# Is atmospheric pressure acting only on the contents or also on the container?

$22~\mathrm{g}$ of dry ice is placed in an an empty $600~\mathrm{ml}$ closed vessel at $298~\mathrm{K}$. Find the final pressure inside the vessel, if all $\ce{CO2}$ gets evaporated?

Now by applying $pV = n\mathcal{R}T$, the value of $p$ comes out to be $20.4~\mathrm{atm}$, but in the solution shown, it adds $1~\mathrm{atm}$ i.e atmospheric pressure pressure, so the answer comes out be $21.4~\mathrm{atm}$. Now why do we need to add atm pressure, the question tells us to find the total pressure inside the vessel, isn't the atmospheric pressure acting only on the container, and not on its contents?

• A quick idea I had was that after placing the dry ice in the container and sealing it, t = 0, the container also contains air at 1 atm. So the $\ce{CO2}$-pressure is 20.4 atm. and the air is 1 atm. Total pressure: 21.4. Otherwise I can't quickly think of an answer. – Eljee Jan 15 '15 at 12:59

## 1 Answer

According to the way this question is worded, adding 1 atmosphere would be incorrect. The reason is that they specify an empty container - if you have a rigid container and it is truly empty (high vacuum), then the absolute pressure would be zero, and the gauge pressure would be -1 atm (see this wikipedia article for reference to these terms).

Once the $\ce{CO2}$ sublimates, the final absolute pressure would be given by $PV=nRT$ - there is no need to add atmospheric pressure. To find the gauge pressure, you would actually subtract one atmosphere.

It is possible that the question wants you to assume that "empty" means "filled with air at 1 atm," but that is a guess and I would argue that it's not safe to assume that based on the way the question is written.