Nitrous acid, HNO2, is a weak acid with a pKa of 3.14. What is the predominant form present in a solution with a pH of 5.00? Write the chemical formula, not just the word acid or base.
The Correct Answer and Explanation is:
Correct Answer:
The predominant form is NO₂⁻.
Explanation:
Nitrous acid (HNO₂) is a weak acid, meaning it only partially dissociates in water. The equilibrium reaction for nitrous acid in water is:HNO2 ⇌ H+ + NO2−\text{HNO}_2 \; \rightleftharpoons \; \text{H}^+ \; + \; \text{NO}_2^-HNO2⇌H++NO2−
To determine the predominant form at a given pH, we compare the solution’s pH to the acid’s pKa, which is 3.14 for HNO₂.
The pKa is the pH at which the concentrations of the acid form (HNO₂) and its conjugate base (NO₂⁻) are equal. If the pH is:
- Lower than the pKa, the acid form (HNO₂) is more prevalent.
- Higher than the pKa, the base form (NO₂⁻) is more prevalent.
In this problem, the pH is 5.00, which is significantly higher than the pKa of 3.14. This means the environment is more basic, and the equilibrium shifts toward the deprotonated form — the conjugate base, NO₂⁻.
We can also use the Henderson-Hasselbalch equation to reinforce our conclusion:pH=pKa+log([NO2−][HNO2])\text{pH} = \text{pKa} + \log\left(\frac{[\text{NO}_2^-]}{[\text{HNO}_2]}\right)pH=pKa+log([HNO2][NO2−])
Plugging in the values:5.00=3.14+log([NO2−][HNO2])5.00 = 3.14 + \log\left(\frac{[\text{NO}_2^-]}{[\text{HNO}_2]}\right)5.00=3.14+log([HNO2][NO2−])log([NO2−][HNO2])=1.86\log\left(\frac{[\text{NO}_2^-]}{[\text{HNO}_2]}\right) = 1.86log([HNO2][NO2−])=1.86[NO2−][HNO2]≈101.86≈72.4\frac{[\text{NO}_2^-]}{[\text{HNO}_2]} \approx 10^{1.86} \approx 72.4[HNO2][NO2−]≈101.86≈72.4
This shows that the concentration of NO₂⁻ is about 72 times higher than HNO₂ at pH 5.00.
Conclusion:
Since the conjugate base NO₂⁻ is present in much higher concentration, NO₂⁻ is the predominant form in a solution of pH 5.00.
