When 2g of a gas A are introduced into an evacuated flask kept at 25 0C, the pressure is found to be one atmosphere, if 3g of another gas B are then added to the same flask, the total pressure becomes 1.5 atm. Assuming ideal gas behaviour, Calculate the ratio of molecular weights M A :M B

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

Partial Pressure of gas nA =1 atm. 

And let moles be nA = 1,‘,

TA 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 MB 

Again the temp and volume are constant therefore, 

PI P2 = 112 
1 0.5 Hence, 2 3 MA MB 0.5MB MA — x 2 3 MA = MB 1 3 MB MA = 3 
or 
MA = -1 1 : 3 MB 3