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Chapter 14: Problem 4

Consider a solution prepared by mixing a weak acid HA and \(\mathrm{HCl}\). What are the major species? Explain what is occurring in solution. How would you calculate the \(\mathrm{pH}\) ? What if you added NaA to this solution? Then added \(\mathrm{NaOH}\) ?

Short Answer

Expert verified
In the initial solution of weak acid HA and strong acid HCl, the major species present are HA, HCl, H鈧僌鈦, and Cl鈦. The pH can be calculated using the H鈧僌鈦 concentration from the dissociation of HCl. When NaA is added, it yields Na鈦 and A鈦 ions, and the pH slightly increases. The major species now include HA, HCl, NaA, H鈧僌鈦, Cl鈦, Na鈦, and A鈦. Upon adding NaOH, the pH significantly increases as OH鈦 ions react with H鈧僌鈦 ions. The major species in the final solution are HA, NaA, NaOH, H鈧侽, Cl鈦, Na鈦, A鈦, and OH鈦. To calculate the pH, determine the pOH using the concentration of OH鈦 ions, and then use the relation \(pH + pOH = 14\).

Step by step solution

01

In a solution prepared by mixing a weak acid (HA) and a strong acid (HCl), the major species present at the beginning are the weak acid HA and the hydronium ion (H鈧僌鈦) from the dissociation of the strong acid. Since the HCl is a strong acid, it will completely dissociate and form H鈧僌鈦 and Cl鈦 ions in the solution. The weak acid, HA, will only partially dissociate into its constituent ions (H鈧僌鈦 and A鈦). The predominant species in this solution are HA, HCl, H鈧僌鈦, and Cl鈦. #Step 2: Determine the pH of the initial solution#

To calculate the pH of the solution, we need to determine the concentration of the H鈧僌鈦 ions. Since HCl is a strong acid and dissociates completely, its concentration can be directly used to determine the concentration of H鈧僌鈦 ions. The dissociation of the weak acid, HA, contributes to the H鈧僌鈦 concentration, but its contribution will be negligible compared to the H鈧僌鈦 concentration from HCl. Therefore, we can approximate the pH of the solution using the concentration of HCl: \(pH = -\log_{10} [H_3O^+]\), where [H鈧僌鈦篯 is the concentration of hydronium ions in the solution. #Step 3: Adding NaA to the solution#
02

When NaA is added to the solution, it will dissociate completely into its constituent ions: Na鈦 and A鈦. As A鈦 is the conjugate base of the weak acid HA, it will react with H鈧僌鈦 ions to form HA. The presence of A鈦 will decrease the concentration of H鈧僌鈦 ions in the solution and, as a result, increase the pH. However, since HCl is present in the solution, the increase in pH will not be significant. The predominant species in this solution will be HA, HCl, NaA, H鈧僌鈦, Cl鈦, Na鈦, and A鈦. #Step 4: Adding NaOH to the solution#

NaOH is a strong base and will dissociate completely when added to the solution, producing Na鈦 and OH鈦 ions. The OH鈦 ions will react with the H鈧僌鈦 ions, forming water. The reaction between H鈧僌鈦 and OH鈦 ions will result in a significant increase in the pH of the solution. To calculate the pH at this point, we need to consider the equilibrium between HA and A鈦 and the concentration of OH鈦 ions present in the solution. The concentration of hydroxide ions can be used to determine the pOH: \(pOH = -\log_{10} [OH^-]\), and subsequently the pH, using the relation: \(pH + pOH = 14\). The predominant species in this solution will be HA, NaA, NaOH, H鈧侽, Cl鈦, Na鈦, A鈦, and OH鈦.

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Most popular questions from this chapter

Determine \(\left[\mathrm{OH}^{-}\right],\left[\mathrm{H}^{+}\right]\), and the \(\mathrm{pH}\) of each of the following solutions. a. \(1.0 \mathrm{M} \mathrm{KCl}\) b. \(1.0 \mathrm{M} \mathrm{KF}\)

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