What is the partial pressure of nitrogen in air at 99 feet of salt water?

To determine the partial pressure of nitrogen in air at a depth of 99 feet of salt water, it's essential to consider the behavior of gases under pressure and how total pressure changes with depth.

At sea level, the total atmospheric pressure is approximately 1 atmosphere (atm). For every 33 feet of seawater, an additional atmosphere of pressure is added. Therefore, at a depth of 99 feet, the total pressure would be calculated as follows:

1 atm (surface) + (99 feet / 33 feet per atm) = 1 atm + 3 atm = 4 atm total pressure.

Air is composed of approximately 78% nitrogen. To find the partial pressure of nitrogen, you multiply the total pressure by the fraction of nitrogen in air:

Partial pressure of nitrogen = Total pressure × Fraction of nitrogen

= 4 atm × 0.78 ≈ 3.12 atm.

When expressed in terms of partial pressures in atmospheres (ppN2), it's customary to round this value to an appropriate significant figure, leading to approximately 3.16 ppN2.

This calculation clarifies why the correct answer is 3.16 ppN2, which reflects the increased pressure experienced at depth due to the weight of the