Chapter 16, Problem 16.148MP

Normal rain has a pH of 5.6 due to dissolved atmospheric carbon dioxide at a current level of 400 ppm. Various mode ls predict that burning fossil fuels will increase the atmos pheric CO₂ concentration to between 500 and 1000 ppm (by volume) by the year 2100. Calculate the pH of rain in a scenario where the CO, concentration is 750 ppm.

1. First use Henry’s Law to calculate the concentration of dissolved CO₂. Solubility $=k\times P$ (Section 12.4) and the Henry’s Law constant (k) for CO₂ at 25 ^oC is $3.2\times {10}^{-2}\text{mol/(L}\cdot \text{atm)}\text{.}$
2. Next calculate the pH of the rain. CO₂reacts with water to produce carbonic acid according to the equation:

   ${\text{CO}}_{\text{2}}(aq)+{\text{H}}_{\text{2}}\text{O(l)}\rightleftharpoons {\text{H}}_{\text{2}}{\text{CO}}_{\text{3}}(aq)$

Assume all the dissolved CO₂ is converted to ${\text{H}}_{\text{2}}{\text{CO}}_{\text{3}}$ . Acid dissociation constants for ${\text{H}}_{\text{2}}{\text{CO}}_{\text{3}}$ are $K_{a1}=4.3\times {10}^{-7};K_{a2}=5.6\times {10}^{-11}.$ (Worked Example Worked Example 16.11 is a model for this calculation.)

c. Will rising CO₂ levels affect the acidity of rainfall?