Optical rotation and specific rotation q.

[coolio_person]

I am having difficulty answering this question, any help would be very much appreciated!! Cheers.

a) A solution has an observed optical rotation of +26 degrees and specific rotation of +130 degrees, what is the concentration, knowing that the polarimeter used had a light wavelength of 589 nm, and a light bath of 1 dm?

b) What is this solution?

I've used the formula C= (obs. rotation/l x spec. rotation)*100

Where C = conc. in g/100ml
l = 1dm

and i got (26/1*130)*100=20 g^ml-1

and i have no idea how to find out what the solution is from here. We were given a table that lists optical rotations of several compounds, but I'm failing to understand the connection between finding the concentration and part a and identifying the actual compound in part b.

[Borek]

If you have table of SPECIFIC rotations just finding it in the table should do the trick.

[coolio_person]

That's what I thought, but then i got confused with why we had to calculate the concentration in the first place because they give use the specific rotation as part of the question in part "a" (130 degrees).

So I don't see the point of part "a", because it hasn't anything to do with part "b"....AND the answer for part "b" is mentioned in the question of part "a". It's honestly really confusing!

The values closest to 130 degrees is melibiose (129.5 degrees) and maltose (130.4 degrees) ...I was taught that we should round up if a decimal was .5 and up, and we should round down if a decimal was .4 or less...so essentially they're both about 130 degrees...which would you say is "correct"?

[Borek]

which would you say is "correct"?

Both. Method is not specific enough to differentiate between these two.

[coolio_person]

Alright, thanks for your help