If we know the values of any three of these quantities, we can calculate the fourth and thereby obtain a full physical description of the gas. Temperature, volume, and amount have been discussed in previous chapters.
We now discuss pressure and its units of measurement. Any object, whether it is your computer, a person, or a sample of gas, exerts a force on any surface with which it comes in contact. The air in a balloon, for example, exerts a force against the interior surface of the balloon, and a liquid injected into a mold exerts a force against the interior surface of the mold, just as a chair exerts a force against the floor because of its mass and the effects of gravity.
Pressure is dependent on both the force exerted and the size of the area to which the force is applied. When we use a hose to wash a car, for example, we can increase the pressure of the water by reducing the size of the opening of the hose with a thumb.
The units of pressure are derived from the units used to measure force and area. Assuming a paperback book has a mass of 2. Given: mass and dimensions of object. The force exerted by the book does not depend on its orientation. In SI units, the force exerted by the book is therefore. A We calculated the force as When the book is lying flat, the area is. B The pressure exerted by the text lying flat is thus. A If the book is standing on its end, the force remains the same, but the area decreases:.
Just as we exert pressure on a surface because of gravity, so does our atmosphere. We live at the bottom of an ocean of gases that becomes progressively less dense with increasing altitude. The pressure exerted by the atmosphere is considerable: a 1 m 2 column, measured from sea level to the top of the atmosphere, has a mass of about 10, kg, which gives a pressure of about kPa:. A barometer may be constructed from a long glass tube that is closed at one end. It is filled with mercury and placed upside down in a dish of mercury without allowing any air to enter the tube.
Gravity is certainly exerting a downward force on the mercury in the tube, but it is opposed by the pressure of the atmosphere pushing down on the surface of the mercury in the dish, which has the net effect of pushing the mercury up into the tube. Because there is no air above the mercury inside the tube in a properly filled barometer it contains a vacuum , there is no pressure pushing down on the column.
Thus the mercury runs out of the tube until the pressure exerted by the mercury column itself exactly balances the pressure of the atmosphere. The pressure exerted by the mercury column can be expressed as:.
This value varies with meteorological conditions and altitude. In Denver, Colorado, for example, at an elevation of about 1 mile, or m ft , the height of the mercury column is mm rather than mm.
Mercury barometers have been used to measure barometric pressure for so long that they have their own unit for pressure: the millimeter of mercury mmHg , often called the torr, after Torricelli. Standard barometric pressure is the barometric pressure required to support a column of mercury exactly mm tall; this pressure is also referred to as 1 atmosphere atm. These units are also related to the pascal:. We are so accustomed to living under this pressure that we never notice it.
What tool is used to measure gas pressure? What is Boyle's law? What is an example of a gas pressure practice problem? What are some common mistakes students make when measuring gas pressure? What is an example of a Boyle's law practice problem? How do Boyle's law and Charles' law differ? How can I measure gas pressure?
How does Boyle's law affect the human body? How does Boyle's law affect hot air balloons? The height of the liquid in the tube is therefore proportional to the pressure exerted by the atmosphere. Figure 3. In a barometer, the height, h , of the column of liquid is used as a measurement of the air pressure.
Using very dense liquid mercury left permits the construction of reasonably sized barometers, whereas using water right would require a barometer more than 30 feet tall.
If the liquid is water, normal atmospheric pressure will support a column of water over 10 meters high, which is rather inconvenient for making and reading a barometer. Because mercury Hg is about Standard atmospheric pressure of 1 atm at sea level , Pa corresponds to a column of mercury that is about mm The torr was originally intended to be a unit equal to one millimeter of mercury, but it no longer corresponds exactly.
The pressure exerted by a fluid due to gravity is known as hydrostatic pressure , p :. Calculation of Barometric Pressure Show the calculation supporting the claim that atmospheric pressure near sea level corresponds to the pressure exerted by a column of mercury that is about mm high. Plugging these values into the equation and doing the necessary unit conversions will give us the value we seek.
The density of water at this temperature is 1. A manometer is a device similar to a barometer that can be used to measure the pressure of a gas trapped in a container. A closed-end manometer is a U-shaped tube with one closed arm, one arm that connects to the gas to be measured, and a nonvolatile liquid usually mercury in between.
As with a barometer, the distance between the liquid levels in the two arms of the tube h in the diagram is proportional to the pressure of the gas in the container. An open-end manometer Figure 4 is the same as a closed-end manometer, but one of its arms is open to the atmosphere. In this case, the distance between the liquid levels corresponds to the difference in pressure between the gas in the container and the atmosphere.
Calculation of Pressure Using a Closed-End Manometer The pressure of a sample of gas is measured with a closed-end manometer, as shown to the right. The liquid in the manometer is mercury. Determine the pressure of the gas in:. Solution The pressure of the gas is equal to a column of mercury of height The pressure at the bottom horizontal line is equal on both sides of the tube.
The pressure on the left is due to the gas and the pressure on the right is due to Check Your Learning The pressure of a sample of gas is measured with a closed-end manometer. Calculation of Pressure Using an Open-End Manometer The pressure of a sample of gas is measured at sea level with an open-end Hg mercury manometer, as shown to the right. Solution The pressure of the gas equals the hydrostatic pressure due to a column of mercury of height Check Your Learning The pressure of a sample of gas is measured at sea level with an open-end Hg manometer, as shown to the right.
It consists of an inflatable cuff to restrict blood flow, a manometer to measure the pressure, and a method of determining when blood flow begins and when it becomes impeded Figure 5. Since its invention in , it has been an essential medical device.
There are many types of sphygmomanometers: manual ones that require a stethoscope and are used by medical professionals; mercury ones, used when the most accuracy is required; less accurate mechanical ones; and digital ones that can be used with little training but that have limitations.
When using a sphygmomanometer, the cuff is placed around the upper arm and inflated until blood flow is completely blocked, then slowly released. As the heart beats, blood forced through the arteries causes a rise in pressure. This rise in pressure at which blood flow begins is the systolic pressure— the peak pressure in the cardiac cycle. As cuff pressure continues to decrease, eventually sound is no longer heard; this is the diastolic pressure— the lowest pressure resting phase in the cardiac cycle.
Blood pressure units from a sphygmomanometer are in terms of millimeters of mercury mm Hg. Throughout the ages, people have observed clouds, winds, and precipitation, trying to discern patterns and make predictions: when it is best to plant and harvest; whether it is safe to set out on a sea voyage; and much more.
We now face complex weather and atmosphere-related challenges that will have a major impact on our civilization and the ecosystem. Several different scientific disciplines use chemical principles to help us better understand weather, the atmosphere, and climate. These are meteorology, climatology, and atmospheric science. Meteorologists seek to understand and predict the weather in the short term, which can save lives and benefit the economy.
Weather forecasts Figure 6 are the result of thousands of measurements of air pressure, temperature, and the like, which are compiled, modeled, and analyzed in weather centers worldwide. Figure 6. Meteorologists use weather maps to describe and predict weather.
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