2. 1 Mr. ShieldsRegents Chemistry U05 L02 Units and the Barometer

3. 2 Pressure Units SI UNITS of Pressure is measured in PASCALS 1 atm = 101.325 KPa  101,325 Pascals Other units of pressure are the following: 1 atm = 14.7 lbs/in 2 1 atm = 14.7 lbs/in 2 1 atm = 760 mm Hg 1 atm = 29.9 in Hg 1 torr = 1 mm Hg Know each of these conversions! English system ex. tire pressure ex. tire pressure

4. 3 Pressure Unit Problems 1) How many Pascals in 2 atm? 2 atm = 2 2 2 202,650 Pascals 2) How many mm Hg in 0.5 atm? 760 mm Hg/atm x 0.5 atm = 380 atm 3) How many mm Hg in 25.33 KPa? (25.33KPa/101.3KPa) x 760 mm Hg/atm = 190 mm Hg 4) How many Torr in 380mm Hg? 1 torr = 1 mm Hg so 380mm Hg = 380 Torr

5. 4 OK. We now know what units are used in measuring Pressure … So how do we actually MEASURE atmospheric pressure? Pressure measurement The instrument is known as a BAROMETER

6. 5 The typical household Barometer Used to forecast A change in weather So how does a barometer Do this?

7. 6 What is STP: What is STP in SI units? But atmospheric Pressure is not always 1 atm; It can vary with weather conditions… Warm fronts (fair weather) have higher pressures Cold Fronts (stormy weather ) have lower pressures And stored gases can vary over any pressure range Pressure measurement 273K and 101.3 KPa 0 deg C and 1 atm

8. 7 The Barometer The lower the barometric pressure and the faster it drops the more severe an approaching storm. Hurricanes for example have very low barometric pressures. Most intense storms to ever hit the US Fla Keys 1935 26.35 in Hg 160 mph Camille 1969 26.84 in Hg 190 mph Katrina C4 2005 27.11 inHg 140 mph Andrew C5 1993 27.23 inHg 165 mph Why are these units expressed in inches of Mercury ?

9. 8 The Origin of the Barometer Units of Torr (or mm of Hg) are associated with an Instrument know as a Torricelli Barometer Named after the inventor Evangelista Torricelli (1608-1647) The first to show that the Atmosphere exerted pressure.

10. 9 Mercuryresevoir Vacuum 760 mm Hg 0r 30” Hg Wt of Atm (P) pushes Down on the Hg A Torricelli Barometer As Atm P changes the height of the supported Hg column changes What happens to the Hg column when atm pressure decreases? Supported Column of Hg