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Chapter 17: Problem 18

Which of the following compounds or ions has the strongest conjugate acid? Briefly explain your choice. (a) \(\mathrm{CN}^{-}\) (b) \(\mathrm{NH}_{3}\) (c) \(\mathrm{SO}_{4}^{2-}\)

Short Answer

Expert verified
\(\mathrm{HSO}_{4}^{-}\) has the strongest conjugate acid.

Step by step solution

01

Identify Conjugate Acids

The conjugate acid of a base is formed by adding a proton (H\(^+\)) to the base. Let's identify the conjugate acids:(a) \/ \(\mathrm{CN}^{-}\) becomes \(\mathrm{HCN}\) when it gains a proton.(b) \/ \(\mathrm{NH}_{3}\) becomes \(\mathrm{NH}_{4}^{+}\) when it gains a proton.(c) \/ \(\mathrm{SO}_{4}^{2-}\) becomes \(\mathrm{HSO}_{4}^{-}\) when it gains a proton.
02

Analyze the Strength of Conjugate Acids

The strength of a conjugate acid is inversely related to the strength of its conjugate base. A weaker base forms a stronger conjugate acid.(a) \/ \(\mathrm{CN}^{-}\) is a relatively strong base, so \(\mathrm{HCN}\) is a weak acid.(b) \/ \(\mathrm{NH}_{3}\) is a weak base, so \(\mathrm{NH}_{4}^{+}\) is a relatively strong acid.(c) \/ \(\mathrm{SO}_{4}^{2-}\) is a very weak base, so \(\mathrm{HSO}_{4}^{-}\) is a strong acid.
03

Compare and Conclude

Compare the strengths of the conjugate acids formed to determine which is the strongest.Among \(\mathrm{HCN}\), \(\mathrm{NH}_{4}^{+}\), and \(\mathrm{HSO}_{4}^{-}\), \(\mathrm{HSO}_{4}^{-}\) is recognized as the strongest conjugate acid due to \(\mathrm{SO}_{4}^{2-}\) being the weakest base.

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Key Concepts

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

Conjugate Acids
In acid-base chemistry, a conjugate acid is formed when a base gains a proton (H\(^+\)). This process is like a handoff during a relay race, where the proton is passed from the acid to the base. Understanding conjugate acids is crucial because it helps predict how substances will behave in different chemical reactions.
To illustrate, let's consider the examples from our original exercise:
  • The ion \(\mathrm{CN}^{-}\) becomes the conjugate acid \(\mathrm{HCN}\) by gaining a proton.
  • Ammonia \(\mathrm{NH}_{3}\) gains a proton to form ammonium \(\mathrm{NH}_{4}^{+}\).
  • Sulfate \(\mathrm{SO}_{4}^{2-}\) turns into hydrogen sulfate \(\mathrm{HSO}_{4}^{-}\).
The transformation reveals how bases can become acids by accepting protons, thus forming conjugate acids.
Strength of Acids
Acid strength refers to an acid's ability to donate a proton, which is a fundamental aspect of acid-base chemistry. A stronger acid releases more protons into a solution. Thus, it affects the acidity of a solution significantly. The strength of an acid is often depicted through its dissociation in water, where it splits into ions.
Based on the examples:
  • \(\mathrm{HCN}\) is a weaker acid because \(\mathrm{CN}^{-}\) holds onto protons tightly, releasing less in solutions.
  • \(\mathrm{NH}_{4}^{+}\) is stronger compared to \(\mathrm{HCN}\), as it's more willing to donate protons.
  • \(\mathrm{HSO}_{4}^{-}\), being the strongest among the given examples, dissociates more readily, increasing its effectiveness in lowering pH.
Each acid's strength is inversely related to its conjugate base's strength, with stronger acids having weaker conjugate bases.
Conjugate Base-Strength Relationship
In acid-base equilibria, the relationship between conjugate acids and bases is like a seesaw. Their strengths balance each other with one being strong if the other is weak. This seesaw relationship plays a critical role in predicting reactions and understanding chemical stability.
For the compounds in the exercise:
  • \(\mathrm{CN}^{-}\) is a strong base, resulting in a weaker conjugate acid \(\mathrm{HCN}\).
  • \(\mathrm{NH}_{3}\) is a weak base, producing a stronger conjugate acid \(\mathrm{NH}_{4}^{+}\).
  • \(\mathrm{SO}_{4}^{2-}\) is very weak as a base, so its conjugate acid \(\mathrm{HSO}_{4}^{-}\) is quite strong.
Understanding this relationship allows us to predict the behavior of substances when mixed in solutions and how they will influence the overall acidity or basicity.

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Most popular questions from this chapter

A \(2.5 \times 10^{-3}\) M solution of an unknown acid has a pH of 3.80 at \(25^{\circ} \mathrm{C}\).(a) What is the hydronium ion concentration of the solution?(b) Is the acid a strong acid, a moderately weak acid \(\left(K_{\mathrm{a}} \mathrm{of} \right.\)about \(\left.10^{-5}\right)\).or a very weak acid ( \(K_{\mathrm{a}}\) of about \(10^{-10}\) )?

Calcium hydroxide, \(\operatorname{Ca}(\mathrm{OH})_{2},\) is almost insoluble in water \(=\) only \(0.50 \mathrm{g}\) can be dissolved in \(1.0 \mathrm{L}\) of water at \(25^{\circ} \mathrm{C} .\) If the dissolved substance is completely dissociated into its constituent ions, what is the pH of a saturated solution?

A 0.015 M solution of a base has a pH of 10.09 (a) What are the hydronium and hydroxide ion concentrations of this solution? (b) Is the base a strong base, a moderately weak base \(\left(K_{\mathrm{b}}\right.\) of about \(10^{-5}\) ), or a very weak base ( \(K_{\mathrm{b}}\) of about \(\left.10^{-10}\right) ?\)

The weak base methylamine, \(\mathrm{CH}_{3} \mathrm{NH}_{2},\) has \(K_{\mathrm{b}}=4.2 \times 10^{-4}\) It reacts with water according to the equation \(\mathrm{CH}_{3} \mathrm{NH}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell) \rightleftarrows \mathrm{CH}_{3} \mathrm{NH}_{3}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})\) Calculate the equilibrium hydroxide ion concentration in a 0.25 M solution of the base. What are the \(\mathrm{pH}\) and \(\mathrm{pOH}\) of the solution?

Which of the following common food additives would give a basic solution when dissolved in water? (a) \(\mathrm{NaNO}_{3}\) (used as a meat preservative) (b) \(\mathrm{NaC}_{6} \mathrm{H}_{5} \mathrm{CO}_{2}\) (sodium benzoate; used as a soft-drink preservative) (c) \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\) (used as an emulsifier in the manufacture of pasteurized cheese)
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