posted by on .

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.

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