Surface Chemistry

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 aceticacid occupies P×10−23m2 surface area on charcoal, the value of P is ______.
[Use given data : Surface area of charcoal =1.5×102m2g−1 ; Avogadro's number (NA)=6.0×1023mol−1 ]