Calculate the density of each of the following gases:?

PV = nRT and n = m/M, where m is mass and M is molar mass
Then, PV = (m/M)RT
PM = (m/V)RT
PM = dRT, where d is density
Hence, d = PM/(RT)


(a)
For C₂H₄ :
Pressure, P = 0.75 atm
Molar mass, M = (12.0×2 + 1.0×4) g/mol = 28.0 g/mol
Gas constant, R = 0.08206 L atm / (mol K)
Absolute temperature, T = (273 + 32) K = 305 K

Density, d = PM/(RT) = 0.75 × 28.0 / (0.08206 × 305) g/L = 0.839 g/L


(b)
For He :
Pressure, P = 791/760 atm
Molar mass, M = 4.0 g/mol
Gas constant, R = 0.08206 L atm / (mol K)
Absolute temperature, T = (273 + 57) K = 330 K

Density, d = PM/(RT) = (791/760) × 4.0 / (0.08206 × 330) g/L = 0.154 g/L