If a particular sample of air is 2.5 percent water vapor, what is its concentration in parts per million (ppm)?

To determine the concentration of water vapor in parts per million (ppm) from a sample of air that contains 2.5 percent water vapor, it's important to understand the relationship between percentage and parts per million.

A percentage is a way of expressing a number as a fraction of 100. Therefore, to convert a percentage to ppm, you can use the formula:

[ \text{ppm} = \text{percentage} \times 10,000 ]

This is because 1 percent is equivalent to 10,000 ppm.

In this case, 2.5 percent can be converted as follows:

[ 2.5 \text{ percent} \times 10,000 = 25,000 \text{ ppm} ]

Thus, the concentration of water vapor in the sample of air is accurately represented as 25,000 ppm, making this the correct answer.

Understanding how to convert between percentage and other concentration units like ppm is crucial in fields such as environmental science, chemistry, and air quality management, where precise measurements of gases in the atmosphere are often required.