Question

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

Hint:

To calculate the percentage composition, divide the mass of the element by the molar mass of the compound and multiply by 100.

Answer 1

Correct Answer: 65

Step 1: Count atoms from structure

Carbon (C) = 15 atoms
Hydrogen (H) = 11 atoms
Oxygen (O) = 4 atoms
Nitrogen (N) = 1 atom
Iodine (I) = 4 atoms

Step 2: Compute mass contribution

\[ \begin{aligned} \text{Mass of C} &= 15 \times 12 = 180 \, \text{g/mol} \\ \text{Mass of H} &= 11 \times 1 = 11 \, \text{g/mol} \\ \text{Mass of O} &= 4 \times 16 = 64 \, \text{g/mol} \\ \text{Mass of N} &= 1 \times 14 = 14 \, \text{g/mol} \\ \text{Mass of I} &= 4 \times 127 = 508 \, \text{g/mol} \end{aligned} \]

Step 3: Total molar mass of thyroxine

\[ \text{Molar mass} = 180 + 11 + 64 + 14 + 508 = 777 \, \text{g/mol} \]

Step 4: Percentage of Iodine

\[ \text{Percentage of Iodine} = \frac{508}{777} \times 100 \approx 65.37\% \]

Correct Answer: 65%