TeXes Science Practice Test

Given the dissociation equation of nitrous acid, what is Ka for a solution with a pH of 3?

1. 1.25 x 10^-4

2. 6.25 x 10^-4

3. 2.5 x 10^-3

4. 4.00 x 10^-5

The correct answer is: 6.25 x 10^-4

To determine the acid dissociation constant (Ka) for nitrous acid from the given pH, it’s important to first understand the relationship between pH, hydrogen ion concentration, and Ka. When a weak acid like nitrous acid (HNO2) dissociates in solution, it can be represented by the following equation: HNO2 ⇌ H⁺ + NO2⁻ The pH of the solution is given as 3, which means the concentration of hydrogen ions, [H⁺], can be calculated using the formula: \[ [H⁺] = 10^{-pH} = 10^{-3} \] This results in a hydrogen ion concentration of 0.001 M. In a weak acid equilibrium, we can express the dissociation in terms of concentrations. Let the initial concentration of nitrous acid be \( C \). For a weak acid at equilibrium, since it only partially dissociates, we can denote the concentration of the dissociated species as \( x \), which in this case is the concentration of H⁺ ions produced from the dissociation. Thus, at equilibrium: - [H⁺] = x = 0.001 M - [NO2