Pressure Units

Understanding Pressure

Concept: Pressure vs. Force 

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Video Transcript

Welcome back guys. In this new video we're going to take a look at pressure, exactly what is pressure and what causes pressure. So, we're going to say that pressure is just defined as the force exerted per unit of a surface area, on the right side we have a container, inside of this container is we have these black balls, we're going to say these black balls represent individual gas particles and we're going to say that from these black balls we have arrows pointing in different directions, those arrows are basically showing us the direction that the gas particles are taking, gases move in a linear fashion, they move in a straight line and so they're going to be moving in this container bouncing everywhere, bouncing off one another and bouncing off the walls of the container, the pressure that the container has the total pressure inside is the result of each of these gas particles hitting the walls of the container, that's what it is, pressure equals the force the impact that these gas particles are doing on the walls inside of the container, that's all pressure is. So, we're going to say pressure is the force of collisions, the force of each one of these gas particles, hitting the walls of the container adding them up and dividing it by the total area inside of the container, that's what pressure really is, and we're going to say here that the SI unit for pressure, when we talk about pressure, is the Pascal, and we're going to say the units for the Pascal are newtons over meters squared. Now, we're going to say the SI unit of force is the Newton. So, this capital n that we talked about just a second ago, and we're going to say the units for the Newton are kilograms times meters over seconds squared. So, just remember those two SI units, that can easily be a theory question as your professor asks you.

Concept: The different units for Pressure

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Video Transcript

Despite the fact that the SI unit for pressure is Pascal, chemist all around the world continue to use different units for pressure, because they're just accustomed to it. The Pascal is the SI unit for pressure, but most of the time, you'll see in your books and in different types of research journals from chemists and scientists, they instead like to use atmospheres, torrs or millimeters of mercury. Sometimes you might see bars as well. The Pascal is the SI unit but they tend to like to use these other types of units for pressure.
Now, here we have the different names for pressure. All of these are just different types of pressure, basically different units for it. They're all saying the same thing. An atmosphere is a unit of pressure just like a torr is, just like a bar is, just like kilopascal is. It's going to be important that you know how to convert from one to the other.
It's a long list, but the ones that professor usually focus on the most are: atmospheres, millimeters of mercury, torrs and bars. Those are usually the ones that we see the most, and, of course, Pascals.
Just realize all of them are equal to one another, so we can say that 760 torrs are equal to 1.01325 bars. You could say that one atmosphere is equal to this many Pascals. All these units are in agreement with one another. They all equal one another. We're going to say that one atmosphere equals 760 millimeters of mercury or 760 torrs.
What I also want you to see is that just look at these two right here, the millimeters of mercury and the torrs, they both have the same number, so 760 millimeters of mercury equals 760 torrs. What this really means is that for every one millimeter -- what this means is for every one millimeter of mercury, we have one torr. They're basically the same units.
Just remember that what the SI units for pressure are, what are the SI units for force, what is pressure defined as and then the connections between the different units for pressure because when it comes to gases, we have to remember this because pressure plays a very large role in the property that a gas has. 

Although the SI unit for pressure is the Pascal, most professors use atm, mmHg or torrs as the everyday units for pressure. 

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Example: A geochemist heats a limestone (CaCO3) sample and collects the CO2 released in an evacuated flask. The CO2 pressure is 283.7 mmHg. Calculate the CO2 pressure in torrs and atmospheres.

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The conversion of units of pressure is simply a dimensional analysis question. Recall the dimensional analysis relationships been volume and length.  

Problem: If the barometer in a laboratory reads 34.2 inHg what is the pressure in bars? (1 in = 2.54 cm)

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Pressure Units Additional Practice Problems

An ordinary gasoline can measuring 30.0 cm by 15.0 cm by 10.0 cm is evacuated with a vacuum pump. 

Assuming that virtually all of the air can be removed from inside the can, and that atmospheric pressure is 14.7 psi, what is the total force (in pounds) on the surface of the can?

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The atmospheric pressure in New York City on a certain day was 768 mm Hg. What was the pressure in kPa?

a. 1.02 x 105 kPa 

b. 584 kPa 

c. 5.76 x 10-3 kPa 

d. 102 kPa 

e. none of the above

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High-pressure liquid chromatography (HPLC) is a method used in chemistry and biochemistry to purify chemical substances. The pressures used in this procedure range from around 500 kilopascals (500,000 Pa) to about 60,000 kPa (60,000,000 Pa). It is often convenient to know the pressure in torr. If an HPLC procedure is running at a pressure of 2.90 x 108 Pa, what is its running pressure in torr? 

Express the pressure numerically in torr.

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The pressure in car tires is often measured in pounds per square inch (lb/in.2), with the recommended pressure being in the range of 25 to 45 lb/in.2. Suppose a tire has a pressure of 37.5 lb/in.2. Convert 37.5 lb/in.2 to its equivalent in atmospheres. 

Express the pressure numerically in atmospheres.

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The atmospheric pressure on the surface of Venus is 9120. kPa. Calculate the atmospheric pressure in atm. Be sure your answer has the right number of significant digits.

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3.90 x 105 gallons of tar (SG = 1.20) is stored in a 23.0-ft tall storage tank. 

