Question

Pregnane is a molecular substance which contains 21 carbon atoms per molecule. If the molar mass of the substance is $360g/mol$, calculate the mass percentage of carbon in pregnane.

• A. $30$
• B. $70$
• C. $25$
• D. $85$

Answer 1

The correct option is B $70$

Since 1 molecule of pregnane contains 21 carbon atoms, 1 mol will contain $21$ mol of carbon atoms.

Let us assume the mass of the compound to be 'x' g and the mass of carbon to be 'y' g.
Equating moles:
$21\times \frac{x}{360}$ = $\frac{y}{12}$
$\frac{y}{x}$ = $\frac{21\times 12}{360}$
$\therefore$ mass percent of carbon = $\frac{y}{x}\times 100$ = $70\%$