How much does the air in the room weigh? Density and specific volume of moist air Examples of problem solving.

What is the density of air at 150 degrees Celsius in kg/m3, g/cm3, g/ml, lb/m3 . Don't forget that this physical quantity, characteristic of air, as its density in kg/m3 (the mass of a unit volume of atmospheric gas, where 1 m3, 1 cubic meter, 1 cubic meter, 1 cubic centimeter, 1 cm3, 1 milliliter, 1 ml or 1 pound is taken as a unit volume), depends on several parameters. Among the parameters describing the conditions for determining the air density (specific gravity of air gas), I consider the following to be the most important and must be taken into account:

1. Temperature air gas.
2. Pressure at which the density of the air gas was measured.
3. Humidity air gas or the percentage of water in it.

If you are interested in the second case air density at T = 150 degrees C, then excuse me, but I have no desire to copy tabular data, a huge special reference book for air density at various pressures. I cannot yet decide on such a colossal amount of work, and I do not see the need for it. See reference book. Narrow profile information or rare special data, density values, should be sought in primary sources. So smarter.

It is more realistic, and probably more practical from our point of view, to indicate what is the density of air at 150 degrees Celsius, for a situation where the pressure is given by a constant and this is Atmosphere pressure (at normal conditions is the most popular question. By the way, do you remember what normal atmospheric pressure is? What does it equal? Let me remind you that normal atmospheric pressure is considered to be equal to 760 mm of mercury, or 101325 Pa (101 kPa), in principle, these are normal conditions adjusted for temperature. Meaning, what is the density of air in kg/m3 at a given temperature air gas you will see, find, learn in table 1.

However, it must be said that the values ​​indicated in the table air density values ​​at 150 degrees in kg/m3, g/cm3, g/ml, will not be true for any atmospheric, but only for dry gas. As soon as we change the initial conditions and change the humidity of the air gas, it will immediately have different physical properties. And its density (weight of 1 cubic meter of air in kilograms) at given temperature in degrees C (Celsius) (kg/m3) will also differ from the dry gas density.

This is the mass (weight) of 1 cubic meter of a mixture of air and water vapor at a certain temperature and relative humidity. Specific volume is the volume of air and water vapor per 1 kg of dry air.

Moisture and heat content

To determine the heat consumption for moisture, you need to know the value heat content of moist air. This value is understood as contained in the mixture of air and water vapor. It is numerically equal to the sum:

DEFINITION

atmospheric air is a mixture of many gases. Air has a complex composition. Its main components can be divided into three groups: constant, variable and random. The former include oxygen (the oxygen content in the air is about 21% by volume), nitrogen (about 86%) and the so-called inert gases (about 1%).

Content constituent parts virtually independent of where the globe a sample of dry air was taken. The second group includes carbon dioxide(0.02 - 0.04%) and water vapor (up to 3%). The content of random components depends on local conditions: near metallurgical plants, noticeable amounts of sulfur dioxide are often mixed into the air, in places where organic residues decay, ammonia, etc. In addition to various gases, air always contains more or less dust.

The density of air is the value equal to the mass gas of the Earth's atmosphere divided by a unit volume. It depends on pressure, temperature and humidity. There is a standard air density value - 1.225 kg / m 3, corresponding to the density of dry air at a temperature of 15 o C and a pressure of 101330 Pa.

Knowing from experience the mass of a liter of air under normal conditions (1.293 g), one can calculate the molecular weight that air would have if it were an individual gas. Since a gram-molecule of any gas occupies under normal conditions a volume of 22.4 liters, the average molecular weight of air is

22.4 × 1.293 = 29.

This number - 29 - should be remembered: knowing it, it is easy to calculate the density of any gas in relation to air.

Density of liquid air

With sufficient cooling, the air becomes liquid. Liquid air can be stored for quite a long time in vessels with double walls, from the space between which air is pumped out to reduce heat transfer. Similar vessels are used, for example, in thermoses.

Freely evaporating at normal conditions liquid air has a temperature of about (-190 o C). Its composition is unstable, since nitrogen evaporates easier than oxygen. As nitrogen is removed, the color of liquid air changes from bluish to pale blue (the color of liquid oxygen).

