To solve this problem we can use the gas law which tells us:
Where,
P is the pressure of the gas
T is the temperature
n is the number of moles
V is the volume of the gas
R is a constant
Now, we are told that the number of moles is reduced. We will assume that the pressure and temperature remain constant.
We have two states, an initial one with the following conditions:
n1=7 moles
V1= 10L
And we have a final state:
n2=5moles
V2=?
If we substitute these values in the ideal gas equation we will have: