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Wize University Chemistry Textbook > Acids and Bases

Acid Equations (Ka, pKa)

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Acids and their Equations

Strong Acids

Strong acids dissociate completely
Example:
HCl(aq) → H+(aq) + Cl-(aq)

Wize Tip
The following are strong acids that you should memorize!
  • HCl (hydrochloric acid)
  • H2SO4 (sulfuric acid)
  • HNO3 (nitric acid)
  • HClO4 (perchloric acid)
  • HBr (hydrobromic acid)
  • HI (hydroiodic acid)


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Weak Acids

Weak acids dissociate incompletely, so they have a Ka value (acid dissociation constant)

Ka is defined as the equilibrium constant for the reaction below:

HA+H2O⇌A−+H3O+HA+H_2O \rightleftharpoons A^-+H_3O^+

Ka=[H3O+][A−][HA]K_a=\cfrac{[H_3O^+][A^-]}{[HA]}

pKa=−log⁡Ka and Ka=10−pKapK_a=-\log K_a\ and\ K_a=10^{-pK_a}
Acid strength follows the following trends:

  • A weaker acid would have a higher/lower Ka value?
    lower
  • A weaker acid would have a higher/lower pKa value?
    higher
Wize Tip
Ka and pKa are like opposite trends.
The higher Ka goes, the lower pKa will go,
and the lower Ka goes, the higher pKa will go!

  • A weaker acid will have a higher/lower pH value:
    higher
  • A stronger acid will have a higher/lower Ka value?
    higher
  • A stronger acid will have a higher/lower pKa value?
    lower
  • A stronger acid will have a higher/lower pH value?
    lower


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In general:
  • If the Ka > 1, pKa < 0 the acid is strong/weak:
    strong
    and reactants/products:
    products
    are favored
  • If the Ka < 1, pKa > 0 the acid is strong/weak:
    weak
    and reactants/products:
    reactants
    are favored

Practice: Strong Acids

Which one of the following statements about strong acids is true?
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Acids With Different Numbers of Protons

Monoprotic Acids
  • These are acids that only have 1 acidic proton (H+) that they can lose in a reaction
  • *think: mono means 1 Examples: HCl, HBr, HI, HNO3, HClO4, etc

Diprotic Acids
  • These are acids that have 2 acidic protons
  • *think: di means 2 Example: H2SO4

Triprotic Acids
  • These are acids that have 3 acidic protons
  • *think: tri means 3 Example: H3PO4

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Polyprotic Acids
  • This term is just referring to any acid that has more than 1 proton
  • *think: poly means many Example: H2SO4, H3BO3, H2C2O4


H3PO4 (aq) + H2O (l) ⇋ H2PO4− (aq) + H3O+ (aq)H_3PO_4\ \left(aq\right)\ +\ H_2O\ \left(l\right)\ \leftrightharpoons\ H_2PO_4^-\ \left(aq\right)\ +\ H_3O^+\ \left(aq\right)

H2PO4− (aq) + H2O (l) ⇋ HPO42− (aq) + H3O+ (aq)H_2PO_4^-\ \left(aq\right)\ +\ H_2O\ \left(l\right)\ \leftrightharpoons\ HPO_4^{2-}\ \left(aq\right)\ +\ H_3O^+\ \left(aq\right)

HPO42− (aq) + H2O (l) ⇋ PO43− (aq) + H3O+ (aq)HPO_4^{2-}\ \left(aq\right)\ +\ H_2O\ \left(l\right)\ \leftrightharpoons\ PO_4^{3-}\ \left(aq\right)\ +\ H_3O^+\ \left(aq\right)



Wize Concept
The main concept to take away from this table is just that each ionization reaction has its own Ka value and Ka1>Ka2>Ka3.

Extra Practice
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What are the Ka and Kb of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
Methyl diethanolamine CH3N(C2H4OH)2 is a weak base which we will abbreviate as mdea. The conjugate acid, mdeaH+, has a pKa = 8.52.
Nitrous acid has a Ka of 7.1 x 10-5. What are [H3O+], [NO2- ] andin 0.50 M HNO2?
Acids and Bases: Weak Acid Practice
Nitrous acid has a Ka of 7.1 x 10-5. What are [H3O+] and [NO2- ] in 0.50 M HNO2?
Nitrous acid has a Ka of 7.1 x 10-5. What are [H3O+], [NO2- ] andin 0.50 M HNO2?
Acids and Bases: Ka
The following occurs when HCl is placed in water:
HCl <----> H+ + Cl-
The Ka for this reaction is known to be Ka = 5.64 once it reaches equilibrium. At equilibrium, 1.35 x 10-7 M of HCl and 1.2 M of Cl- can be found. What is the pH of the solution at equilibrium?
The following reaction occurs in water:
CH3OH <----> CH3O- + H+
Once the reaction was allowed to reach equilibrium, 0.5 M of CH3OH, 0.3 M of CH3O+, and 0.3 M of H+ were found in solution. What is the Ka of this solution?
The equilibrium constant for the auto-ionization of water at 10 °C is 3.0 x 10-15. What is the concentration of H3O+ ions in pure water at this temperature?
What is the pH of a 1.1M solution of nitrous acid (HNO2). The Ka of nitrous acid is 7.2 x 10-4
Ammonia(NH3) is a weak base, Kb = 1.8 x 10-5 and benzoic acid (C6H5CH2O-) is a weak acid, Ka = 6.3 x 10-5. A solution of ammonium benzoate (NH4)(C6H5CH2O-) is:
To 4.00 L of water was added 100 mg of HF (6.6 x 10-4). What is the pH?
CH3COOH is a weak acid with a Ka of 1.8 x 10-5. In what pH range would you most likely an aqueous solution of Na(CH3COO).
Acids and Bases: Henderson Hasselbalch
To a solution of acetic acid (Ka=1.8×10−5\rm K_a = 1.8 \times 10^{-5}) was added 0.1 mol of NaOH\rm NaOH. After the addition of NaOH\rm NaOH the volume was 1.4L and the pH\rm pH was 5.2.
a) This solution a buffer? (True/False)
b) Calculate the number of moles of acetic acid that were originally in solution. Report your answer in moles to three significant figures. Do not include units in the answer field
Calculate the pH of the solution resulting from the addition of 10.0 mL of a 0.10 M NaOH to 50.0 mL of 0.10 M HCN (Ka = 4.9 x 10-10) solution
A solution of [CH3NH3][CH3COO] is made in an undergraduate laboratory. Use the information below to predict the acidity of the solution.
CH3COOH: Ka = 1.8 x 10-5
CH3NH3+: Ka = 2.3 x 10-11
Strength of Acids and Bases
What is the strongest acid below? What is the weakest acid?
Strongest acid:
Cl3CCOOH
Weakest acid:
HCN
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}
Acids and Bases: Acidity
Nitrous acid has a Ka of 7.1 x 10-5. What are [H3O+] and [NO2- ] in 0.50 M HNO2?
Which of the solutions below would have the HIGHEST pH value?
Methyl diethanolamine CH3N(C2H4OH)2 is a weak base which we will abbreviate as mdea. The conjugate acid, mdeaH+, has a pKa = 8.52.
Practice: Relating [H3O+], [OH-], pH and pOH
Compare two acids and their 1M solution:
Acid 1:Ka=1.0×10−3Acid 2:Ka=1.0×10−6\rm Acid\ 1:K_a=1.0\times10^{-3}\qquad Acid\ 2:K_a=1.0\times10^{-6}