To form a complete monolayer of acetic acid on 1 ~g of charcoal, 100 ~mL of 0.5 M acetic acid was used. Some of the acetic acid remained unadsorbed. To neutralize the unadsorbed acetic acid, 40 mL of 1 M NaOH solution was required. If each molecule of acetic acid occupies P 10⁻²³ ~m ^2 surface area on charcoal, the value of P is _______ [Use given data: Surface area of charcoal =1.5 10^2 ~m ^2 ~g ⁻¹ ; Avogadro's numb