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NVAEasyJEE 2025Hormones (Basics)

JEE Chemistry 2025 Question with Solution

Thyroxine, the hormone has the given structure.

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

Answer

Correct answer:65

Step-by-step solution

Standard Method

Given: Thyroxine contains 44 iodine atoms. From the structure used in the solution, the atom count is C=15\text{C}=15, H=11\text{H}=11, O=4\text{O}=4, N=1\text{N}=1, I=4\text{I}=4.

Find: Percentage of iodine in thyroxine.

Use percentage composition:

Percentage of iodine=mass of iodine in one mole of compoundmolar mass of compound×100\text{Percentage of iodine} = \frac{\text{mass of iodine in one mole of compound}}{\text{molar mass of compound}} \times 100

Mass contributions are:

Mass of C=15×12=180 g/molMass of H=11×1=11 g/molMass of O=4×16=64 g/molMass of N=1×14=14 g/molMass of I=4×127=508 g/mol\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}

Calculation and rounding

Total molar mass of thyroxine:

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

Now calculate the iodine percentage:

Percentage of iodine=508777×100≈65.37%\text{Percentage of iodine} = \frac{508}{777} \times 100 \approx 65.37\%

Rounding to the nearest integer gives 6565. Therefore, the percentage of iodine in thyroxine is 65%65\%.

The second approach on the page uses a different assumed formula and gives 64.6%64.6\%, which also rounds to 65%65\%. The correct numerical value answer is 6565.

Common mistakes

  • Counting the number of iodine atoms incorrectly from the structure leads to the wrong iodine mass. First count the atoms carefully, then multiply by 127127 for each iodine atom.

  • Using the mass of iodine alone as the answer is incorrect because the question asks for percentage composition. Divide the iodine mass by the total molar mass of thyroxine and then multiply by 100100.

  • Rounding too early can change the final integer answer in other questions. Complete the fraction calculation first, then round the final percentage to the nearest integer.

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