Draw charge-separated resonance structures to give a representation of the bonding in $\mathrm{PF}_5$ such that the octet rule is strictly obeyed.

Confirm that the octet rule is obeyed by each of the atoms in the following molecules: (a) $\mathrm{CF}_4$, (b) $\mathrm{O}_2$, (c) $\mathrm{AsBr}_2$, (d) $\mathrm{SF}_7$.

Potassium can be determined by flame emission spectrometry (flame photometry) using a lithium internal standard. The following data were obtained for standard solutions of $\mathrm{KCl}$ and an unknown containing a constant known amount of $\mathrm{LiCl}$ as the internal standard. All the intensities were corrected for background by subtracting the intensity of a blank.

$$\begin{array}{ccc} \begin{array}{c} \text { Concentration } \\ \text { of K, ppm } \end{array} & \begin{array}{c} \text { Intensity of } \\ \text { K Emission } \end{array} & \begin{array}{c} \text { Intensity of } \\ \text { Li Emission } \end{array} \\ \hline 1.0 & 10.0 & 10.0 \\ 2.0 & 15.3 & 7.5 \\ 5.0 & 34.7 & 6.8 \\ 7.5 & 65.2 & 8.5 \\ 10.0 & 95.8 & 10.0 \\ 20.0 & 110.2 & 5.8 \\ \text { Unknown } & 47.3 & 9.1 \\ \hline \end{array}$$

(a) Plot the K emission intensity vs. the concentration of $\mathrm{K}$ and determine the linearity from the regression statistics.

(b) Plot the ratio of the $\mathrm{K}$ intensity to the Li intensity vs. the concentration of $\mathrm{K}$ and compare the resulting linearity to that in part (a). Why does the internal standard improve linearity?

(c) Calculate the concentration of $\mathrm{K}$ in the unknown.

(a) Construct an MO diagram for the formation of $\mathrm{O}_2$; use only the valence orbitals of the oxygen atoms. (b) Use the diagram to rationalize the following trend in $\mathrm{O}-\mathrm{O}$ bond distances: $\mathrm{O}_2, 121 \mathrm{pm} ;\left[\mathrm{O}_2\right]^{+}, 112 \mathrm{pm}$; $\left[\mathrm{O}_2\right]^{-}, 134 \mathrm{pm} ;\left[\mathrm{O}_2\right]^{2-}, 149 \mathrm{pm}$. (c) Which of these species are paramagnetic?

Show that $\left[\mathrm{SiF}_6\right]^{2-}$ and $\left[\mathrm{PF}_6\right]^{-}$are iscelectronic.

a) Using the data in Table $1.5$, determine a value for $\Delta H$ for the process:

$$\mathrm{O}(\mathrm{g})+2 \mathrm{e}^{-} \rightarrow \mathrm{O}^{2-}(\mathrm{g})$$

(b) Comment on the relevance of the sign and magnitude of your answer to pert (a) in the light of the fact that many metal oxides with ionic lattices are thermodynamically stable.

Why does a $\sigma$-MO formed by the overlap of two $2 p_x$ orthitak (Fig. 2.7a) have the label $\sigma_g$ ?

In Fig. 1.16, identify the trends in the first ionization energies of the elements in (a) descending group 1 , (b) descending group 13, (c) crossing the first row of the $d$-block, (d) crossing the row of elements from B to Ne, (e) going from Xe to $\mathrm{Cs}$, and (f) going from $\mathrm{P}$ to $\mathrm{S}$. Rationalize each of the trends you have described.

Draw the structures of the two isomers of octahedral $\left[\mathrm{Cr}\left(\mathrm{OH}_2\right)_4 \mathrm{Cl}_2\right]^{+}$and give labek that distinguish the isomers.

Use the following data to estimate the bond dissociation enthalpy of $\mathrm{BrCl}: D(\mathrm{Br}-\mathrm{Br})=224 \mathrm{~kJ} \mathrm{~mol}^{-1}$; $D(\mathrm{Cl}-\mathrm{Cl})=242 \mathrm{~kJ} \mathrm{~mol}^{-1} ; \chi^{\mathrm{P}}(\mathrm{Br})=3.0 ; \chi^{\mathrm{P}}(\mathrm{Cl})=3.2$. $\left[\right.$ Ans. $\approx 237 \mathrm{~kJ} \mathrm{~mol}^{-1} ;$ actual experimental value $=$ $\left.218 \mathrm{~kJ} \mathrm{~mol}^{-1}\right]$

Using Fig. $2.10$ as a basis, account for the fact that $\left[\mathrm{N}_2\right]^{+}$ is paramagnetic.

Given that $D(\mathrm{H}-\mathrm{H}), D(\mathrm{CH}-\mathrm{Cl}), D(\mathrm{Br}-\mathrm{Br})$ and $D(\mathrm{I}-\mathrm{I})$ in $\mathrm{H}_2, \mathrm{Cl}_2, \mathrm{Br}_2$ and $\mathrm{I}_2$ are $436,242,193$ and $151 \mathrm{~kJ} \mathrm{~mol}^{-1}$ respectively, estimate (by the above method) values of $D(\mathrm{H}-\mathrm{X})$ in $\mathrm{HCl}, \mathrm{HBr}$ and $\mathrm{HI}$. $\left[\right.$ Ans. $\left.339 ; 315 ; 294 \mathrm{kJmol}^{-1}\right]$

Sewage and industrial pollutants dumped into a body of water can reduce the dissolved oxygen concentration and adversely affect aquatic species. In one study, weekly readings are taken from the same location in a river over a 2-month period.

$$\begin{array}{cc} \text { Week Number } & \text { Dissolved } \mathrm{O}_2, \mathrm{ppm} \\ \hline 1 & 4.9 \\ 2 & 5.1 \\ 3 & 5.6 \\ 4 & 4.3 \\ 5 & 4.7 \\ 6 & 4.9 \\ 7 & 4.5 \\ 8 & 5.1 \\ \hline \end{array}$$

Some scientists think that $5.0 \mathrm{ppm}$ is a dissolved $\mathrm{O}_2$ level that is marginal for fish to live. Conduct a statistical test to determine whether the mean dissolved $\mathrm{O}_2$ concentration is less than $5.0 \mathrm{ppm}$ at the $95 \%$ confidence level. State clearly the null and alternative hypotheses.

Using VB theory and the Lewis structure model, determine the bond order in (a) $\mathrm{H}_2$, (b) $\mathrm{Na}_2$, (c) $\mathrm{S}_2$, (d) $\mathrm{N}_2$ and (e) $\mathrm{Cl}_2$. Is there any ambiguity with finding the bond orders by this method?