When 2g of a gas A is introduced into an evacuated flask kept at 25ºC, the pressure is found to be 1 atmosphere. If 3g of another gas is then added to the same flask, the total pressure becomes 1.5 atm. Assuming ideal behaviour, the ratio of their molecular weights M_A : M_B is

When 2 grams of a gas A is introduced in an evacuated flask kept at 25^oC; the pressure was found to be 1 atm .i.e.

Partial Pressure of gas A = 1 atm.

And let moles be n_A​=2​/M_A​

if 3 g of another gas B is then added to the same flask;

Partial Pressure of gas B = 1.5 atm –  1 atm. = 0.5 atm

moles of gas _B  =​3/M_B​

Again the temp and volume are constant therefore,

$$\begin{gathered} \frac{P_1}{n_1}=\frac{ P_2}{n_2} \\ \frac{M_A}{M_B}=\frac{1}{3} \end{gathered}$$