As force of attraction between molecules disappears, then molecules will hit the wall with more speeds, hence, rate of change of momentum will increase. As we know F = ∆p / ∆t, where F is average force on the wall due to molecules. ∆p is change in momentum and At is the time duration. Due to increase in ∆p force F will also increase, hence F . . .
pressure p = F/A will increase. Here, A is area of one wall.