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]}Ka=[HNO2​][H3​O+][NO2−​]​

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

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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]}Kb=[NH3​][NH4+​][OH−]​

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

Practice: Conjugate Acid-Base Pairs
In the reaction HSO4-(aq) + OH-(aq) ⇌ SO42-(aq) + H2O(l), the conjugate acid-base pairs are:

HSO4− and OH− ; SO42− and H3O+HSO_4^-\ and\ OH^-\ ;\ SO_4^{2-}\ and\ H_3O^+HSO4−​ and OH− ; SO42−​ and H3​O+

HSO4− and SO42− ; H2O and OH−HSO_4^{-\ }and\ SO_4^{2-}\ ;\ H_2O\ and\ OH^-HSO4− ​and SO42−​ ; H2​O and OH−

HSO4− and OH−; SO42− and H2O.HSO_4^-\ and\ OH^-;\ SO_4^{2-}\ and\ H_2O.HSO4−​ and OH−; SO42−​ and H2​O.

HSO4− and H2O ; OH and SO42−HSO_4^-\ and\ H_2O\ ;\ OH\ and\ SO_4^{2-}HSO4−​ and H2​O ; OH and SO42−​

HSO4− and H3O+ ; SO42− and OH−HSO_4^-\ and\ H_3O^+\ ;\ SO_4^{2-}\ and\ OH^-HSO4−​ and H3​O+ ; SO42−​ and OH−

I don't know

Extra Practice

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

Weak Acids and Bases

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

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?

 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)HSO4−​(aq) + OH−(aq) ⇌ SO42−​(aq) + H2​O (l) the conjugate acid-base pairs are 

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

Consider the weak bases below and their Kb values:  
C6H7O            Kb = 1.3 x 10-10
C2H5NH2       Kb = 5.6 x 10-4

Consider the following reaction:
NH4++OH−⇌H2O+NH3NH^{4+}+OH^-\rightleftharpoons H_2O+NH_3NH4++OH−⇌H2​O+NH3​
Choose the INCORRECT statement

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}}HCl+H2​O​
 
 **#1 and #2 designate the two sets of conjugate acid base pairs present in each equilibrium**   

  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}C6​H7​OKb​=1.3×10−10
C2H5NH2Kb=5.6×10−4C_2H_5NH_2 \hspace{30pt}K_b=5.6 \times 10^{-4}C2​H5​NH2​Kb​=5.6×10−4

 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)}HSO4(aq)−​+OH(aq)−​⇌SO4(aq)2−​+H2​O(l)​the conjugate acid-base pairs are 

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

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,

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?

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?

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}}HCl+NH3​​
 
 **#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.  
+CH3COOH⇌H2O+Na+CH3COO−\rm \quad+\quad {CH_3COOH}\quad \xrightleftharpoons{\hspace{1.5cm}}\quad H_2O\quad+\quad Na^+CH_3COO^-+CH3​COOH​H2​O+Na+CH3​COO−
 
 **#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 
NH3+H3PO4↔NH4++H2PO4−NH_3+H_3PO_4 \leftrightarrow NH^+_4+H_2PO^-_4NH3​+H3​PO4​↔NH4+​+H2​PO4−​
CH3O−+NH3↔CH3OH+NH2−CH_3O^-+NH_3 \leftrightarrow CH_3OH+NH^-_2CH3​O−+NH3​↔CH3​OH+NH2−​

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}}HCl+H2​O​
 
 **#1 and #2 designate the two sets of conjugate acid base pairs present in each equilibrium**   

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}AcidHOAcHCHO2​HClOHF​​​Ka​1.8×10−51.8×10−43.0×10−86.8×10−4​​

Acids and bases: Conjugate identification

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

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

Acids and Bases: Conjugate Acid-base Pairs

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

Wize University Chemistry Textbook > Acids and Bases

Conjugate Acid-Base Pairs

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Identifying Conjugate Acid and Base Pairs

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

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

Practice: Conjugate Acid-Base Pairs

Extra Practice

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