What is the pH of pure water in equilibrium with 1 atmosphere of carbon dioxide at 25˚C? (Hint, what does 1 atmosphere of CO2 mean?)
To determine the pH of pure water in equilibrium with 1 atmosphere of carbon dioxide (CO2) at 25˚C, we need to understand what "1 atmosphere of CO2" means and how it affects the water.
Atmospheric pressure is the force exerted by the Earth's atmosphere on a given area. It is typically measured in units of atmospheres (atm). 1 atmosphere (1 atm) is approximately equal to the average atmospheric pressure at sea level (around 101.325 kilopascals or 14.696 pounds per square inch).
Now, when we say "1 atmosphere of CO2," it means that the partial pressure of carbon dioxide is 1 atm. In other words, the concentration of CO2 is equivalent to the pressure exerted by the CO2 molecules in the atmosphere.
In aqueous solution, carbon dioxide reacts with water to form carbonic acid (H2CO3) through a reversible reaction:
CO2 + H2O ⇌ H2CO3
Carbonic acid then further dissociates to release hydrogen ions (H+) in the solution:
H2CO3 ⇌ H+ + HCO3-
The concentration of H+ ions in the solution determines the pH value. pH is a measure of the acidity or basicity of a solution, ranging from 0 (most acidic) to 14 (most basic). A pH of 7 is considered neutral.
Pure water without any dissolved substances has a pH of 7 because the concentration of H+ ions and OH- ions is equal. However, when carbon dioxide dissolves in water, it forms carbonic acid and increases the concentration of H+ ions, making the solution more acidic.
To determine the pH, we need to know the concentration of H+ ions in the solution. For simplicity, we can use the equilibrium constant expression for the dissociation of carbonic acid (H2CO3) in water:
K = [H+][HCO3-] / [H2CO3]
At equilibrium, the concentration of H2CO3 is related to the partial pressure of CO2 (Pco2) through Henry's law. At 25˚C, the equation is:
[H2CO3] = α * Pco2
Where α is the Henry's law constant for CO2 in water.
By substituting these values into the equilibrium expression, we can solve for the concentration of H+ ions in the solution, which will give us the pH of water in equilibrium with 1 atmosphere of CO2 at 25˚C.
However, determining the exact pH value requires knowing the values of α and Pco2, which may vary depending on the conditions. Therefore, to calculate the precise pH, additional information such as the Henry's law constant and the partial pressure of CO2 is needed.