Poly(lactic acid), -[O-CH(CH3)-CO]_n- (72 g/mol per unit), is made by polycondensation of lactic acid, CH3CH(OH)COOH (90 g/mol). For a very long chain (end groups neglected, 100 percent conversion), the mass of lactic acid needed for 720 g of PLA is

JEE Advanced levelSingle correct optionDifficulty 3 of 3pl107

Poly(lactic acid), $- [O - CH (CH X_{3}) - CO] X_{n} X^{-}$ (72 g/mol per unit), is made by polycondensation of lactic acid, $CH X_{3} CH (OH) COOH$ (90 g/mol). For a very long chain (end groups neglected, 100 percent conversion), the mass of lactic acid needed for 720 g of PLA is

720 g of PLA = 720/72 = 10.0 mol of units, which needs 10.0 mol of lactic acid = 900 g. The 180 g difference is the 10.0 mol of water lost.