In a compound C, H and N atoms are present in 9 : 1 : 3.5 by weight . Molecular weight of the compound is 108 g mol-1 . Molecular formula of compound is

Some Basic Concepts of Chemistry PYQ — JEE Main 2002 · Shift 0 · Q53 with Solution

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Correct answer: C

Given mass ratio of elements in the compound: $\text{C : H : N} = 9 : 1 : 3.5$
Convert mass ratio to mole ratio using atomic masses:
- Carbon: atomic mass $=12$
- Hydrogen: atomic mass $=1$
- Nitrogen: atomic mass $=14$
So, $\text{moles of C} = \frac{9}{12} = 0.75$ $\text{moles of H} = \frac{1}{1} = 1$ $\text{moles of N} = \frac{3.5}{14} = 0.25$
Find simplest whole-number ratio by dividing by the smallest value $0.25$ : $\text{C : H : N} = \frac{0.75}{0.25} : \frac{1}{0.25} : \frac{0.25}{0.25} = 3 : 4 : 1$

Hence, the empirical formula is: $C_3H_4N$
Calculate empirical formula mass: $3(12) + 4(1) + 1(14) = 36 + 4 + 14 = 54$
Use molecular mass: Given molecular mass $=108$ g/mol.

$\frac{108}{54} = 2$

Therefore, molecular formula is: $(C_3H_4N)_2 = C_6H_8N_2$
Check options:
- A: $C_2H_6N_2$ → molar mass $=58$ , incorrect
- B: $C_3H_4N$ → empirical formula only, molar mass $=54$ , incorrect
- C: $C_6H_8N_2$ → molar mass $=108$ , correct
- D: $C_9H_{12}N_3$ → molar mass $=162$ , incorrect

Therefore, the correct option is C.