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Question: our air contains carbon dioxide co2 at a concentration of...

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Our air contains carbon dioxide (CO2) at a concentration of approximately 400 parts per million by volume (ppmv). (This concentration has been rising over the past 60 years – it was kind of a big deal when we surpassed the 400 ppmv threshold.)

(a) What is the partial pressure of CO2 in the air? Report your answer in units of atm, Pa, and torr.

(b) Convert the concentration to units of mg/m3 , assuming an air temperature of 20 ◦C. Hint: use the ideal gas law along with the partial pressure you found in part (a).

(c) This concentration of CO2 in the atmosphere has an effect on the pH of rain drops that form in the atmosphere. Consider the following chemical reactions:

CO2 (g) + H2O <==> H2CO-3 (aq)

KH = 3.91 × 10−2 M/atm [Note the units!!]

H2CO3 <==> H+ + HCO-3

pKA = 6.35 [so what is KA?]

Based on these reactions, and given the CO2 concentration of 400 ppmv, determine the pH of rain water that is in equilibrium with the air. Is rain water acidic or basic? Hint: you have three unknowns: [H2CO3], [H+], and [HCO-3]. You are given two equilibrium equations. Therefore you need one more equation to solve the problem. The third equation is based on electroneutrality, and the important result is that [H+] = [HCO-3]. With this third equation, you should be able to solve the problem. Note that you won’t always have [H+] = [HCO-3], so don’t assume that is a general rule, but it works in this particular case (for reasons we won’t go into here)

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