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CH 302 Lecture Notes - Lecture 1: Weak Base, Ionic Compound, Sodium Chloride

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School
UT-Austin
Department
Chemistry
Course
CH 302
Professor
Fakhreddine Fatima

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Document Summary

A salt is an ionic compound in which the anion is neither o2- nor oh-. A salt is formed in the reaction of an acid and a base. The cation part of the salt comes from the base and the anion part comes from the acid. Nacl: na+, the cation, comes from the base (naoh) and cl-, the anion comes from the acid (hcl). Salts can be classified based on the acid and the base from which they were formed. 1- salts of strong bases and strong acids. 2- salts of strong bases and weak acids. 3- salt of weak bases and strong acids. 4- salts of weak bases and weak acids. Based on their classification, when dissolved in water, salts can give neutral, acidic, or basic aqueous solutions. When we dissolve a salt of a strong base and a strong base in water the resulting solution is always neutral. When placed in water, solid nacl completely dissociates:

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Related Questions

Salts formed by reaction of a strong acid with a weak base areacidic, and salts formed by reaction of a weak acid with a strongbase are basic. This is due to the equilibrium the conjugate haswith its acid or base. For example, sodium bicaronate NaHCO3contains the conjugate of the weak acid H2CO3. The conjugate willreform the acid through the equilibrium

HCO3?(aq)+H2O(aq)?H2CO3(aq)+HO?(aq)

and, thus, acts as a weak base. A solution of NaHCO3yields abasic solution.

If a salt is derived from a weak acid and a weak base, the pH ofthe solution depends on the relative acid strength (Ka) ofthe cation and base strength (Kb) of the anion. IfKa>Kb, the solution of the salt is acidic. IfKa<Kb, it is basic. If Ka?Kb, it is neutral.

Given that

  • Ka for HCN is 4.9

For conjugate acid-base pairs, how are Ka and Kb related?

Consider the reaction of acetic acid in water:

CH3CO2H (aq) + H2O (l) ⇌ CH3CO2− (aq) + H3O+ (aq)             where Ka = 1.8 × 10−5

  1. Which two bases are competing for the proton?
  2. Which is the stronger base?
  3. In light of your answer to part b. why do we classify the acetate ion (CH3CO2−) as a weak base? Use an appropriate reaction to justify your answer.
  4. In general, as base strength increases, conjugate acid strength decreases. Explain why the conjugate acid of the weak base NH3 is a weak acid.

 

To summarize, the conjugate base of a weak acid is a weak base and the conjugate acid of a weak base is a weak acid (weak gives you weak). Assuming Ka for a monoprotic strong acid is 1× 106, calculate Kb for the conjugate base of this strong acid.

Why do conjugate bases of strong acids have no basic properties in the water?

List the conjugate bases of the six common strong acids.

To tie it all together, some instructors have students think of Li+, K+, Rb+, Cs+, Ca2+, Sr2+, and Ba2+ as the conjugate acids of the strong bases LiOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, and Ba(OH)2. Although not technically correct, the conjugate acid strength of these cations is similar to the conjugate base strength of the strong acids. That is, these cations have no acidic properties in water; similarly, the conjugate bases of strong acids have no basic properties (strong gives you worthless). Fill in the blanks with the correct response. The conjugate base of a weak acid is a_____base. The conjugate acid of a weak base is a_____acid. The conjugate base of a strong acid is a_____base. The conjugate acid of a strong base is a_____ acid. ( Hint: Weak gives you weak and strong gives you worthless.)