2 g of gas A introduced in a evacuated flask at 25°C. The pressure of the gas is 1 atm. Now 3 g of another gas B is introduced in the same flask so total pressure becomes 1.5 atm. The ratio of molecular mass of A and B is

Partial pressure of $\mathrm{A}\to {\mathrm{P}}_{\mathrm{A}}=1\mathrm{atm}$
Partial pressure of $\mathrm{B}\to {\mathrm{P}}_{\mathrm{B}}=(1.5-1) \mathrm{atm} = 0.5 \mathrm{atm}$
Then, 
${\mathrm{P}}_{\mathrm{A}}={\mathrm{P}}_{\text{total }}{\mathrm{\chi }}_{\mathrm{A} ...\left(1\right)}$
and 
${\mathrm{P}}_{\mathrm{B}}={\mathrm{P}}_{\text{total }}{\mathrm{X}}_{\mathrm{B}} ..\left(2\right)$
$\text{ where, }{\mathrm{\chi }}_{\mathrm{A}}\text{ is mole fraction of }\mathrm{A} \text{ and }{\mathrm{\chi }}_{\mathrm{B}}\text{ is mole fraction of }\mathrm{B}$
Dividing (2) by (1), we get
$\frac{{\mathrm{P}}_{\mathrm{B}}}{{\mathrm{P}}_{\mathrm{A}}}=\frac{{\mathrm{\chi }}_{\mathrm{B}}}{{\mathrm{\chi }}_{\mathrm{A}}}=\frac{\frac{{\mathrm{n}}_{\mathrm{B}}}{{\mathrm{n}}_{\mathrm{A}}+{\mathrm{n}}_{\mathrm{B}}}}{\frac{{\mathrm{n}}_{\mathrm{A}}}{{\mathrm{n}}_{\mathrm{A}}+{\mathrm{n}}_{\mathrm{B}}}}=\frac{{\mathrm{n}}_{\mathrm{B}}}{{\mathrm{n}}_{\mathrm{A}}}$
${\mathrm{n}}_{\mathrm{B}}=\frac{{\mathrm{W}}_{\mathrm{B}}}{{\mathrm{M}}_{\mathrm{B}}};{\mathrm{n}}_{\mathrm{A}}=\frac{{\mathrm{W}}_{\mathrm{A}}}{{\mathrm{M}}_{\mathrm{A}}}\left[\text{ where, }{\mathrm{W}}_{\mathrm{A}},{\mathrm{W}}_{\mathrm{B}}=\right.$
$= \mathrm{Weight} \mathrm{of} \mathrm{A},\mathrm{B} \mathrm{and} {\mathrm{M}}_{\mathrm{A}},{\mathrm{M}}_{\mathrm{B}}= \mathrm{Molecular} \mathrm{mass}]$
$\therefore \frac{{\mathrm{n}}_{\mathrm{B}}}{{\mathrm{n}}_{\mathrm{A}}}=\frac{\frac{{\mathrm{W}}_{\mathrm{B}}}{{\mathrm{M}}_{\mathrm{B}}}}{\frac{{\mathrm{W}}_{\mathrm{A}}}{{\mathrm{M}}_{\mathrm{A}}}}=\frac{{\mathrm{W}}_{\mathrm{B}}}{{\mathrm{W}}_{\mathrm{A}}}\times \frac{{\mathrm{M}}_{\mathrm{A}}}{{\mathrm{M}}_{\mathrm{B}}}=\frac{{\mathrm{P}}_{\mathrm{B}}}{{\mathrm{P}}_{\mathrm{A}}}$
Putting the values, we get
$\frac{{\mathrm{M}}_{\mathrm{A}}}{{\mathrm{M}}_{\mathrm{B}}}=\frac{{\mathrm{P}}_{\mathrm{B}}}{{\mathrm{P}}_{\mathrm{A}}}\times \frac{{\mathrm{W}}_{\mathrm{A}}}{{\mathrm{W}}_{\mathrm{B}}}=\frac{0.5 \mathrm{atm}}{1 \mathrm{atm}}\times \frac{2 \mathrm{g}}{3 \mathrm{g}}=\frac{1}{3}$