Question

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15. Calculate the observed rotation expected in the following situation: a standard 2.00-dm cell is used and ten grams of a 9:1 mixture of enantiomers is dissolved in 100 mL of solvent. The major enantiomer has a specific rotation of +200°.(4 pts)

Answer 1

Ques The specific rotation of a molecule is the rotation (og observed upon bassing polarized light through a bath length of cell of 1 dem at a concentration of tolml. The mathematical formula of specific rotation is given by Cara Robs 1 [a] = exl where [a] = specific rotation. (°) Toby = observed rotation (²) - conentration (g/ml) = path length (dom) Ginen, ath length of cello a dom Amount of the enantiomer mixture = 10g Volume of the solvent = 100ml Now, concentration of the mixture = 1 g/mo ca 0.1 g/ml We have , 9:1 mixture major part having specific rotation of + 2000 : Firstly, find the optical purity. % Optical purity = total amount major part - minor part. 100 % Optical burity -

%o optical burity - 92 x 100 = 80% 10 The "optical purity" is a comparison of the optical rotation of a pure sample of unknown stereochemistry versus the optical rotation of the of pore Chantiomer. % optical purity =- [x] mix x100 [a pure [&] mg = (a) prore % optical purity X 100 Camin = 80 X (+200) Loo Camix = + 160° Now, observed rotation a = ? (a) mia = cool Lobs= (a) mia X cxl cobs = 160° xong/ml x 2 dcm Lobs = +32°