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Dalton's law expresses the fact that the total pressure of a mixture of gases is equal to the sum
of the partial pressures of the individual gases in the mixture. This equality arises from the fact
that in an ideal gas the molecules are so far apart that they do not interact with each other.
Most actual real-world gases come very close to this ideal.
See this Wikipedia article
https://en.wikipedia.org/wiki/Partial_pressure#:~:text=In%20a%20mixture%20of%20gases,mixture%20at%20the%20same%20temperature.&text=The%20partial%20pressure%20of%20a,activity%20of%20the%20gas's%20molecules.
to learn the subject from there.