Gases and Pressure Chapter 11 Section 1

Vocabulary Pressure: the force per unit area on a surface Atmospheric pressure: the force per unit area exerted against a surface by the weight of the air above that surface.

Measuring Pressure Barometer: device used to measure atmospheric pressure First introduced by Evangelista Torricelli during the early 1600s Discovered that atmospheric pressure supports a column of mercury about 760 mm above the surface of Hg in a dish

Units of Pressure There are several units for pressure. mm Hg, also called torr in honor of Torricelli Average atmospheric pressure at sea level at 0°C is 760 mm Hg, or 760 torr Atmospheres: one atm is defined as being exactly equivalent to 760 mm Hg

More about Units The SI unit of measurement for pressure is the pascal. One pascal (Pa) is the pressure exerted by a force of one newton acting on an area of one square meter. Usually expressed as kPa 1 atm = 101.325 kPa

STP STP = standard temperature and pressure STP has conditions of exactly 0°C and 1 atm of pressure.

Dalton’s Law of Partial Pressures The pressure of each individual gas in a mixture of gases is called the partial pressure of that gas. Dalton’s law states that the total pressure of a gas mixture is the sum of the partial pressures of the component gases. PT = P1 + P2 + P3 + …

Problem 1 A mixture of O2, CO2, and N2 has a total pressure of 0.97 atm. What is the partial pressure of O2 if the partial pressure of CO2 is 0.70 atm and the partial pressure of N2 is 0.12 atm? PT = Poxygen + Pcarbon dioxide + Pnitrogen Poxygen = PT – (Pcarbon dioxide + Pnitrogen) Poxygen = 0.97 atm – (0.70 atm + 0.12 atm) Poxygen = 0.15 atm

Problem 2 Find the total pressure for a mixture that contains four gases with partial pressure of 5.00 kPa, 4.56 kPa, 3.02 kPa, and 1.20 kPa. PT = P1 + P2 + P3 + P4 PT = (5.00 + 4.56 + 3.02 + 1.20) kPa PT = 13.78 kPa

Water Displacement Gas can be collected by water displacement, but some water vapor will be present. The total pressure will be the sum of the partial pressures of the gas collected and partial pressure of water vapor. The partial pressure of water vapor is found on a table (dependent on temperature).

Gas Collected by Water Displacement Figure 5- page 366 Pgas = Patm – Pwater vapor

Problem 3 O2(g),from the decomposition of KClO3, was collected by water displacement. The barometric (atmospheric) pressure and the temperature during the experiment were 97.5 kPa and 20.0° C. What was the partial pressure of the O2 collected?

Solution PT = Patm = 97.5 kPa Pwater vapor = 2.34 kPa Unknown = Poxygen gas Poxygen gas = PT – Pwater vapor Poxygen gas = 97.5 kPa – 2.34 kPa Poxygen gas = 95.2 kPa