How much does air weigh? At first glance, this is a completely meaningless question, because air cannot be touched, it, as we used to think, does not put pressure on us, it is completely invisible, not felt, and in general, how can 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, it comes from somewhere.
The question of how much air weighs is far from being as absurd as it seems at first glance. It is not at all meaningless when it comes to the weight of air in the context of chemical reactions and calculations. Usually chemists operate with the 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? How to measure it? And what if you need to calculate the weight of a very large volume of air?
What is molar mass?
Molar mass is the ratio (division sign) of the mass of a substance to the number of moles of that substance. In other words, the molar mass of a substance is the mass of one mole of thissubstances.
The generally accepted form of molar mass in chemical formulas is the capital letter "M". That is, if you need to write the phrase "the molar mass of a substance is equal" as a formula, it will look like this: "M=…"
Usually, the subscript indicates the molar mass of which substance is meant. For a complex substance, such as air, which does not have a specific short form, it can also be indicated in brackets. Then the molar mass of air can be denoted as Mair or as M (air). Still, the option of writing with a lower index is more preferable.
How is molar mass measured?
The unit of molar mass in the International System of Units (SI) is the kilogram per mole. In abbreviated form in the Russian version, this will look like "kg / mol", and the accepted international abbreviation is written as kg / mol. Historically, however, molar masses have been measured in grams per mole, since, as a rule, we are talking about very small quantities and quantities of a substance, 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 a substance. But what is this mole? How to calculate it? Who and when decided to calculate the mass in moles?
Mol, according to the Resolution of the XIV General Conference on Weights and Measures, adopted in 1971, and the Regulation on Units of Values Allowed for Use in the Russian Federation, is defined as the numbersubstances of a system containing as many structural elements as there are atoms in carbon-12 weighing 0.012 kg. The building blocks 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 air weigh? Chemists can give an exact answer to this question. The average molar mass of air is 28.98 grams per mole. For ease of calculation for educational purposes, this number is usually rounded up to 29 grams per mole. This is written when solving chemical equations as 28.98 g / mol or as 29 g / mol. The molar mass of air for typical 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 that make up it. That is, to find it, it is necessary to find the arithmetic weighted average of the molar mass of the mass fractions that are part of the individual substances that form air.
For convenienceCalculations by 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 28.98 grams per mole is obtained by finding the arithmetic weighted average using these data.
Will a mole of air always weigh exactly that much?
Because air is a mixture of gases, it is an unstable compound in which the exact proportions of substances can change depending on various conditions.
So, for example, the content of carbon dioxide in the air is higher in large cities than in rural areas, or even more so in forests, where it is consumed by trees, on the contrary, bringing a greater percentage of oxygen into its composition. In general, the composition of the air in the urban environment is also greatly changed due to exhaust gases, the operation of factories and enterprises, the uneven concentration of green areas and areas rolled into concrete and cement, as well as industrial and recreational areas.
Another manifestation of differences in the composition of the air in different places is well known to climbers. It is due to the fact that oxygen molecules have a large mass, and therefore, with height, its concentration in the air decreases. Consequently, in the highlands, oxygen in the air is much less than in the plains or lowlands. At the same time, the concentration of nitrogen in the air becomes higher with height due to a decrease in the concentration of oxygen, since this gas has a lower molar mass than the molar mass of oxygen. That's whyconquerors of mountain peaks have to carry oxygen cylinders on themselves, and a person who first gets into a mountainous area can 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 areas.
How can you find a mass of more air?
Knowing the molar mass of air, you can calculate how much a larger volume of air weighs. To do this, you need to know the amount of air.
The mass of air is calculated by multiplying the amount of air by its molar mass. If you write this statement as a formula, the calculation scheme will look like this: m=V × M. In this formula, m denotes the mass of air, V is the amount of air in moles, and M is the molar mass of air.
