Question

I need to calculate the concentration (g/mL) of limonene in ethanol in order to calculate the specific rotation. (Observed rotation was 34.65 deg.)

We made a solution by adding 2.33g of limonene oil to a 10-mL flask and then filling the flask to the 10 mL line with ethanol.

My question is what figures do I use to calculate the concentration in g/mL?

Is it 2.33 g limonene /10 mL total volume?

Or: I used the known density of limonene to calculate its volume and got 2.771 mL. Which means we added 7.229 mL ethanol. Would I then do 2.33 g/ 7.229 mL?

Answer 1

We can calculate the molar concentration as

M = No. of moles / volume in solution by liters

No. of moles = Given mass / molar mass = 2.33g / 136.4 = 0.017 moles

Volume in liters = 10 / 1000 = 0.01L

M = 0.017 moles / 0.01L = 1.7M

CONCEPT: Volume of solution is = Volume of solvent + volume of solute. Volume of flask is 10mL so the total volume of solution is 10mL

No need to sepaartely measure the volume on density or any other bases.

The concentration in units of g/mL is 2.33g/10mL = 0.233 g/mL

Volume of solution = volume of solvent + volume of solute = 10mL