TIP
When a gas is collected over water, subtract the water vapor pressure at the given
temperature from the atmospheric pressure to find the partial pressure of the gas.CORRECTION OF DIFFERENCE IN THE HEIGHT OF THE FLUID. When
gases are collected in eudiometers (glass tubes closed at one end), it is not
always possible to get the level of the liquid inside the tube to equal the level on
the outside. This deviation of levels must be taken into account when determining
the pressure of the enclosed gas. There are then two possibilities: (1) When the
level inside is higher than the level outside the tube, the pressure on the inside is
less, by the height of fluid in excess, than the outside pressure. If the fluid is
mercury, you simply subtract the difference from the outside pressure reading
(also in height of mercury and in the same units) to get the corrected pressure of
the gas. If the fluid is water, you must first convert the difference to an equivalent
height of mercury by dividing the difference by 13.6 (since mercury is 13.6 times
as heavy as water, the height expressed in terms of Hg will be 1/13.6 the height of
water). This is shown pictorially in Figure 21. Again, care must be taken that this
equivalent height of mercury is in the same units as the expression for the outside
pressure before it is subtracted to obtain the corrected pressure for the gas in the
eudiometer. (2) When the level inside is lower than the level outside the tube, a
correction must be added to the outside pressure. If the difference in height
between the inside and the outside is expressed in terms of water, you must take
1/13.6 of this quantity to correct it to millimeters of mercury. This quantity is then
added to the expression of the outside pressure, which must also be in millimeters
of mercury. If the tube contains mercury, then the difference between the inside
and outside levels is merely added to the outside pressure to get the corrected
pressure for the enclosed gas.
Figure 21. Same Pressure Exerted on Both Liquids