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Chapter 2: Problem 44

Nitrous acid, HNO, \(_{2},\) has a \(\mathrm{pK}_{\mathrm{a}}\) value of \(3.3 .\) If a solution of nitrous acid is found to have a pH of \(4.2,\) what can be said about the concentration of the conjugate acid/base pair found in solution? (A) \(\left[\mathrm{HNO}_{2}\right]>\left[\mathrm{NO}_{2}^{-}\right]\) (B) \(\left[\mathrm{NO}_{2}^{-}\right]>\left[\mathrm{HNO}_{2}\right]\) (C) \(\left[\mathrm{H}_{2} \mathrm{NO}_{2}^{+}\right]>\left[\mathrm{HNO}_{2}\right]\) (D) \(\left[\mathrm{HNO}_{2}\right]>\left[\mathrm{H}_{2} \mathrm{NO}_{2}^{+}\right]\)

Short Answer

Expert verified
The correct answer is (B) \([NO2^-] > [HNO2]\). When the pH of the solution is greater than the pKa of the acid, the concentration of the conjugate base is greater than the concentration of the acid.

Step by step solution

01

Understand the Chemistry Involved

In an acid-base equilibrium, HNO2 (nitrous acid) can donate a proton to become its conjugate base NO2-, while water acts as the base, accepting the proton to become its conjugate acid H3O+. The equation should look like this: HNO2 + H2O ⇌ H3O+ + NO2-. The pH is given by -log10[H3O+]. If the pH is greater than the pKa, the base will predominate. If the pH is less than the pKa, the acid form will predominate. If the pH equals the pKa, there will be equal amounts of acid and base.
02

Compare the pKa and the pH

In the given problem, the pKa of nitrous acid is 3.3 and the pH of the solution is 4.2. Since 4.2 > 3.3, we can say that the base (NO2-) will predominate. This allows us to understand that the concentration of NO2- is greater than the concentration of HNO2.
03

Select the correct answer

Given the findings in Step 2, the answer matches with option (B) which states that \([NO2^-] > [HNO2]\). Hence, (B) is the correct answer.

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Most popular questions from this chapter

Which of the following best explains why the ionization of atoms can occur during photoelectron spectroscopy, even though ionization is not a thermodynamically favored process? (A) It is an exothermic process due to the release of energy as an electron is liberated from the Coulombic attraction holding it to the nucleus. (B) The entropy of the system increases due to the separation of the electron from its atom. (C) Energy contained in the light can be used to overcome the Coulombic attraction between electrons and the nucleus. (D) The products of the ionization are at a lower energy state than the reactants.

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Consider the Lewis structures for the following molecules: $$\begin{equation} \mathrm{CO}_{2}, \mathrm{CO}_{3}^{2-}, \mathrm{NO}_{2}^{-}, \text {and } \mathrm{NO}_{3}^{-} \end{equation}$$ Which molecule would have the smallest bond angle between terminal atoms? (A) \(\mathrm{CO}_{2}\) (B) \(\mathrm{CO}_{3}^{2-}\) (C) \(\mathrm{NO}_{2}^{-}\) (D) \(\mathrm{NO}_{3}^{-}\)
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