Measuring Pressure

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Measuring Pressure
Units and the Barometer
Mr. Shields Regents Chemistry U05 L02
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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/in2 1 atm 760 mm Hg 1 atm
29.9 in Hg 1 torr 1 mm Hg
English system ex. tire pressure
Know each of these conversions!
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Pressure Unit Problems
1) How many Pascals in 2 atm? 2 atm 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
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Pressure measurement
OK. We now know what units are used in measuring
Pressure So how do we actually MEASURE
atmospheric pressure?
The instrument is known as a BAROMETER
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The typical household Barometer
Used to forecast A change in weather
So how does a barometer Do this?
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Pressure measurement
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
0 deg C and 1 atm
273K and 101.3 KPa
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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
in Hg 140 mph Andrew C5 1993 27.23 in Hg
165 mph
Why are these units expressed in inches of
Mercury ?
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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.
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A Torricelli Barometer
Vacuum
760 mm Hg 0r 30 Hg
Wt of Atm (P) pushes Down on the Hg
Supported Column of Hg
Mercury resevoir
As Atm P changes the height of the supported Hg
column changes What happens to the Hg column
when atm pressure decreases?