For some reason, I can only find the answer in ppm and I don’t know how to do this conversion.
2 Answers
What I think you are asking for is the partial pressure of carbon dioxide in the atmosphere.
The partial pressure of carbon dioxide can be determined, where,
$P_{CO_2} \ = \ x_{CO_2}\cdot P_{atm}$
One mole of any substance is equal to exactly 6.022⋅1023 molecules of that substance (Avogadro's number, $N_A$).
The number of moles of carbon dioxide is,
$Moles\ of\ CO_2 \ = \ Number\ of\ molecules\ of\ CO_2 \cdot N_A$
If the atmosphere has a carbon dioxide concentration of 400 ppm, then
$n_{CO_2}\ = \ \frac{400}{1\ 000\ 000} \cdot N_A \ = \ 0.0004 \cdot N_A$
Thus, the partial pressure of carbon dioxide in the atmosphere will be,
$P_{CO_2} \ = 0.0004 \cdot 101\ 325 \ = \ 40.53 \ Pa \ = 0.0004 \ atm \ = \ 0.304 \ mmHg$
Not nearly so elegant as @fred answer: 400 ppm = 0.04% , then CO 2 is 0,04 % or 0.0004 atm.