10.2 NOTES Gas Pressure

C. Gas Pressure 1. Pressure force per unit area - air pressure at higher altitudes is slightly lower than at sea level (due to decreased mass in upper atmosphere) - examples of force per area - snow and snow shoes; ax vs. hammer

2. Measuring air pressure - Torricelli was first to demonstrate air exerted pressure Barometer instrument used to measure atmospheric pressure - height of mercury is a barometer is always about 760mm - actual height based on two factors: gravity and atmospheric pressure - increase air pressure, causes Hg to rise, decrease AP, Hg will fall;

3. Units of pressure SI unit of pressure is the Pascal (Pa) - Pa = 1 N/m2 (Newton = kgXm/s2) - At sea level (temp. of 0°C) – avg air pressure = 760 mmHg – known as 1atm; 1 atm = 101.3kPa = 760 mmHg (∞ sig figs); 760 torr (∞ sig figs); 14.7 psi

4. Dalton’s Law of Partial Pressures - each gas in a mixture exerts pressure independently of the other gases present; Ptotal = PA + PB + PC

Partial pressure of any gas depends on the # of moles of the gas, size of the container and temperature of the mixture it does not depend on the identity of the gas! At a given T and P, the partial pressure of one mole of any gas is the same if the percentage of a mixture is known as well as the total pressure, it is possible to determine the partial pressure of each gas multiply the % times the total pressure = partial pressure of gas b/c partial pressures of gases at the same temp. are related to their concentration;

Collecting over water certain gases must be collected over water (oxygen) when this happens, water vapor will also be collected;   vapor pressure – pressure exerted by a vapor in equilibrium with its liquid liquid w/ strong attractive forces (glycerol) have a low VP; while those with weak attractive forces (ether) have high VP; water vapor exerts a pressure and must be accounted for; water vapor pressure dependent on temperature of the system, can look it up;

Sample Problem – A sample of oxygen is collected over water at a temperature of 22°C. The pressure of the system was 764.2 mmHg. What is the pressure of the DRY oxygen? Ptotal = P1 + P2 + P3 + … 764.2 = PO2 + 19.8 PO2 = 744.4 mmHg