1 mole of an ideal gas is contained in a cubical box of volume V, ABCDEFGH at 300 K. One face of the box (EFGH) is made up of a material which totally absorbs any gas molecule incident on it.

At any given time :

the pressure on EFGH would be half the pressure on ABCD

The pressure on face EFGH would be half that on face ABCD. This is because force EFGH absorbs totally all the molecules incident on it. These molecules do not rebound. Change in momentum causing force and hence pressure is reduced to half compared to pressure on face ABCD from where molecules rebound with an equal velocity.