To form a complete monolayer of acetic acid on 1g 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 m² surface area on charcoal, the value of P is _____. [Use given data: Surface area of charcoal = 1.5 × 10² m²g⁻¹; Avogadro’s number (N_A) = 6.0 × 10²³ mol⁻¹]