"If two or more then gases, chemically react with each
other, in the same space, then at a constant temperature, the pressure of the
mixture is equal to the addition of the pressures of each gas would have
separately as it is only in that space used to be. "

**P total = P.a + P.b + P.c + ... + Pn**

In This formula:

- P.total is the total pressure of the mixture, also expressed as Ptot or just P
- P.a or others are the partial pressure of gas "g", sometimes also expressed as PPG
- Through n are the various gases in the gas mixture

Here we take as an example.

We keep it simple; air consists of

**21%**oxygen and**79%**nitrogen, other gases we exclude.
Now we take three rigid, equal spaces and stop there the gas
mixture, the proportion of oxygen and nitrogen in the share.

According to the law of Dalton the pressure of the mixture
is equivalent to the sum of the pressures of the gases.

The pressure that would have one of the gases, as it would
only occupy the space of the mixtures, we call the Partial Pressure.

From this follows:

**Pg = Fg x P**

In which:

- Pg is partial pressure, expressed in bars
- FGIS the fraction that the gas occupies in the mixture, expressed as a decimal (21% = 0.21)
- P is the total pressure of the mixture, expressed in bar

Now we calculate the partial pressure of the oxygen in
atmospheric air:

**Fg x P = Pg**

**FO2 x P = PO2**

**0.21 x 1 = 0,21bar**

Now we calculate the oxygen partial pressure of air at 35m
depth:

**FO2 x P = PO2**

**0.21 x 4.5 = 0.945 bar**