Question

The density of a gas A is thrice that of a gas B at the same temperature. The molecular weight of gas B is twice that of A. What will be the ratio of the pressures acting on B and A?

• A. $\frac{1}{4}$
• B. $\frac{7}{8}$
• C. $\frac{2}{5}$
• D. $\frac{1}{6}$

Answer 1

Answer: (D)

$\frac{d}{p}=\frac{M}{RT}$
Let density of gas B be d.
$\therefore$ density of gas A = 3d
and let molecular weight of A be M.
$\therefore$ molecular weight of B = 2M
Since, R is gas constant and T is same for both gases, so
$p_A=\frac{d_{A}RT}{M_A}$ and $p_B=\frac{d_{B}RT}{M_B}$
$\frac{p_B}{p_A}=\frac{d_B}{d_A}\times \frac{M_A}{M_B}=\frac{d}{3d}\times \frac{M}{2M}=\frac{1}{6}$