Question

What is the observed rotation when 0.1 M solution of (R)-2 butanol is mixed with an equal volume of 0.05 M solution of racemic 2-butanol and the resulting solution is taken in a cell of 5 cm log tube in a polarimeter? The specific rotation of (R)-2-butanol is +13.9${}^o$

• A. 0.26${}^o$
• B. 0.56${}^o$
• C. 0.45${}^o$
• D. 0.09${}^o$

Answer 1

The correct option is A 0.26${}^o$

The racemic 2-butanol ha no contribution to optical activity. So the
observed rotation will be due to (R)-2-butanol only. Race mic butanol
helps in diluting the solution. Therefore, the concentration of(R)_2
butanol after dilution is 0.05 M. Molecular weight of 2-butanol is 74
l = 5 cm = 0.5 dm
concentration = 740.051000 = 3.7 103 gm ml1
[]obs = []pure l c = 13.9 0.5 3.7 103
0.26