Air consits of 21% oxygen, 78% nitrogen and 1% argon by volume. Calculate the partial pressures in Pa if total pressure is 1 atm.

My teacher just converted 1atm in pa and then did 21% of that for oxygen etc etc. Can you explain why she did this, is volume directly proportional to pressure? isnt it inversely proportional?

And for oxygen she just called it the oxygen mocule O2. But why? the question just said oxygen. Please someone shed some light.

Oxygen in the air is O2 like wise nitrogen is N2.

mole fraction = n (mole of 1 component) / N (total mole of all components)

Oxygen mole = 21 g /(32g/mol)=0.66 mole
Nitrogen mole = 78 g / (28g/mol) =2.78 mole
Argon mole = 1g / (40g/mol) =0.025

total mole =0.66 + 2.78 + 0.025 =3.465 mol

mole fraction oxygen = mole/total mole =0.19
mole fraction N =2.78/3.465 =0.8
mole fraction Ar =0.01

Partial pressure = mole fraction x pressure

oxygen partial pressure =0.19 x 1 atm = 0.19 atm. since 1 atm=101325 Pa; 0.19 atm =19251.75 Pa

and follow the same step for partial pressure of N2 and Ar. hope this helps.

since all the gases are in % (100 percent) you could easily convert those percentages to grams to find the moles of O2, N2, and Ar.

The atmosphere composition is % by volume, not by mass. If we take the gases as ideal gases then because the same volume of any gas contains the same number of moles the % by volume is also the % by mole, and hence fraction by mole.

0.21 mole
0.78 mole
0.1 mole

Thus because

Partial pressure = mole fraction x pressure

It is straightforward to calculate the partial pressure, so in atmospheres the partial pressure are

0.21 atm
0.78 atm
0.1 atm

which can be conveted to Pa by multiplying by 10^4

Oxygen exists in the atmosphere as O2, but you do not need to know this to answer the question.

To calculate the partial pressures of oxygen, nitrogen, and argon in the given air mixture, you need to understand the relationship between volume and pressure, as well as the concept of partial pressure.

1. Converting atmospheres (atm) to pascals (Pa):
Your teacher converted 1 atm to pascals (Pa) because atmospheric pressure is commonly measured in atm, while scientific calculations often use SI units such as pascals. The conversion factor from atm to Pa is 1 atm = 101,325 Pa.

2. Volume and pressure relationship:
The relationship between volume and pressure is defined by Boyle's law, which states that at a constant temperature, the pressure of a gas is inversely proportional to its volume. This means that as the volume of a gas decreases, its pressure increases, and vice versa.

However, in the context of partial pressures, the volume does not have a direct impact. Partial pressure represents the pressure exerted by a single gas in a mixture, assuming all other gases are removed. It is independent of the volume as long as the temperature remains constant.

So, even though volume and pressure have an inverse relationship, it is not directly relevant when calculating partial pressures.

3. Identifying oxygen as O2:
In the context of air composition, when we refer to oxygen, it is implied that it exists as diatomic oxygen, O2. Oxygen gas (O2) is the most common molecular form of oxygen in the Earth's atmosphere. It is often assumed that when we talk about the presence of oxygen, we are referring to O2 molecules.

Therefore, when calculating the partial pressure of oxygen in the air mixture, it is appropriate to consider it as O2.

Now, let's calculate the partial pressures:
Given:
Total pressure (P) = 1 atm
Oxygen percentage = 21%
Nitrogen percentage = 78%
Argon percentage = 1%

Step 1: Convert atmospheric pressure to pascals:
1 atm = 101,325 Pa

Step 2: Calculate partial pressures:
Partial pressure of oxygen (PO2) = 21% of 101,325 Pa
Partial pressure of nitrogen (PN2) = 78% of 101,325 Pa
Partial pressure of argon (PAr) = 1% of 101,325 Pa

By following these steps, you can determine the partial pressures of each gas in the given air mixture.