1. 7-3 Collecting a Gas over Water using the Eudiometer (Section 13.6)
And you

2. Dalton’s Law of Partial Pressures: in a mixture of gases, each gas exerts a pressure (partial pressure) and the total pressure is the sum of all the partial pressures.
Notice how, in all three pictures, the gas is collected over water and will therefore contain a small amount over water vapor (water gas). The water vapor pressure must be subtracted from the total pressure to obtain the pressure of the gas (Dalton’s Law).

3. Let’s discuss the pressure inside the tube shall we?
If the water level inside the tube is higher than the water level in the tank, then how does the gas pressure inside the tube compare to the atmospheric pressure???
Pin < Patmo.

4. What if the water level inside the tube is the same as the level outside the tube??
Pin = Patmo.
But recall that the Pin = Pwater vapor + P collected gas

5. Finally, what if the water level inside is lower than the water level outside the tube?
Pin > Patmo
Once again, recall that Pin = Pvapor + Pgas

6. Here the water level in the tube is higher than that of the container
Here the water level in the tube is higher than that of the container. To calculate the Pgas collected we first find the height of the water column from the meniscus in the eudiometer (example: 28.2cm high).
Then this height can be changed to Hg units by dividing by the density of Hg:
Height: cm H2O = mm H2O x mm Hg mm H2O 
= 20.7 mm Hg
The water column represents a height of 20.7 mm Hg. Since the atmospheric pressure is greater than the collected gas pressure, we need to subtract this difference from the known Patmo..
Finally, we subtract Pwater vapor (given: dep. On Temp.):
Pgas collected = Patmo. – mm Hg – Pwater vapor