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Some Basic Concepts of Chemistry question

2023 · 25 Jan · Shift 2 · Q21
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Some Basic Concepts of Chemistry question

2023 · 25 Jan · Shift 2 · Q21

JEE MainChemistrySome Basic Concepts of ChemistryNumerical+4 / −1
Number of hydrogen atoms per molecule of a hydrocarbon A having 85.8% carbon is ‾\underline{\hspace{2cm}}​ (Given : Molar mass of A = 84 g mol −1^{-1}−1)
Numerical answer
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Correct answer: 12

  1. Let the hydrocarbon be CxHy\mathrm{C_xH_y}Cx​Hy​.

  2. Use the molar mass condition: 12x+y=8412x + y = 8412x+y=84 since molar mass of carbon is 121212 and hydrogen is 111.

  3. Use the percentage of carbon: Carbon contributes 85.8%85.8\%85.8% of the total mass.

    So, mass of carbon in 111 mole of the hydrocarbon is 0.858×84=72.072 g0.858 \times 84 = 72.072 \text{ g}0.858×84=72.072 g

  4. Find number of carbon atoms: 12x≈72.07212x \approx 72.07212x≈72.072 x≈72.07212=6.006≈6x \approx \frac{72.072}{12} = 6.006 \approx 6x≈1272.072​=6.006≈6

    Since xxx must be an integer, we take x=6x=6x=6

  5. Now find hydrogen atoms using molar mass: 12(6)+y=8412(6) + y = 8412(6)+y=84 72+y=8472 + y = 8472+y=84 y=12y = 12y=12

  6. Check the percentage: For C6H12\mathrm{C_6H_{12}}C6​H12​, %C=7284×100=85.714%≈85.8%\%C = \frac{72}{84} \times 100 = 85.714\% \approx 85.8\%%C=8472​×100=85.714%≈85.8% which matches the given data.

Therefore, the number of hydrogen atoms per molecule is: 12\boxed{12}12​

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