How much does air weigh? At first glance, this is a completely pointless question, because the air cannot be touched, it, as we used to think, does not press on us, it is completely invisible, not felt, and in general, can the air weigh something? They even sometimes say about thin and bony people: "Yes, he (or she) is lighter than air!". How much air weighs seems to be a completely absurd question. And yet, from somewhere it is taken.
The question of how much air weighs is not as absurd as it seems at first glance. It is not without meaning at all when it comes to the weight of air in the context of chemical reactions and calculations. Usually chemists operate with a molar mass of air.
What is it and how did scientists manage to weigh the air? Did they use any special weights? And what is the mass of air equal to? How to measure it? And what if you need to calculate the weight of a very large volume of air?
What is the molar mass?
Molar mass is the ratio (sign of division) of the mass of a substance to the number of moles of this substance. In other words, the molar mass of a substance is the mass of one mole of this substance.
The common form for recording molar mass in chemical formulas is the capital letter "M". That is, if you need to write down the formula the phrase "the molar mass of the substance is", it will look like this: "M = ..."
Usually, the lower index indicates the molar mass of which substance is meant. For a complex substance, such as air, which does not have a specific short form of writing, it can also be indicated in parentheses. Then the molar mass of air can be designated as M air or as M (air). Still preferable is the option of writing with a subscript.
What is the molar mass measured?
The unit of measurement of molar mass in the International System of Units (SI) is kilogram per mole. In abbreviated form in the Russian-language version, it will look like "kg / mol", and the accepted international abbreviation is written as kg / mol. Historically, however, it turned out that the molar mass is measured in grams per mole, since, as a rule, we are talking about very small quantities and quantities of matter, which means that kilograms here would only complicate the calculations without performing any other function.
What is a mole?
As mentioned above, the molar mass expresses the weight of one mole of substance. But what is this mole? How to count it? Who and when decided to count the mass in moles?
Moth, according to the Resolution of the XIV General Conference on Weights and Measures, adopted in 1971, and the Regulation on Units Allowed for Use in the Russian Federation, is defined as the amount of substance in a system containing as many structural elements as there are atoms in carbon-12 mass 0.012 kg The structural elements can be atoms, molecules, ions, electrons or other particles and specified groups of particles.
The name of this quantity comes from the Latin moles, meaning "quantity, mass, countable set."
What is the molar mass of air?
So how much does the air weigh? Chemists can give an exact answer to this question. The average molar mass of air is 28.98 grams per mole. For simplicity, calculations for educational purposes, this number is usually rounded to 29 grams per mole. This is written when solving the chemical equations as 28.98 g / mol or as 29 g / mol. The molar mass of air for standard calculations that do not require high accuracy is unchanged.
How did you manage to weigh a mole of air?
Air is a mixture of various gases. It mainly consists of nitrogen and oxygen. Together, their share in the air is more than 98 percent. In addition to them, the air contains hydrogen, carbon dioxide, argon and very tiny impurities of other gases that form the Earth’s atmosphere, as well as the smallest particles of water vapor.
The molar mass of air is calculated as the molar mass of a mixture of several substances in its composition. That is, to find it, it is necessary to find the arithmetic mean weighted molar mass of mass fractions that are part of the individual substances that make up the air.
For the convenience of calculations, chemists take some ideal values of the molar masses of the gases that make up the air, as well as some absolute fractions of these gases in the air. The number of 28.98 grams per mole is obtained by finding the arithmetic mean weighted using these data.
Will a mole of air always weigh exactly that much?
Since air is a mixture of gases, it is an unstable compound in which the exact proportions of substances can vary depending on various conditions.
For example, the carbon dioxide content in the air is higher in large cities than in rural areas, and even more so in the forests where it is consumed by trees, on the contrary, introducing a larger percentage of oxygen into its composition. In general, the composition of air in an urban environment is also very much changed due to exhaust gases, the operation of factories and enterprises, the uneven concentration of green and rolled into concrete and cement plots, as well as industrial and recreational areas.
Another manifestation of differences in the composition of air in different places is well known to climbers. It is connected with the fact that oxygen molecules have a large mass, and therefore its concentration in air decreases with height. Consequently, in high mountain regions, oxygen in the air contains much less than on the plain or in the lowland. The nitrogen concentration in the air in this case becomes higher with a height due to a decrease in the oxygen concentration, since this gas has a lower molar mass than the molar mass of oxygen. That is why conquerors of mountain peaks have to carry oxygen tanks on themselves, and a person who first gets into a mountainous area may feel dizzy.
Also affects the concentration of gases in the air water vapor. The proportion of its content in the air depends on humidity, temperature, climate, season, and some other conditions. Its share is usually not very significant, but it can reach several percent in some places.
How can I find a mass of more air?
Knowing the molar mass of air, we can calculate how much a larger volume of air weighs. To do this, you need to know the amount of air.
Air mass is calculated by multiplying the amount of air by its molar mass. If this statement is written in the form of a formula, the calculation scheme will look as follows: m = V × M. In this formula, m stands for air mass, V is the amount of air in moles, and M is the molar mass of air.