The density of a gas at STP is $1.5 g/L$ at STP. Its molecular weight is _________ .

At STP the conditions are:

Pressure (P) = 1 atm

Temperature (T) = 273.15 K

Gas constant (R) = $0.0821 \mathrm{atm} \mathrm{mol}{{}^{-1} \mathrm{K}}^{-1}$

Using the formula $\mathrm{\rho }=\frac{\mathrm{PM}}{\mathrm{RT}}$, we get:

$\mathrm{M}=\frac{\mathrm{\rho RT}}{\mathrm{P}}$

$\mathrm{M}=\frac{1.5\times 0.0821\times 273.15}{1}$

$\mathrm{M}=33.6 \mathrm{g}$

Therefore, the molecular weight of the gas is 33.6 grams.