How do I calculate the density of propane assuming an ideal gas?
As you have already figured out that we have to use the ideal gas law $PV= nRT$, which is you have modified as $$\frac{n}{V}= \frac{P}{RT}$$

To find the density, we can further modify the equation as,
\begin{align}
\frac{m}{MV} &amp;= \frac{P}{RT}\\
\frac{\rho}{M}&amp;= \frac{P}{RT}\\
\rho &amp;= \frac{PM}{RT}\\
\end{align}

Now we just have to replace the parameters with their respective values.

The value of temperature should be converted to Kelvin as $(0 + 273)~\mathrm{K} = 273~\mathrm{K}$.

Likewise, the value of gas constant $R$ should be chosen as  $0.8206~\mathrm{L~atm~K^{-1}{mol}^{-1}}$.
\begin{align}
\rho &amp;= \frac{1~\mathrm{atm} \cdot 44.09562~\mathrm{g~mol^{-1}}}{{0.8206~\mathrm{L~atm~K^{-1}{mol}^{-1}} \cdot 273~\mathrm{K}}}\\
&amp;=1.9683~\mathrm{g/L}\\
&amp;=0.0019683~\mathrm{g/cm^3}\\
\end{align}