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Homework Help: Specific Rotation Problem

  1. Oct 5, 2013 #1
    1. The problem statement, all variables and given/known data

    The specific rotation of a tetrasaccharide was measured to be -20.5°. Upon complete hydrolysis in acid the optical rotation of the solution was found to be -36.9°. Knowing that the specific rotations of n-acetylgalacosamine,3-deoxy-l-fucose and 2-deoxy-D-ribose are -105°,-75.7° and 18.7°. What is the specific rotation of 4-methoxy-d-mannose?
    b]2. Relevant equations[/b]

    3. The attempt at a solution


    and solve for x=14.2 for 4-methoxy-d-mannose

    Can anyone tell me If Im doing this correctly? It seems too simple...
  2. jcsd
  3. Oct 6, 2013 #2


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    Staff: Mentor

    Close, but not necessarily correct. You can't assume hydrolyzing tetrasaccharide solution that is 1 g/mL of the original compound you will get 0.25 g/mL of each produced monosaccharide, their concentrations will be in general higher (hydrolysis consumes water, so the mass of products is higher than the mass of the main reactant). To find exact concentrations you need to follow the reaction stoichiometry and molar masses.

    I don't like the question. It says "optical rotation of the solution was found", not defining the solution. Either we are expected to do some unlikely assumptions, or whoever worded the question didn't think about it for long enough. We can assume it is a hydrolyzed solution that was used for the specific rotation measurement (so it was initially 1 g/mL of tetrasaccharide) but I find it hard to believe in practice nothing has changed - hydrolysis requires adding of an acid, heating and so on - final solution is definitely different, unless we did some unmentioned extra tricks.
  4. Oct 6, 2013 #3
    Thanks, hmmm..im not sure if we're expected to make some assumptions but that's how the Question was worded exactly.
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