Thyroxine, the hormone has given below structure
The percentage of iodine in thyroxine is __________ %. (nearest integer)
(Given molar mass in g mol⁻¹ C: 12, H: 1, O: 16, N: 14, I: 127 )

Solution

Chemistry ma’am · 2 steps

Count every atom from the structure, find the molar mass, then see what share the four iodines carry.

1. Read the formula. Each benzene ring gives 6 C, and the side chain CH₂-CH(NH₂)-COOH adds 3 more. Each ring carries 2 I and 2 ring H. Oxygen comes from the OH, the ether bridge and the COOH group, so thyroxine is C₁₅H₁₁I₄NO₄.

   Element	Atoms	Mass (g mol⁻¹)
   C	15	15 × 12 = 180
   H	4 + 1 + 2 + 1 + 2 + 1 = 11	11 × 1 = 11
   I	4	4 × 127 = 508
   N	1	1 × 14 = 14
   O	4	4 × 16 = 64
   Total	35 atoms	777

   % I = 508/777 × 100 = 65.38%

2. So. The percentage of iodine in thyroxine is 65.38%, which rounds to 65.

The idea to keep. Mass percent of an element is (n × atomic mass)/(molar mass) × 100. From a structure, count hydrogens patiently: ring H, OH, NH2, CH2, CH and COOH all count.

Answer from the NTA final answer key. Solution by PrepWiser.