Type Problem
Hydrogen gas was collected in a eudiometer tube over water. It was impossible
to level the outside water with that in the tube, so the water level inside the tube
was 40.8 mm higher than that outside. The barometric pressure was 730. mm of
Hg. The water vapor pressure at the room temperature of 29°C was found in a
handbook to be 30.0 mm. What is the pressure of the dry hydrogen?
Step 1 To find the true pressure of the gas, we must first subtract the water-level
difference expressed in mm of Hg:
Then 730. mm − 3.00 mm = 727 mm total pressure of gases in the eudiometer
Step 2 Correcting for the partial pressure due to water vapor in the hydrogen, we
subtract the vapor pressure (30.0 mm) from 727 mm and get our answer:
697 mm.
TIP
Know how to use the Ideal Gas Law:PV = nRTIdeal Gas Law
The preceding laws do not include the relationship of number of moles of a gas to
the pressure, volume, and temperature of the gas. A law derived from the Kinetic-
Molecular Theory relates these variables. It is called the Ideal Gas Law and is
expressed as
PV = nRTP, V, and T retain their usual meanings, but n stands for the number of moles of the
gas and R represents the ideal gas constant.
Boyle’s Law and Charles’s Law are actually derived from the Ideal Gas
Law. Boyle’s Law applies when the number of moles and the temperature of the
gas are constant. Then in PV = nRT, the number of moles, n, is constant; the gas
constant (R) remains the same; and by definition T is constant. Therefore, PV = k.
At the initial set of conditions of a problem, P 1 V 1 = a constant (k). At the second