Question $2$

a$)$ Mention the four $(4)$ steps carried out during solid phase extractions $($SPE$)\text{'}$

b$)$ Give any four $(4)$ reasons why SPE is a better and more preferable technique to liquidliquid extraction $($LLE$)$

c$)$ Benzene is one of the Toxic Air Pollutants $($TAPs$)$ found in car exhaust fumes. A $50\mathrm{.}00mL$ aqueous sample containing a $0\mathrm{.}05000M$ benzene $($molecular weight $=78\mathrm{.}1I\frac{g}{m}ol)$ is extracted once with chloroform. Given that the $K_D$ value of this extraction is $5\mathrm{.}00,$ calculate the volume of the chloroform which will extract $1\mathrm{.}50{10}^{-3}$ moles of the original compound at equilibrium.

d$)$ A weak acid, HA$,$ with a $K_a$ of $1\mathrm{.}0{10}^{-5},$ has a partition coefficient, $K_D($between water and an organic solvent$)$ of $1200.$ What restrictions on the sample's $pH$ are necessary to ensure that a minimum of $99\mathrm{.}9\%$ of the weak acid is extracted in a single step from $50\mathrm{.}00mL$ of water using $50\mathrm{.}00mL$ of the organic solvent?

e$)$ The equilibria occurring in a liquid liquid extraction which involves the extraction of an aqueous solution containing a molecular weak acid, $HA,$ are depicted by the scheme below

Show that the resulting distribution ratio is given by the following equation

$D=\frac{K_D}{1+(\frac{K_a}{H_3{O}_{aq}^{+}})}$