$($c$)($S$)-$Amlodipine is used to treat high blood pressure and angina. The specific rotation of clinical grade $(S)-$amlodinine is $-25\mathrm{.}1$

The experimentally determined specific rotation of a batch prepared in the development laboratory using an alternative synthesis route is $-19\mathrm{.}3.$

$($i$)$ Determine the optical purity and enantiomeric excess for this sample.

$($ii$)$ Use these values to calculate the percentage of the $(+)$ and $(-)$ enantiomer present in the laboratory sample. Show clearly how you reached your answers.