Assuming that air contains 78 percent \(\mathrm{N}_{2}, 21\) percent \(\mathrm{O}_{2}\), and 1 percent Ar, all by volume, how many molecules of each type of gas are present in \(1.0 \mathrm{~L}\) of air at STP?

Understanding the concept of Standard Temperature and Pressure (STP) is crucial in chemistry, especially in gas-related calculations. STP refers to the condition of a gas at 0 degrees Celsius (273.15 Kelvin) and 1 atmosphere pressure. At STP, one mole of an ideal gas occupies 22.4 liters of volume. This is a standard reference that chemists use to compare various gas volumes and to perform calculations under controlled conditions, like finding out how many molecules are present in a given volume of air.

When working with the molecular composition of air, as in the example problem, knowing that the volume of 1 mole of gas at STP is 22.4 liters helps us calculate the number of moles in any volume of gas easily. This allows students and scientists alike to predict the behavior of gases under STP, making it a foundational concept in understanding gas laws and their applications.

Avogadro's number is another fundamental concept in chemistry and is directly related to the mole concept. It is defined as the number of constituent particles, usually atoms or molecules, that are contained in one mole of a substance. The value of Avogadro's number is approximately \(6.022 \times 10^{23}\) particles per mole. This constant enables us to convert between the number of moles and the number of particles.

In the provided example, we apply Avogadro’s number to convert moles of nitrogen, oxygen, and argon into the actual number of molecules present in a liter of air. Knowing Avogadro's number is essential for quantifying substances on the molecular level and is used widely in stoichiometry, which is the calculation of reactants and products in chemical reactions.

The molar volume of a gas is the volume occupied by one mole of the gas at specified conditions of temperature and pressure. The molar volume is especially useful for gases because it can be used to relate the gas's volume with the number of moles, regardless of the type of gas. At STP, the molar volume of any ideal gas is 22.4 liters. This is a critical point because it simplifies the calculation of volumes of gases involved in chemical reactions at STP.

In our exercise, by knowing the molar volume, we calculate that 1.0 liter of air at STP contains 0.0446 moles, and from there, we determine the number of moles for each individual gas in the mixture. It’s this property that allows us to use the same molar volume for nitrogen, oxygen, and argon when they are part of the air mixture.

Percent composition is the percentage by mass or volume of each element present in a compound or mixture. In the context of gas mixtures like air, percent composition is usually expressed in terms of volume. It allows chemists to determine the composition of a mixture and calculate the amounts of each component present.

In the air mixture example, the percent composition is given by volume: 78% nitrogen, 21% oxygen, and 1% argon. With these percentages, we can calculate the volume of each gas in a given sample of air. Knowing the percent composition is fundamental in analyzing the makeup of mixtures and helps in predicting the outcome of chemical reactions where various reactants are combined proportionally.