In liquid air, ethyl alcohol, diethyl ether and many gases easily turn into a solid state. If, for example, carbon dioxide is passed through liquid air, then it turns into white flakes, similar in appearance to the snow. Mercury immersed in liquid air becomes solid and malleable.

Many substances cooled by liquid air change their properties dramatically. Thus, chink and tin become so brittle that they easily turn into powder, a lead bell makes a clear ringing sound, and a frozen rubber ball shatters if dropped on the floor.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Air density is a physical quantity that characterizes the specific mass of air at vivo or the mass of gas in the Earth's atmosphere per unit volume. The value of air density is a function of the height of the measurements, its humidity and temperature.

A value equal to 1.29 kg/m3 is taken as the air density standard, which is calculated as the ratio of its molar mass(29 g / mol) to the molar volume, the same for all gases (22.413996 dm3), corresponding to the density of dry air at 0 ° C (273.15 ° K) and a pressure of 760 mm Hg (101325 Pa) at sea level ( i.e. under normal conditions).

Not so long ago, information on air density was obtained indirectly through observations of polar lights, propagation of radio waves, meteors. Since the advent artificial satellites Earth's air density began to be calculated thanks to the data obtained from their braking.

Another method is to observe the spreading of artificial clouds of sodium vapor created by meteorological rockets. In Europe, the air density at the Earth's surface is 1.258 kg/m3, at an altitude of five km - 0.735, at an altitude of twenty km - 0.087, at an altitude of forty km - 0.004 kg/m3.

There are two types of air density: mass and weight (specific gravity).

The weight density determines the weight of 1 m3 of air and is calculated by the formula γ = G/V, where γ is the weight density, kgf/m3; G is the weight of air, measured in kgf; V is the volume of air, measured in m3. Determined that 1 m3 of air at standard conditions (barometric pressure 760 mmHg, t=15°C) weighs 1.225 kgf, based on this, the weight density (specific gravity) of 1 m3 of air is equal to γ ​​= 1.225 kgf/m3.

It should be taken into account that the weight of air is a variable and varies according to various conditions such as geographical latitude and the force of inertia that occurs when the Earth rotates around its axis. At the poles, the weight of air is 5% more than at the equator.

The mass density of air is the mass of 1 m3 of air, denoted by the Greek letter ρ. As you know, body weight is a constant value. A unit of mass is considered to be the mass of a weight made of platinum iridide, which is located in the International Chamber of Weights and Measures in Paris.

Air mass density ρ is calculated using the following formula: ρ = m / v. Here m is the mass of air, measured in kg×s2/m; ρ is its mass density, measured in kgf×s2/m4.

The mass and weight density of air are dependent: ρ = γ / g, where g is the acceleration coefficient free fall, equal to 9.8 m / s². Whence it follows that the mass density of air under standard conditions is 0.1250 kg×s2/m4.

As barometric pressure and temperature change, air density changes. Based on the Boyle-Mariotte law, the greater the pressure, the greater will be the density of the air. However, as the pressure decreases with height, the air density also decreases, which introduces its own adjustments, as a result of which the law of vertical pressure change becomes more complicated.

The equation that expresses this law of change in pressure with height in an atmosphere at rest is called basic equation of statics.

It says that with increasing altitude, the pressure changes to a smaller side and when ascending to the same height, the decrease in pressure is the greater, the more more strength gravity and air density.

An important role in this equation belongs to changes in air density. As a result, we can say that the higher you climb, the less pressure will drop when you rise to the same height. The density of air depends on temperature as follows: in warm air, the pressure decreases less intensively than in cold air, therefore, by the same equal height in warm air mass the pressure is higher than in the cold.

With changing values ​​of temperature and pressure, the mass density of air is calculated by the formula: ρ = 0.0473xV / T. Here B is the barometric pressure, measured in mm of mercury, T is the air temperature, measured in Kelvin.

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Density is also determined by air humidity. The presence of water pores leads to a decrease in air density, which is explained by the low molar mass of water (18 g/mol) against the background of the molar mass of dry air (29 g/mol). Humid air can be considered as a mixture of ideal gases, in each of which the combination of densities allows one to obtain the required density value for their mixture.

