Determine the [OH-] and pH of a solution that is 0.250 M in HCO3⁻.

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Identify the relevant equilibrium reaction for bicarbonate ion (HCO₃⁻) in water: HCO₃⁻ + H₂O ⇌ H₂CO₃ + OH⁻.

Write the expression for the equilibrium constant (K_b) for the reaction: K_b = [H₂CO₃][OH⁻]/[HCO₃⁻].

Use the relationship between K_w, K_a, and K_b to find K_b for HCO₃⁻: K_b = K_w / K_a, where K_w is the ion-product constant of water (1.0 x 10⁻¹⁴) and K_a is the acid dissociation constant for H₂CO₃.

Set up an ICE (Initial, Change, Equilibrium) table to determine the changes in concentration of HCO₃⁻, H₂CO₃, and OH⁻ at equilibrium.

Solve for [OH⁻] using the K_b expression and the values from the ICE table, then calculate pH using the relationship pH + pOH = 14.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Bicarbonate Ion (HCO3⁻)

The bicarbonate ion (HCO3⁻) is a key component in the bicarbonate buffer system, which helps maintain pH in biological systems. It acts as a weak base, capable of accepting protons (H⁺) to form carbonic acid (H2CO3), thus influencing the acidity of the solution. Understanding its role is crucial for calculating hydroxide ion concentration and pH.

pH and pOH Relationship

pH is a measure of the hydrogen ion concentration in a solution, while pOH measures the hydroxide ion concentration. They are related by the equation pH + pOH = 14 at 25°C. This relationship is essential for determining the pH of a solution when the concentration of hydroxide ions is known, or vice versa.

Equilibrium and Acid-Base Reactions

Acid-base reactions often reach a state of equilibrium, where the rates of the forward and reverse reactions are equal. In the case of bicarbonate, it can act as both an acid and a base, depending on the conditions. Understanding how to set up and solve equilibrium expressions, such as the dissociation of HCO3⁻, is vital for calculating [OH⁻] and pH.

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Acid-Base Reaction

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