Problem: Consider the following gases, all at STP: Ne, SF6, N2, CH4.Which one has the highest root-mean-square molecular speed at a given temperature?

root mean square speed (μ_rms)

$\boxed{{\mathbf{\mu }}_{\mathbf{rms}}\mathbf{=}\sqrt{\frac{\mathbf{3}\mathbf{RT}}{\mathbf{M}}}}$

µ_rms = rms speed,
R = 8.314 J/(mol·K)
T = temperature, K
M = molar mass, kg/mol

Based on the equation for μ_rms, 3 and R are constants. All of the gases are at the same temperature (STP). The only difference the gases have is their molar masses (M).