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Chapter 5: Problem 160

Which of the following mixtures can act as buffer? a. \(\mathrm{H}_{2} \mathrm{CO}_{3}+\mathrm{KOH} \quad(2: 5: 1\) molar ratio \()\) b. \(\mathrm{H}_{2} \mathrm{CO}_{3}+\mathrm{KOH} \quad(2: 5: 3\) molar ratio \()\) c. \(\mathrm{NH}_{4} \mathrm{OH}+\mathrm{HNO}_{3}\) ( \(6: 4\) molar ratio) d. \(\mathrm{NH}_{4} \mathrm{OH}+\mathrm{HNO}_{3}(3: 5\) molar ratio \()\)

Short Answer

Expert verified
Mixtures b and c can act as buffers.

Step by step solution

01

Define a Buffer Solution

A buffer solution is a solution that can resist changes in pH when an acid or base is added. It is typically made from a weak acid and its conjugate base or a weak base and its conjugate acid.
02

Analyze Mixture a

Mixture a involves \(\mathrm{H}_{2} \mathrm{CO}_{3}\) and \(\mathrm{KOH}\) in a 2:5:1 molar ratio. In this scenario, \(\mathrm{KOH}\) is used to neutralize \(\mathrm{H}_{2} \mathrm{CO}_{3}\). The reaction would ideally convert all of \(\mathrm{H}_{2} \mathrm{CO}_{3}\) into \(\mathrm{HCO}_{3}^{-}\) and compete fully due to the small amount of \(\mathrm{H}_{2} \mathrm{CO}_{3}\) relative to \(\mathrm{KOH}\) present.
03

Analyze Mixture b

Mixture b also involves \(\mathrm{H}_{2} \mathrm{CO}_{3}\) and \(\mathrm{KOH}\) in a 2:5:3 molar ratio. Here, more \(\mathrm{H}_{2} \mathrm{CO}_{3}\) remains after the reaction with \(\mathrm{KOH}\), resulting in the presence of both \(\mathrm{H}_{2} \mathrm{CO}_{3}\) (a weak acid) and \(\mathrm{HCO}_{3}^{-}\) (its conjugate base), forming a buffer.
04

Analyze Mixture c

Mixture c involves \(\mathrm{NH}_{4} \mathrm{OH}\) and \(\mathrm{HNO}_{3}\) in a 6:4 molar ratio. Here, the weak base \(\mathrm{NH}_{4} \mathrm{OH}\) has some of its conjugate acid \(\mathrm{NH}_{4}^{+}\) present due to incomplete neutralization by the strong acid \(\mathrm{HNO}_{3}\), forming a buffer.
05

Analyze Mixture d

Mixture d involves \(\mathrm{NH}_{4} \mathrm{OH}\) and \(\mathrm{HNO}_{3}\) in a 3:5 molar ratio. In this case, there is more \(\mathrm{HNO}_{3}\) than \(\mathrm{NH}_{4} \mathrm{OH}\), leading to complete neutralization and excess \(\mathrm{HNO}_{3}\), which cannot act as a buffer.
06

Conclusion

Based on the analysis, mixtures b and c have the correct proportions to act as buffers. Mixture b has a weak acid and its conjugate base, while mixture c has a weak base and its conjugate acid.

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Key Concepts

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

Weak Acid
A weak acid is a type of acid that partially dissociates in water. This means that only a small fraction of the acid molecules ionize to produce hydrogen ions ((H^+) in solution. Unlike strong acids, which fully dissociate, weak acids help maintain stability in a solution by not releasing all their protons at once.

Common examples of weak acids include:
  • Acetic acid (CH鈧僀OOH)
  • Carbonic acid (H鈧侰O鈧�)
  • Citric acid (C鈧咹鈧圤鈧�)
When a weak acid like H鈧侰O鈧� is present in excess in a mixture with a base like KOH, part of it reacts to form a conjugate base. As seen in mixture b from the exercise, not all of the H鈧侰O鈧� is neutralized, thus creating an ideal buffer mixture with H鈧侰O鈧� and its conjugate base, HCO鈧冣伝.
Conjugate Base
In chemistry, a conjugate base forms when an acid donates a proton (H鈦�) in a reaction. This means that every acid has a corresponding conjugate base.

