# Dalton's Law Of Partial Pressures

I'm given a container of volume of 4.0 Litres that containes Nitrogen at $25 ^\circ C$ and 803 kPa. This container is connected by a valve to another container of 10.0 litres volume that containes Aragon at $25 ^\circ C$ and $47.2 kPa$ .

We now open the valve so that the two gas are mixing .

I need to calculate the partial pressure of every gas after the mixing and the total pressure of the mixture.

My question is - using the Dalton's law of partial pressures , isn't the partial pressure of every gas if excatly $803 kPa$ and $47.2 kPa$ and the total pressure if $803+47.2 kPa$ ?

What am I missing here?

Dalton's law of partial pressures means that the total pressure in the final volume $V=V_1+V_2$ is equal to the sum of the partial pressures P(N2) + P(Ar) in this volume. However, the partial pressure of N2 in the total volume $V$ is not the same as the pressure of N2 in its initial volume $V_1$. You have to account for the expansion. This probably means you also need to use the ideal gas law for that.