Question

Number of hydrogen atoms per molecule of a hydrocarbon A having $858 \%$ carbon is_____ (Given: Molar mass of $A =84 g mol ^{-1}$ )

Answer 1

Correct Answer: 12

The correct answer is 12.

Element	Percentage	Mole	Mole ratio
C	85.8	\(\frac{85.8}{12}=7.5\)	1
H	14.2	\(\frac{14.2}{1}=14.2\)	2

Empirical formula (CH₂​)
14×n=84
n=6
∴ Molecular formula C₆​H_12​

Answer 2

1. Calculate the moles of each element: To find the moles of each element, divide the percentage composition by the atomic mass of the respective element. Moles of Carbon (C) = $\frac{85.8}{12} = 7.15$ moles \item Moles of Hydrogen (H) = $\frac{14.2}{1} = 14.2$ moles 

2. Determine the mole ratio: Divide the moles of each element by the smallest number of moles calculated. \begin{itemize}  Mole ratio of Carbon (C) = $\frac{7.15}{7.15} = 1$  Mole ratio of Hydrogen (H) = $\frac{14.2}{7.15} \approx 2$ 

3. Write the empirical formula: The empirical formula represents the simplest whole-number ratio of atoms in a compound. Based on the mole ratio, the empirical formula is: $CH_2$ 

4. Calculate the empirical formula mass: Add the atomic masses of the elements in the empirical formula. Empirical formula mass = (1 × Atomic mass of C) + (2 × Atomic mass of H) Empirical formula mass = (1 × 12) + (2 × 1) = 12 + 2 = 14 g/mol 

5. Calculate the value of n: Divide the molecular weight of the compound by the empirical formula mass. $n = \frac{\text{Molecular weight}}{\text{Empirical formula mass}} = \frac{84}{14} = 6$ 

6. Determine the molecular formula: Multiply the subscripts in the empirical formula by the value of n. Molecular formula = $(CH_2)_n = (CH_2)_6 = C_6H_{12}$ 

Results:  Empirical formula: $CH_2$ \item Molecular formula: $C_6H_{12}$