What is the total mass of the liquid in the tank? 

What is the pressure at the bottom of the tank? 

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The density of mercury is 13.5 g/mL and the density of water is 1.00 g/mL. 

lf a mercury barometer reads 727 mmHg, what is the barometric pressure in cm H 2O?

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The pressure 70.0 meters under water is 788 kPa.

What is this pressure in atm?

What is this pressure in mmHg? 

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The atmospheric pressure at the summit of Mt. McKinley is 606 mmHg on a certain day. What is the pressure in atm and in kPa?

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Convert 562 mmHg to atm.

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Why do astronauts have to wear protective suits when they are on the surface of the moon?

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Why is it that if the barometer reading falls in one part of the world, it must rise somewhere else?

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If the maximum distance that water may be brought up a well by a suction pump is 34 ft (10.3 m), how is it  possible to obtain water and oil from hundreds of feet below the surface of Earth?

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Explain how a unit of length (mmHg) can be used as a unit for pressure.

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Explain why the height of mercury in a barometer is independent of the cross-sectional area of the tube. Would the barometer still work if the tubing were tilted at an angle, say 15° ?

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Why is mercury a more suitable substance to use in a barometer than water?

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When you are in a plane flying at high altitudes, your ears often experience pain. This discomfort can be temporarily relieved by yawning or swallowing some water. Explain.

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Define pressure and give the common units for pressure.

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The pressure on top of Mt. Everest averages about 235 mmHg.

1.) Convert this pressure to torr.

2.) Convert this pressure to psi.

3.) Convert this pressure to inHg.

4.) Convert this pressure to atm.

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Which of the following indicates the greatest pressure?

a. 1 atm

b. 777 torr

c. 5.5 mm Hg

d. 1000 Pa

e. 12 psi

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On a flight to Europe, you notice that the packages of peanuts are puffed up. Why?

1. The peanuts have greater velocities at high altitudes, thus causing the peanuts to exert more force on the packaging.

2. The jet is over-pressurized at high altitudes, thus causing the gas inside the packages to expand.

3. The air in the jet is approximately ten times hotter than on the ground, thus increasing the temperature of the gas in the packages.

4. The packages are puffed up on the ground due to overinflation with helium. This type of packaging provides extra buoyancy for aircrafts.

5. They were packaged at a higher pressure on the ground, thus causing the gas inside the packages to expand in the sky where the jet is at a lower pressure.

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Which of the following is  NOT a unit for pressure?

A) Pa        

B) mmHg       

C) atm            

D) torr            

E) liter

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Convert 1.50 atm to torr.

A) 760 torr

B) 875 torr

C) 1140 torr

D) 1000 torr

E) 1520 torr

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The atmospheric pressure is 700 mm Hg. What is the pressure in inches of Hg?

A) 16.0 in Hg

B) 0.921 in Hg

C) 13.5 in Hg

D) 27.6 in Hg

E) 32.5 in Hg

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Convert 1.50 atm to torr.

A) 760 torr

B) 875 torr

C) 1140 torr

D) 1000 torr

E) 1520 torr

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The air pressure in a volleyball is 75 psi. What is this pressure in torr ( 1 psi = 14.7 atm, 1 atm = 101,325 Pa = 760 torr)?

a. 75,000 torr

b. 393,300 torr

c. 76,500 torr

d. 837,900 torr

e. 562 torr

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A sample of N2O gas has a density of 2.85 g/L at 298 K. What is the pressure of the gas (in mmHg)? 

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The world record for lowest pressure (at sea level) was 652.5 mmHg recorded inside Typhoon Tip on October 12,1979, in the Western Pacifie Ocean. Convert this pressure to 

d. psi

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The world record for lowest pressure (at sea level) was 652.5 mmHg recorded inside Typhoon Tip on October 12,1979, in the Western Pacifie Ocean. Convert this pressure to 

c. in Hg

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The world record for lowest pressure (at sea level) was 652.5 mmHg recorded inside Typhoon Tip on October 12,1979, in the Western Pacifie Ocean. Convert this pressure to 

b. atm

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The pressure on top of Mt. Everest averages about 235 mmHg. Convert this pressure to

d. atm

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The pressure on top of Mt. Everest averages about 235 mmHg. Convert this pressure to

c. in Hg

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The pressure on top of Mt. Everest averages about 235 mmHg. Convert this pressure to

b. psi

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The pressure in Denver. Colorado (elevation 5280 ft). averages about 24.9 in Hg. Convert this pressure to

d. Pa

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The pressure in Denver. Colorado (elevation 5280 ft). averages about 24.9 in Hg. Convert this pressure to

c. psi

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The pressure in Denver. Colorado (elevation 5280 ft). averages about 24.9 in Hg. Convert this pressure to

b. mmHg

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The world record for lowest pressure (at sea level) was 652.5 mmHg recorded inside Typhoon Tip on October 12,1979, in the Western Pacifie Ocean. Convert this pressure to 

a. torr

Watch Solution

The pressure on top of Mt. Everest averages about 235 mmHg. Convert this pressure to

a. torr

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The pressure in Denver. Colorado (elevation 5280 ft). averages about 24.9 in mmHg. Convert this pressure to

a. atm

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