1) Ideal gas equation:
pV = nRT
2) Transform the equation to compute density, d = m / V
n = mass in grams / molar mass = m / MM
pV = (m/MM) RT
=> pV = mRT/MM
=> m/V = pMM / (RT)
d = pMM / (RT)
p = 1.21 atm
MM = 83.80 g/mol (this is the atomic mass of krypton element)
T = 50 + 273.15 K = 323.15K
3) Compute:
d = (1.25 atm * 83.80 g/mol) / (0.0821 atm*liter /K*mol * 323.15K)
d = 3.95 g / liter