Question

What evidence do you have that you actually isolated limonene? If Ig of limonene dissolved has been dissolved into 10mL of isopropanol solution. The optical rotation is 12.5. what is specific rotation, presuming path length to be Idm. At 60°C vapor pressure of limonene is 125 pa, and remaining is water vapor (latm 101kpa). Find the mole % of each component.

Answer 1

Isolated compound is limonene can be ascertained by measuring of specific rotation of isolated compound and it can be matched with available standard value of limonene specific rotation. Spectroscopic techniques like NMR can be used with using chiral solvent.

• Specific rotation is defined as the observed rotation at a specific cell length and sample concentration.

Specific rotation = Observed rotation (degrees) / Length (dm) * Concentration(g/mL)

Concentration is expressed in grams per milliliter (g/mL) : 1g is in 10 ml = 0.1 g /ml

Using given values :

Specific rotation = +12.5 / 1 * 0.1 = +125

• V.P. of limonene at 60 C (in water ) is 125 Pa

Raoult law : ratio of the partial vapour pressure of each component to its vapour pressure as a pure liquid, pi/pi*, is equal to it  mole fraction in the liquid mixture.
pi = x_i * pi*

V.P. of Pure limonene at 60 C = ~1.25 k Pa (online source)

so, mole fraction of limonene = 125 Pa / 1.25 k Pa = 0.10 (1 k Pa = 1000 Pa )

or mole % of limonene = ~10 %

​​​​​​​so, mole % of water = ~ 90 %.