To an evacuated vessel with a movable piston under a total pressure of 1 atm, 0.1 mol of He and an unknown compound (vapour pressure 0.68 atm at 0℃) are introduced. Considering the ideal gas behaviour, the total volume (in litre) of the gases at 0℃ is close to:

 For a better understanding of the Ideal Gas Equation, let us mention the four gas variables and one constant. Pressure (P), Volume (V), Number of Moles (n), and Temperature (T) are the four gas variables (T). Finally, R, often known as the Gas constant, is a constant in the equation presented below.

nRT = PV

Assume the unknown substance is x and its vapour pressure is 0.68 atm, as stated in the question.

Pressure of helium will be = 1 – 0.68 = 0.32 atm Now according to the Ideal gas Equation: PV = nRT V = nRT/P

Here, P = pressure of helium,

n = number of moles of Helium.

Now, V = 0.1×0.082×273/0.32 = 7 L