JEE Advanced levelSingle correct optionDifficulty 3 of 3ev203
Use C 12, H 1, O 16, Cl 35.5. A field receives 4.42 kg of 2,4-D (formula CX8HX6ClX2OX3). In the soil it degrades by first order kinetics with half-life 10 days. The mass of chlorine still bound in unchanged 2,4-D after 20 days is
A0.355 kg
B1.42 kg
C0.178 kg
D0.710 kg
Answer: A
Molar mass is 96+6+71+48=221, so Cl is 71/221 of the mass. Initial Cl is 4.42×71/221=1.42 kg. After 20 days (2 half-lives) one quarter of 2,4-D remains, so Cl remaining is 1.42/4=0.355 kg.