Wize University Chemistry Textbook > Acids and Bases

Conjugate Acid-Base Pairs

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Identifying Conjugate Acid and Base Pairs

Two substances that differ from each other only by one proton (H+) are referred to as a conjugate acid-base pair.



Identify the acid, base, conjugate acid, and conjugate base in the following reactions:

1) HNO2(aq) + H2O(l) ⇌ H3O+(aq) + NO2-(aq)

acid base conj. acid conj. base


Write the Ka or Kb expression for this reaction:


Ka=[H3O+][NO2−][HNO2]Ka=\frac{\left[H_3O^+\right]\left[NO_2^-\right]}{\left[HNO_2\right]}


  • If we were to increase Ka, would that lead to more/less dissociation into ions? ________
  • Would this mean we have a stronger/weaker acid? ________


2) NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)

base acid conj. acid conj. base


Write the Ka or Kb expression for this reaction:



Kb=[NH4+][OH−][NH3]Kb=\frac{\left[NH_4^+\right]\left[OH^-\right]}{\left[NH_3\right]}



  • If we were to increase Kb, would that lead to more/less OH- ions? ________
  • Would we have a stronger/weaker base? ________



Practice: Conjugate Acid-Base Pairs

In the reaction HSO4–(aq) + OH–(aq) ⇌ SO42–(aq) + H2O(l), the conjugate acid-base pairs are:

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Extra Practice

Acids and Bases: Conjugate Acid-base Pairs

Indicate how many of these combinations are conjugate acid-base pairs. Select all that apply.

B is a weak base. Which equilibrium corresponds to the equilibrium constant Ka for HB+?

Acids and bases: Conjugate identification

In the reaction HSO4-(aq) + OH-(aq) ⇌ SO42-(aq) + H20(l), the conjugate acid-base pairs are:

Acids and Bases: Conjugate

Using the data in the table, which of the conjugate bases below is the strongest base?

AcidKaHOAc1.8×10−5HCHO21.8×10−4HClO3.0×10−8HF6.8×10−4\def\arraystretch{1.5} \begin{array}{c} \hline \rm Acid &&&K_a\\ \hline HOAc&&&1.8 \times 10^{-5}\\ HCHO_2&&&1.8 \times 10^{-4}\\ HClO &&& 3.0 \times 10^{-8}\\ HF &&& 6.8 \times 10^{-4}\\ \hline \end{array}

Using the incomplete reaction below, choose the completed equilibrium which indicates the correct association of conjugate acid base pairs.

HCl+H2O⇌\rm HCl\quad+\quad H_2O\quad \xrightleftharpoons{\hspace{1.5cm}}

**#1 and #2 designate the two sets of conjugate acid base pairs present in each equilibrium**

Acids and Bases: Conjugate

In each equation, label the acids, bases and conjugate acid-base pairs

  1. NH3+H3PO4↔NH4++H2PO4−NH_3+H_3PO_4 \leftrightarrow NH^+_4+H_2PO^-_4
  2. CH3O−+NH3↔CH3OH+NH2−CH_3O^-+NH_3 \leftrightarrow CH_3OH+NH^-_2

Using the incomplete reaction below, choose the completed equilibrium which indicates the correct association of conjugate acid base pairs.

+CH3COOH⇌H2O+Na+CH3COO−\rm \quad+\quad {CH_3COOH}\quad \xrightleftharpoons{\hspace{1.5cm}}\quad H_2O\quad+\quad Na^+CH_3COO^-

**#1 and #2 designate the two sets of conjugate acid base pairs present in each equilibrium**

Using the incomplete reaction below, choose the completed equilibrium which indicates the correct association of conjugate acid base pairs.

HCl+NH3⇌\rm HCl\quad+\quad {NH_3}\quad \xrightleftharpoons{\hspace{1.5cm}}

**#1 and #2 designate the two sets of conjugate acid base pairs present in each equilibrium**

Pure water has a pH of 7.00. 1.1g of MgBr2 is added to 15L of pure water. The solubility of MgBr2 in water is approximatly 101.5g/L. What is the pH of the resulting solution?

A series of solutions are formed with strontium salts of the form SrX2 where X=F, Cl, Br, I.

Which of the solutions will have the highest pH?

Triemethylamine (N(CH3)3) is a base (pKb=4.19) used in a variety of organic reactions. It can also be used as an effective Lewis base due to it's small steric profile.

if a solution of [N(CH3)3H]2[SO4]is dissolved in neutral water the pH will be,

To 2L of pure water is added Mg(ClO4)2. What would the effect on pH be?

In the reaction HSO4(aq)−+OH(aq)−⇌SO4(aq)2−+H2O(l)HSO^-_{4(aq)}+OH^-_{(aq)} \rightleftharpoons SO^{2-}_{4(aq)}+H_2O_{(l)}the conjugate acid-base pairs are

Consider the weak bases below and their Kb values:

C6H7OKb=1.3×10−10C_6H_7O \hspace{48pt}K_b=1.3 \times 10^{-10}

C2H5NH2Kb=5.6×10−4C_2H_5NH_2 \hspace{30pt}K_b=5.6 \times 10^{-4}

Using the incomplete reaction below, choose the completed equilibrium which indicates the correct association of conjugate acid base pairs.

HCl+H2O⇌\rm HCl\quad+\quad H_2O\quad \xrightleftharpoons{\hspace{1.5cm}}

**#1 and #2 designate the two sets of conjugate acid base pairs present in each equilibrium**

Consider the following reaction:

NH4++OH−⇌H2O+NH3NH^{4+}+OH^-\rightleftharpoons H_2O+NH_3

Choose the INCORRECT statement

Consider the weak bases below and their Kb values:

C6H7O Kb = 1.3 x 10-10
C2H5NH2 Kb = 5.6 x 10-4

Select which of the following WOULD NOT be a conjugate acid/base pair.

In the reaction HSO4−(aq) + OH−(aq) ⇌ SO42−(aq) + H2O (l)HSO_4^-\left(aq\right)\ +\ OH^-\left(aq\right)\ ⇌\ SO_4^{2-}\left(aq\right)\ +\ H_2O\ \left(l\right) the conjugate acid-base pairs are

Weak Acids and Bases

Sulfurous acid (H2SO3) is a diprotic weak acid (pKa1 = 1.85, pKa2 = 7.20). Bisulfite (HSO3-) is the conjugate base of sulfurous acid and is an amphoteric species (can act as an acid or a base). Will HSO3- act as an acid or as a base?

Weak Acids and Bases

Will HCO3- act as an acid or as a base?

In a reaction, which of the following could be conjugate acid-base pairs?