Estimate the mass of air in a room whose dimensions are `5\ m times 3\ m times 2.5\ m` high, at STP. Given: `V = 5\ m times 3\ m times 2.5\ m = 37.5\ m^3` under STP `T = 273 K` `P = 1.013 times 10^3 N//m^2` Known: `R = 8.314\ J//mol•K` Equation: `PV = nRT` Solution: `n = (PV)/(RT) = ((1.013 times 10^3 N//m^2)(37.5\ m^3))/((8.314\ J//mol•K)(273 K))` `n = 1.7 times 10^3 mol` Air is a mixture of about 20%(`O_2`) and 80% (`N_2`). thus 1 mol fo air has a mass of `20%(2 times 16\ g//mol) + 80% (2 times 14\ g//mol) = 29 g//mol` `m = 1.7 times 10^3 mol times 29\ g//mol`