Calculation of VOCs concentration (ppm) in the headspace of a bottle
Usage of ppm/ppb etc units is discouraged, as they are ambiguous without the explicit context. As generally, 1 ppm (w/w) &lt;> 1 ppm (V/V) &lt;> 1 ppm (n/n) &lt;> 1 ppm (w/V).

It is recommended to use explicit units, like e.g mg/L.

Expecting ideal gas behaviour $pV = nRT$ is not reasonable.   Ideal gases are not in equilibrium with their liquid phase.

There should be rather used a real gas equation.The easiest to use is the van Der Waals equation, but even that has significant error near condensation state. 

Additionally, the sample(!) temperature  has to be strictly controlled, otherwise $n$ would be variable.

Why not rather to use small volume of ethanol to be known to fully evaporate in given volume ? Or perhaps other solvent, unless ethanol is waterless.
Note that ethanol has significant deviation from the Raoult's law.



I prefer $R = \pu{8.31446 J K-1 mol-1}$

Note that by May 2019 redefinition, 
 It's exact value is $\pu{8.31446261815324 J K-1 mol-1}$