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Chapter 16: Problem 68

Compare the \(\mathrm{pH}\) values for \(0.10-\mathrm{M}\) solutions of \(\mathrm{NaOH}\) and of \(\mathrm{NH}_{3}\) to illustrate the difference between a strong base and a weak base.

Short Answer

Expert verified
NaOH has a pH of 13 (strong base), while 狈贬鈧� has a pH of 11.13 (weak base).

Step by step solution

01

Determine the pH of NaOH Solution

NaOH is a strong base, meaning it fully dissociates in water. For a \(0.10\,\text{M}\) NaOH solution, the hydroxide ion concentration \([ ext{OH}^-]\) is \(0.10\,\text{M}\). To find the \(\text{pH}\), first calculate the \(\text{pOH}\): \(\text{pOH} = -\log [\text{OH}^-] = -\log(0.10) = 1\). Then use the relation \(\text{pH} + \text{pOH} = 14\) to find \(\text{pH} = 14 - 1 = 13\).
02

Determine the pH of 狈贬鈧� Solution

狈贬鈧� is a weak base, it partially dissociates in water. Use the equilibrium equation: \(\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-\). \(K_b\) for 狈贬鈧� is \(1.8 \times 10^{-5}\). Use the expression \(K_b = \frac{[\text{NH}_4^+][\text{OH}^-]}{[\text{NH}_3]}\) and assume \(x = [\text{OH}^-]\), \(K_b = \frac{x^2}{0.10 - x} \approx \frac{x^2}{0.10}\), solving the quadratic gives \(x \approx 1.34 \times 10^{-3}\,\text{M}\). Now, \(\text{pOH} = -\log (1.34 \times 10^{-3}) \approx 2.87\), and \(\text{pH} = 14 - 2.87 = 11.13\).
03

Compare the pH Values

The \(\text{pH}\) of \(0.10\,\text{M}\) NaOH is 13, indicating a strong base with complete dissociation. The \(\text{pH}\) of \(0.10\,\text{M}\) 狈贬鈧� is 11.13, showing a weaker base with incomplete dissociation. This illustrates the difference in pH levels between strong and weak bases at the same concentration.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Strong Base
A strong base is a substance that completely dissociates into its ions in water. This means that every molecule of a strong base that dissolves in water splits into its respective ions. As a result, strong bases drastically increase the [OH^-] concentration in a solution. One of the most common examples of a strong base is sodium hydroxide (NaOH).
  • Complete Dissociation: In an aqueous solution, NaOH will fully dissociate to form Na+ and OH- ions. Therefore, the concentration of OH- ions in the solution is equal to the concentration of the NaOH solution you start with.
  • Effect on pH: Because of this complete dissociation, strong bases will have high pH values. In our example, a 0.10 M NaOH solution has a pH of 13, indicating a very basic (or alkaline) solution.
Weak Base
Contrary to strong bases, weak bases only partially dissociate in water. This means that only a small fraction of the base molecules release ions into the solution. Ammonia (狈贬鈧�) is a classic example of a weak base.
  • Partial Dissociation: 狈贬鈧� does not entirely dissociate into ions in aqueous solutions. Instead, it establishes an equilibrium between the ammonia molecules and the ions (狈贬鈧� + H鈧侽 鈬� NH鈧勨伜 + OH鈦�).
  • Effect on pH: Due to partial dissociation, the concentration of OH鈦� ions in a weak base solution is lower compared to strong bases with the same concentration. For example, a 0.10 M solution of 狈贬鈧� results in a pH of 11.13, signifying a less basic solution compared to a strong base.
NaOH
Sodium hydroxide (NaOH) is not just any base; it is one of the most common strong bases used in laboratory and industrial processes. It is important to understand its properties to handle it safely and effectively.
  • Powers of NaOH: In water, NaOH completely dissociates into sodium (Na+) and hydroxide (OH鈦�) ions. Because of this property, NaOH is highly effective at raising the pH of a solution.
  • Handling NaOH: Due to its strong basic nature, NaOH solutions are corrosive and can cause severe burns. It's crucial to use protective gear when handling NaOH solutions.
  • Applications: NaOH is used in the manufacturing of soap, paper, and as a cleaning agent to dissolve grease and oils.
狈贬鈧�
Ammonia (狈贬鈧�) is a weak base that finds a range of applications due to its properties in solution. Unlike strong bases, it only partially dissociates in water, which affects its behavior in chemical reactions.
  • Properties: Ammonia interacts with water to form ammonium (NH鈧勨伜) and hydroxide (OH鈦�) ions, establishing an equilibrium in the solution. Its base nature allows it to be used in household cleaning products and fertilizer production.
  • Role in pH: As a weak base, 狈贬鈧� will increase the pH of a solution, though not as significantly as a strong base like NaOH. This makes it suitable for applications where moderate pH adjustment is needed.
  • Application: It is widely used as a refrigerant, in the production of fertilizers, and in water purification processes due to its effectiveness to neutralize acids and its balance between reactivity and safety.

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Most popular questions from this chapter

Calculate the \(\mathrm{pH}\) at \(25^{\circ} \mathrm{C}\) of a \(0.25-M\) aqueous solution of oxalic acid \(\left(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\right) \cdot\left(K_{\mathrm{a}_{1}}\right.\) and \(K_{\mathrm{a}_{2}}\) for oxalic acid are \(6.5 \times 10^{-2}\) and \(6.1 \times 10^{-5}\), respectively.)

Calculate the \(\mathrm{pH}\) of a \(0.36 \mathrm{M} \mathrm{CH}_{3} \mathrm{COONa}\) solution. \(\left(K_{\mathrm{a}}\right.\) for acetic acid \(\left.=1.8 \times 10^{-5} .\right)\)

Which of the following is a stronger base: \(\mathrm{NH}_{3}\) or \(\mathrm{PH}_{3}\) ? (Hint: The \(\mathrm{N}-\mathrm{H}\) bond is stronger than the \(\mathrm{P}-\mathrm{H}\) bond. \()\)

Calculate the \(\mathrm{pH}\) of a \(0.082 \mathrm{M} \mathrm{NaF}\) solution. \(\left(K_{a}\right.\) for \(\left.\mathrm{HF}=7.1 \times 10^{-4} .\right)\)

\(\mathrm{Al}(\mathrm{OH})_{3}\) is insoluble in water. It dissolves in concentrated NaOH solution. Write a balanced ionic equation for this reaction. What type of reaction is this?
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