Such a kind of interpretation allows density values ​​to be determined with an error level of less than 0.2% in the temperature range from −10 °C to 50 °C. The density of air allows you to get the value of its moisture content, which is calculated by dividing the density of water vapor (in grams), which is contained in the air, by the density of dry air in kilograms.

The basic equation of statics does not allow solving constantly emerging practical problems in real conditions of a changing atmosphere. Therefore, it is solved under various simplified assumptions that correspond to the actual real conditions, by putting forward a number of particular assumptions.

The basic equation of statics makes it possible to obtain the value of the vertical pressure gradient, which expresses the change in pressure during ascent or descent per unit height, i.e., the change in pressure per unit vertical distance.

Instead of the vertical gradient, the reciprocal of it is often used - the baric step in meters per millibar (sometimes there is still an outdated version of the term "pressure gradient" - the barometric gradient).

The low air density determines a slight resistance to movement. Many terrestrial animals, in the course of evolution, used the ecological benefits of this property of the air environment, due to which they acquired the ability to fly. 75% of all land animal species are capable of active flight. For the most part, these are insects and birds, but there are mammals and reptiles.

Video on the topic "Determination of air density"

03.05.2017 14:04 1392

How much does air weigh.

Despite the fact that we cannot see some things that exist in nature, this does not mean at all that they do not exist. It is the same with air - it is invisible, but we breathe it, we feel it, so it is there.

Everything that exists has its own weight. Does the air have it? And if so, how much does air weigh? Let's find out.

When we weigh something (for example, an apple, holding it by a twig), we do it in the air. Therefore, we do not take into account the air itself, since the weight of air in air is zero.

For example, if we take an empty glass bottle and weigh it, we will consider the result obtained as the weight of the flask, without thinking that it is filled with air. However, if we tightly close the bottle and pump out all the air from it, we will get a completely different result. That's it.

Air consists of a combination of several gases: oxygen, nitrogen and others. Gases are very light substances, but they still have weight, although not much.

In order to make sure that the air has weight, ask an adult to help you carry out the following simple experiment: Take a stick about 60 cm long and tie a rope in the middle of it.

Next, to both ends of our stick we will attach 2 inflated ones of the same size balloons. And now we will hang our structure by a rope tied to its middle. As a result, we will see that it hangs horizontally.

If we now take a needle and pierce one of the inflated balloons with it, air will come out of it, and the end of the stick to which it was tied will rise up. And if we pierce the second ball, then the ends of the stick will be equal and it will again hang horizontally.

What does it mean? And the fact that the air in the inflated balloon is denser (that is, heavier) than the one that is around it. Therefore, when the ball was blown away, it became lighter.

The weight of the air depends on various factors. For example, air above a horizontal plane is atmospheric pressure.

Air, as well as all objects that surround us, is subject to gravity. It is this that gives the air its weight, which is equal to 1 kilogram per square centimeter. In this case, the air density is about 1.2 kg / m3, that is, a cube with a side of 1 m, filled with air, weighs 1.2 kg.

An air column rising vertically above the Earth stretches for several hundred kilometers. This means that straight standing man, on his head and shoulders (the area of ​​\u200b\u200bwhich is approximately 250 square centimeters, a column of air weighing about 250 kg presses!

If such a huge weight were not opposed by the same pressure inside our body, we would simply not be able to withstand it and it would crush us. There is another interesting experience, which will help to understand everything that we said above:

We take a sheet of paper and stretch it with both hands. Then we will ask someone (for example, a younger sister) to press on it with a finger from one side. What happened? Of course, there was a hole in the paper.

And now we will do the same thing again, only now it will be necessary to press on the same place with two index fingers, but from different sides. Voila! The paper is intact! Do you want to know why?

Just pressure us sheet of paper on both sides was the same. The same thing happens with the pressure of the air column and the counter pressure inside our body: they are equal.

Thus, we found out that: air has weight and presses it on our body from all sides. However, it cannot crush us, since the counter pressure of our body is equal to the external one, that is, atmospheric pressure.

Our last experiment showed this clearly: if you press on a sheet of paper from one side, it will tear. But if you do it on both sides, this will not happen.