The density of krypton gas at 1.21 atm and 50.0°c is g/l.

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

:DD