-, two oxygen atoms (bonded to H) have a formal charge of zero, while the other tow have a formal charge of -1/2.

6. For the molecules OCS, ICl₃, IF₅, SO₂, PF₃, and CCl₂Br₂:

a) determine the AX_nE_m class;

b) decide which have dipole moments, and for those that do, decide the direction of the dipole moment.

OCS has 16 valence electrons. Its Lewis structure (with bond moments shown) is
 
 

It is an AX₂ molecule, with a net dipole moment pointing to O.

ICl₃ has 28 valence electrons. Its Lewis structure (with bond moments shown) is
 
 

It is an AX₃E₂ molecule, with a net dipole moment pointing towards the equatorial Cl (the two axial bond moments cancel).

IF₅ has 42 valence electrons. Its Lewis structure is
 
 

It is an AX₅E molecule, with a net dipole moment pointing to the fluorine that is situated 180^o from the nonbonding pair.

SO₂ has 18 valence electrons. Its Lewis structure is
 
 

It is an AX₂E molecule, with a net dipole moment pointing from S to a point midway between the O atoms.

PF₃ has 26 valence electrons. Its Lewis structure is
 
 

It is an AX₃E molecule, with a net dipole moment pointing from P to the center of the triangle formed by the three fluorines.

CCl₂Br₂ has 32 valence electrons. Its Lewis structure is
 
 

It is an AX₄ molecule, with a net dipole moment pointing from C and bisecting the Cl-C-Cl angle.

7. A gaseous molecule "X" is made by burning a yellow elemental solid in air. "X" is converted to "Y" by reaction with oxygen in the presence of a catalyst. "Y" reacts vigorously with water to produce "Z" which turns litmus red.

a) Identify "X", "Y", and "Z".

"X" = SO₂, "Y" is SO₃, and "Z" is H₂SO₄.

b) Write a balanced equation for each reaction.

S(s) + O₂(g) --> SO₂(g)

2SO₂(g) + O₂(g) --> 2SO₃(g)

SO₃(g) + H₂O(l) --> H₂SO₄(aq)

c) Draw Lewis diagrams and indicate the shapes of the molecules "X" and "Y".

SO₂ has 18 valence electrons. Its Lewis structure is
 
 

The molecule is AX₂E, and is bent.

SO₃ has 24 valence electrons. Its Lewis structure is
 
 

The molecule is AX₃, and is triangluar planar.

d) State whether "X" and "Y" possess dipole moments and, if so, in which direction they point.

SO₂ has a dipole moment pointing from S to midway between the oxygen atoms.

SO₃ has no dipole moment.

e) When 1 mole of "Z" is titrated with 2 moles of NaOH, a soluble salt "W" forms. Write an appropriate Lewis electron dot structure for the anion of this salt.

"W" is SO₄^2-. There are six resonance structures. One of them is