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.
