Thyroxine, the hormone has the given below structure.

The percentage of iodine in thyroxine is .......... %. (nearest integer)
(Given molar mass in g mol$$^{-1}$$ of C:12, H:1, O:16, N:14, I:127)

Thyroxine is 3,3′,5,5′-tetraiodo-L-thyronine. Its molecular formula is $$C_{15}H_{11}I_{4}NO_{4}$$ (15 carbons, 11 hydrogens, 4 iodines, 1 nitrogen and 4 oxygens).

Molar mass contributed by each element:
$$\text{C}: 15 \times 12 = 180 \text{ g mol}^{-1}$$
$$\text{H}: 11 \times 1 = 11 \text{ g mol}^{-1}$$
$$\text{I}: 4 \times 127 = 508 \text{ g mol}^{-1}$$
$$\text{N}: 1 \times 14 = 14 \text{ g mol}^{-1}$$
$$\text{O}: 4 \times 16 = 64 \text{ g mol}^{-1}$$

Total molar mass of thyroxine:
$$M = 180 + 11 + 508 + 14 + 64 = 777 \text{ g mol}^{-1}$$

Percentage of iodine:
$$\%\ I = \frac{508}{777}\times 100 = 65.37\%$$

Nearest integer ≈ 65.

Answer: 65