Solid white phosphorus melts and then vaporizes at high temperature. gaseous white phosphorus effuses at a rate that is 0.404 times that of neon in the same apparatus under the same conditions. how many atoms are in a molecule of gaseous white phosphorus?

Rate of effusion of phosphorus / rate of effusion of neon = 0,404.
rate of effusion of neon = 1/√M(Ne).
rate of effusion of neon = 1/√20,18 = 0,222.
rate of effusion of phosphorus = rate of effusion of neon · 0,404.
rate of effusion of phosphorus = 0,222 · 0,404 = 0,0897.
rate of effusion of phosphorus = 1/√M(Pₓ).
√M(Pₓ) = 1 ÷ 0,0897 = 11,14.
M(Pₓ) = (11,15)² = 124.
N(P) = 124 g/mol ÷ 31 g/mol = 4.
There are 4 atoms in a molecule of gaseous white phosphorus.

Density and pressure