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Question

# An organic compound of molar mass greater than 100 contains only C, H and N. The percentage of C is 6 times that of H while the sum of percentages of C and H is 1.5 times that of N. What is the least molar mass of the compounds?

A
175 g/mol
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B
140 g/mol
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C
105 g/mol
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D
210 g/mol
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Solution

## The correct option is B 140 g/molLet us assume percent of H as x %, ∴Percent of C is 6x% As per question % C +% H = 1.5(% N) ∴ Percent of N is 143x% Assuming 100 g of the total compound, Mass of H is x g ⇒ moles of H is x mol Mass of C is 6x g ⇒ moles of C is x2 mol Mass of N is 14x3 g ⇒ moles of N is x3 mol Taking simplest mole ratio of all three i.e. C:H:N comes out to be x2:x:x3. On simplifying it comes out to be 3:6:2 Empirical formula: C3H6N2 Empirical mass = 70 g Since the molar mass is greater than 100, least mass would have the formula (on multiplying empirical formula by 2) C6H12N4, whose molar mass is 140 g i.e. greater than 100 g.

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