Question

You have an unknown quantity of oxygen at a pressure of 2.2 atam, a volume of 21 liters and a temperature of 87 Celsius. How many miles of gas are left for you to breath

Answer 1

Let's assume that the oxygen gas has ideal gas behavior. 
Then we can use ideal gas formula,
      PV = nRT

Where, P is the pressure of the gas (Pa), V is the volume of the gas (m³), n is the number of moles of gas (mol), R is the universal gas constant ( 8.314 J mol⁻¹ K⁻¹) and T is temperature in Kelvin.

P = 2.2 atm = 222915 Pa
V = 21 L = 21 x 10⁻³ m³

n = ?

R = 8.314 J mol⁻¹ K⁻¹

T = 87 °C = 360 K

By substitution,
222915 Pa x 21 x 10⁻³ m³ = n x 8.314 J mol⁻¹ K⁻¹ x 360 K
                                       n = 1.56 mol

Hence, 1.56 moles of the oxygen gas are left for you to breath.