Final answer:
The equilibrium pH of CO2 dissolved in water is indeed about 5.6, as it forms carbonic acid which slightly increases the hydronium ion concentration, leading to a pH between 5 and 6 which is typically observed in normal rainwater.
Step-by-step explanation:
The equilibrium pH of dissolved CO2 in water, also known as carbonic acid (H2CO3), is indeed around 5.6 under normal atmospheric conditions. When CO2 dissolves in water, it forms carbonic acid through the reaction:
CO2 (aq) + H2O(l) ⇒ H2CO3(aq)
The carbonic acid (H2CO3) then dissociates into hydronium (H3O+) and bicarbonate (HCO3−) ions:
H2CO3(aq) ⇒ H+ (aq) + HCO3− (aq)
The pH of a solution is determined by the concentration of hydronium ions. Normal rainwater has a pH between 5 and 6 because of the presence of dissolved CO2, which when forming carbonic acid slightly increases the hydronium ion concentration in water. The provided data indicates that air-saturated water has a hydronium ion concentration of 2.0 × 10−6 M at 25 °C, which is about 20-times larger than that of pure water. To calculate the pH of this solution, we would use the formula:
pH = -log[H3O+]
Therefore, at 25 °C:
pH = -log(2.0 × 10−6) ≈ 5.7
This calculation helps to support the notion that normal dissolved CO2 levels in water would result in an equilibrium pH of around 5.6.