[john2520]

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. Know that the specific rotation of N-acetyl-glucosamine (-105), 4-methoxy-D-mannose (14.2), and 2-deoxy-D-ribose (18.7) what would be the specific rotation of the last monosaccharide unit, 3-deoxy-l-fucose.

So I was thinking we completely ignore the -20.5 and just total the 4 monosaccharide units to 36.9.

-105+14.2+18.7+x = -36.9 and then solve for x but i dont think its suppost to be that simple, can anyone give me any insight on what to do.

[Dan]

i dont think its supposed to be that simple

It is almost that simple. You've got the right idea, -36.9 is the average (note: not the sum) rotation of the four equimolar monosaccharides.