Q3. Glucose and sucrose dissolve in water, but cyclohexane and benzene, also six-membered rings, do not. Explain. (NCERT Intext Question 10.1, page 290.)
Glucose and sucrose carry many –OH groups, which form hydrogen bonds with water molecules. Cyclohexane and benzene are hydrocarbons, with no such groups, so they cannot hydrogen-bond with water (reasoned).
Q4. What mass of glucose can 40.5 g of starch give on complete hydrolysis? (C = 12, H = 1, O = 16.)
Each C6H10O5 unit, 72 + 10 + 80 = 162 g mol−1, takes up one water and gives one C6H12O6, 180 g mol−1. 40.5/162 = 0.250 mol of units, so 0.250 × 180 = 45.0 g of glucose (reasoned from NCERT page 283).
Q5. Amylose has 200–1000 glucose units. Estimate the range of its molar mass, taking each chain as H–(C6H10O5)n–OH.
M = 162n + 18. For n = 200: 32 400 + 18 = 32 418 g mol−1, about 3.24 × 104; for n = 1000: 162 018 g mol−1, about 1.62 × 105 (reasoned from NCERT page 288).
Q6. Glucose pentaacetate is C16H22O11. What mass of it can 1.80 g of glucose give?
1.80/180 = 0.0100 mol of glucose gives 0.0100 mol of pentaacetate, M = 192 + 22 + 176 = 390 g mol−1: 3.90 g (reasoned from NCERT page 283).