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JEE Advanced 2025 · Paper 2, Chemistry question 16

A linear octasaccharide (molar mass =1024 g mol−1= 1024\ \mathrm{g\,mol^{-1}}) on complete hydrolysis produces three monosaccharides: ribose, 2-deoxyribose and glucose. The amount of 2-deoxyribose formed is 58.26 % (w/w) of the total amount of the monosaccharides produced in the hydrolyzed products. The number of ribose unit(s) present in one molecule of octasaccharide is _____.

Use: Molar mass (in g mol−1\mathrm{g\,mol^{-1}}): ribose =150= 150, 2-deoxyribose =134= 134, glucose =180= 180; Atomic mass (in amu): H = 1, O = 16

Answer and worked solution

Official answer: 2

Total mass of monosaccharides. Eight units are joined by 7 glycosidic bonds, each losing one water:
∑Mi=1024+7×18=1150.\sum M_i=1024+7\times18=1150.

Deoxyribose units. 0.5826×1150=670=5×1340.5826\times1150=670=5\times134, so there are 5 units of 2-deoxyribose.

The other 3 units. They weigh 1150−670=4801150-670=480. With aa ribose and 3−a3-a glucose:
150a+180(3−a)=480  ⟹  a=2.150a+180(3-a)=480\implies a=2.

Topics:CarbohydratesStoichiometry
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