Question

The atmospheric pressure is $760\, mm\, Hg$ at the sea level. Which of the following ranges is nearest to the partial pressure of $CO_2$ in mm Hg?

• A. 0.30-0.31
• B. 0.60-0.61
• C. 3.0-3.1
• D. 6.0-6.1

Answer 1

Answer: (A)

$pCO_2 = 0.30 - 0.31\, mm\, Hg$ in air.
Air contains 0.04% of carbon dioxide.
This means that in every 100 molecules of air, 0.04 will be $CO_2$ molecules.
The number of moles of carbon dioxide in 100 molecules of air will be
$nCO_2 = 0.04$ molecules $\times N_A = 0.04 \times N_A$
The total number of moles in the sample of air will be
$n_{total} = 100$ molecules $\times N_A = 100\,N_A$
This means that mole fraction of carbon dioxide in the mixture will be
$XCO_{2}=\frac{0.04\,N_{A}}{100\,N_{A}}=0.0004$
Carbon dioxide’s partial pressure in air will thus be
$pCO_2 = 0.0004 \times 760\, mm\, Hg$
$= 0.304\, mm\, Hg$ $\therefore $ We can say it ranges between $0.30-0.31\, mm\, Hg$