Here are key points about conjugate bases:
  • They are the species that remains after an acid has donated a proton.
  • Conjugate bases can accept protons, which is the reverse action of acids.
  • The strength of a conjugate base depends on the strength of its original acid.
In the exercise, HCO鈧冣伝 is the conjugate base of carbonic acid (H鈧侰O鈧�). This balance between a weak acid and its conjugate base in the solution creates a chemical equilibrium and resists drastic changes in pH levels when acids or bases are added.
Neutralization
Neutralization is a fundamental chemical reaction where an acid and a base react to form water and a salt. This process is key to understanding buffer solutions, as partial neutralization contributes to buffer capacity.

The reaction can be represented as:\[\text{Acid} + \text{Base} \rightarrow \text{Water} + \text{Salt}\]Important aspects of neutralization:
  • If balanced precisely, neutralization results in a neutral pH solution.
  • Partial neutralization creates a buffer with both the acid and its conjugate base present.
In mixture b and c of the exercise, partial neutralization of H鈧侰O鈧� with KOH and NH鈧凮H with HNO鈧� leads to the formation of both an acidic and basic component, crucial for buffer formation.
Weak Base
A weak base is a base that partially ionizes in water. This means it does not fully dissociate into its ions in solution, much like a weak acid.

Characteristics of weak bases include:
  • They typically produce fewer hydroxide ions (OH鈦�) compared to strong bases.
  • Weak bases form conjugate acids upon accepting protons.
A classic example of a weak base is ammonia (NH鈧�). In the exercise, NH鈧凮H acts as the weak base. In mixture c, partial neutralization of this weak base with the strong acid (HNO鈧�) results in a buffer solution, as both NH鈧凮H and its conjugate acid NH鈧勨伜 are present. When in such a balance, the solution can resist changes in pH when more acid or base is introduced.

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Most popular questions from this chapter

Find the pH of \(0.012 \mathrm{M}\) solution of \(\mathrm{NH}_{4} \mathrm{CN}\) if \(\mathrm{K}_{\mathrm{a}}\) for \(\mathrm{HCN}\) and \(\mathrm{K}_{\mathrm{b}}\) for \(\mathrm{NH}_{3}\) are \(6.2 \times 10^{-10}\) and \(1.6 \mathrm{x}\) \(10^{-5}\) respectively a. \(8.2\) b. \(9.2\) c. \(4.8\) d. \(5.8\)

For the reaction \(\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \leftrightarrow 2 \mathrm{NH}_{3}(\mathrm{~g}), \Delta \mathrm{H}=-93.6 \mathrm{~kJ}\) \(\mathrm{mol}^{-1}\), the concentration of \(\mathrm{H}_{2}\) at equilibrium can be increased by I. lowering the temperature II. increasing the volume of the system III. adding \(\mathrm{N}_{2}\) at constant volume IV. adding \(\mathrm{H}_{2}\) at constant volume a. II, IV b. only II is correct c. I, II and III are correct d. III and IV are correct e. only IV is correct

The hexaammine cobalt (III) ion is very unstable in acidic aqueous solution: \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}(\mathrm{aq})+6 \mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq}) \rightarrow\) \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{4+}(\mathrm{aq})+6 \mathrm{NH}_{4}^{+}(\mathrm{aq})\) However, solutions of hexaammine cobalt(III) can be stored in acidic solution for months without noticeable decomposition. Which statement below about the equilibrium constant and the activation energy for the reaction is true? a. \(\mathrm{Keq}>10^{3}\) and \(\mathrm{Ea}\) is very large b. Keq \(<10^{3}\) and Ea is very small c. Keq \(<10^{3}\) and Ea is very large d. \(\mathrm{Keq}>10^{3}\) and Ea is very small

Match the following: Column I Column II A. \(0.5 \mathrm{M} \mathrm{NO}_{3}+0.1 \mathrm{M} \quad\) (p) 7 \(\mathrm{NH}_{4} \mathrm{OH}\) B. \(0.1 \mathrm{M} \mathrm{KCl}+0.1 \mathrm{M} \quad\) (q) greater then 7 \(\mathrm{KNO}_{3}\) C. \(0.2 \mathrm{M} \mathrm{NaOH}+0.5 \mathrm{M}(\mathrm{r})\) between 1 to 7 \(\mathrm{HCOOH}\) D. \(0.1 \mathrm{NH}_{4} \mathrm{Cl}+0.1 \mathrm{M}(\mathrm{s}) 0.7\) \(\mathrm{KOH}\)

(A): The value of \(\mathrm{K}\) gives us a relative idea about the extent to which a reaction proceeds. \((\mathbf{R}):\) The value of \(\mathrm{K}\) is independent of the stoichiometry of reactants and products at the point of equilibrium.
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