# Question #e9de5

##### 1 Answer

Here's what I got.

#### Explanation:

You can try to find a solution by using an *intuitive* approach.

As you know, air pressure is essentially the **force** with which molecules of air push on a given surface area. For example, the pressure at *sea level* is equal to

#P_"sea level" = "101,325 Pa"#

A *pascal*, which is the SI unit for pressure, is defined as a force of

This means that if you draw a

Now, this force is actually the **weight of the molecules** present in the column. This implies that if you have **fewer molecules of air** in the column, the pressure will be **lower**.

This is why atmospheric pressure **decreases** when altitude **increases** -- fewer and fewer molecules of air are present as you move away from sea level, i.e. the air is *less dense*.

*So, what would you expect to see as you go at altitudes that are below sea level?*

The number of molecules of air present in the column would **increase**, i.e. the air will become *more dense*. This implies that the **total pressure** of the column of air will **increase**.

Consequently, the **partial pressure** of oxygen would be *higher*, if you assume that oxygen's * mole fraction*,

You can use **Dalton's Law of Partial Pressures** to write

#P_ ("O"_ 2)^' = chi_ ("O"_ 2) xx P_"sea level" -># the partial pressure of oxygen at sea level

#P_ ("O"_ 2)^('') = chi_ ("O"_ 2) xx P_"below sea" -># the partial pressure of oxygen below sea level

Divide these equations to get rid of the mole fraction

#P_( "O"_ 2)^'/P_ ("O"_ 2)^('') = color(red)(cancel(color(black)(chi_ ("O"_ 2))))/color(red)(cancel(color(black)(chi_ ("O"_ 2)))) * P_"sea level"/P_"below sea"#

This will get you

#P_ ("O"_ 2)^('') = P_"below sea"/P_"sea level" * P_("O"_ 2)^'#

You know that the pressure below sea level is **higher** than the pressure at sea level, which means that

#P_"below sea"/P_"sea level" > 1#

and so

#P_( "O"_ 2)^('') > P_("O"_2)^'#

The partial pressure of oxygen is **higher** at altitudes that are below sea level than it is at sea level.