Calculate the amount of hcn that gives the lethal dose in a small laboratory room measuring 12.0 ft×15.0 ft×9.00ft.

The lethal dose of Hydrogen cyanide (HCN ) is approximately 300.mg/kg of air when inhaled. Its density at 26 C is 0.00118 g/cm3 .  Mass is the product of density and volume. 

Given are the following:

m= dv

d = 0.00118 g/cm3

v = 12.00 ft x 15.00 ft x 9.00 ft 
v = 365.8 cm x 457.2 cm x 274 cm
v =  45,824,790 cm3

Substitute:
m= dv = (0.00118 g/cm3)( 45,824,790 cm3)
m= 54,073 g

To find the lethal dose in the room:
(300 mg/kg)(1kg/1000g) (54,073 g) = 0.016g of HCN will be considered lethal.