I know that to calculate the specific rotation for a compound you do
observed rotation / path length(dm) * conc. (g/ml).
I am confused about how to calculate the specific optical rotation for a compound.
Thank you for your help!
I'm not understanding. You know the formula; substitute the numbers and solve for alpha.
Oh ok, the question asks me to find the specific rotation for D-erythrose 4-phosphate which I determined was 50 and then it asks me to find the specific optical rotation for L-erythrose 4-phosphate so I thought that it would be a different formula.
The path of length was given in cm so I converted it to dm but I am not sure if I calculated the concentration correctly. A 50 gram sample was dissolved in 10 mL solution so would I do 50/10?
Yes, path length is in dm and cncn is g/mL. In your case that is 50g/10 mL = 5 g/mL.
ok thank you and for the specific rotation of L-erythrose 4-phosphate would it be the same as for D-erythrose 4-phosphate which I determined was 50 or would it be -50?
If one is + the optical isomer will be - but don't confuse D and L with d and l.
d is for dextro-rotatory(right handed) and l is for levo-rotatory(left handed) and they don't have anything to do with D- and L-.
Here is a LONG article where you can read more.
http://en.wikipedia.org/wiki/Chirality_%28chemistry%29
To calculate the specific optical rotation for a compound, you need a few pieces of information:
1. Observed rotation: This is the value obtained by measuring the rotation of polarized light as it passes through the compound in a polarimeter. The observed rotation is typically given in degrees (°).
2. Path length: The path length refers to the distance the polarized light travels through the sample in the polarimeter. It is usually expressed in decimeters (dm) and represents the distance from the light source to the detector.
3. Concentration: The concentration of the compound is represented by its mass per unit volume, often measured in grams per milliliter (g/ml).
Once you have these values, you can use the formula you mentioned to calculate the specific optical rotation:
Specific rotation (α) = Observed rotation / (Path length × Concentration)
By substituting the values into the formula, you will obtain the specific optical rotation for the compound you are analyzing. Remember to ensure that the units are consistent throughout the calculation.
It's important to note that the specific optical rotation is a characteristic property of a compound, which means that it can be used to identify and compare different compounds.