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Ch.18 - Chemistry of the Environment
Chapter 18, Problem 13b

Air pollution in the Mexico City metropolitan area is among the worst in the world. The concentration of ozone in Mexico City has been measured at 441 ppb (0.441 ppm). Mexico City sits at an altitude of 7400 feet, which means its atmospheric pressure is only 0.67 atm. (b) How many ozone molecules are in 1.0 L of air in Mexico City? Assume T = 25 °C.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Ideal Gas Law

The Ideal Gas Law relates the pressure, volume, temperature, and number of moles of a gas through the equation PV = nRT. This law is essential for calculating the number of gas molecules in a given volume under specific conditions of temperature and pressure. In this case, it will help determine how many ozone molecules are present in 1.0 L of air in Mexico City.
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Ideal Gas Law Formula

Molar Volume of a Gas

At standard temperature and pressure (STP), one mole of an ideal gas occupies 22.4 liters. However, since the conditions in Mexico City differ (lower pressure and a specific temperature), the molar volume will need to be adjusted accordingly. Understanding how to calculate the molar volume under non-STP conditions is crucial for determining the number of ozone molecules in the air.
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Concentration Units (ppb and ppm)

Parts per billion (ppb) and parts per million (ppm) are units used to express the concentration of a substance in a solution or mixture. In this context, the concentration of ozone is given in ppb, which must be converted to a more usable form (like moles or molecules) to find the total number of ozone molecules in 1.0 L of air. Understanding these units is vital for accurate calculations.
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