Chemistry 104 Study Groups
ACIDS & BASES
 

CONCEPTS

 

Water

    Water is more complicated than it looks! In a solution of pure water, some of the water molecules ionize according to:
    2H2O(aq) <---> OH-(aq) + H3O+(aq)
    The equilibrium constant for this reaction is given by:
    [OH-][H3O+] = Kw = 1.0 x 10-14
    This equation can be used to convert between [H3O+] and [OH-] in a solution and is very handy!

pH

    The strength of an acidic or basic solution is measured on the pH scale.
    The pH scale ranges from 0-14, with 7 indicating a neutral solution.
    pH values < 7 are acidic; pH values > 7 are basic.
    pH is determined by [H3O+] of the solution.
    pH is calculated from [H3O+]:
    pH = -log[H3O+]

    [H3O+] can be determined from the pH:
    [H3O+]=10-pH

pOH

    In basic solutions, the [OH-] is generally easier to determine than the [H3O+].
    The pOH can be related to the [OH-] concentration by:
    pOH = -log[OH-]
    [OH-] = 10-pOH

    pH and pOH are related:
    pH + pOH = 14
    Derivation of this formula.

Acids

    An acid is a proton donor
    All acids may be represented as HA
    Acids react with water as follows:
    HA(aq) + H2O <---> A-(aq) + H3O+(aq)

    Strong acids react completely with water (Ka=). Strong acids include:
    • HCl, HBr, HI
    • HNO3
    • HClO4
    • H2SO4 (first ionization to HSO4- only)
    All other acids are weak acids. The size of Ka indicates the extent to which they react with water:

Bases

    A base is a proton acceptor
    All bases may be represented as B
    Bases react with water as follows:
    B(aq) + H2O <---> BH+(aq) + OH-(aq)

    Strong bases react completely with water (Kb=). Strong bases include:
    • LiOH, NaOH, KOH, RbOH,CsOH
    • Mg(OH)2,Ca(OH)2,Sr(OH)2, Ba(OH)2
    All other bases are weak bases. The size of Kb indicates the extent to which they react with water:

Ions can sometimes act as Acids or Bases

    Some ions react with water (hydrolysis) and produce either H3O+ or OH- ions.
    Salts which are formed from the reaction of strong acids with strong bases do not hydrolyse water.
    • Na+,Ba2+ and K+ are some examples of cations that do not hydrolyze water.
    • Cl-,Br-, I- and NO3- are examples of anions that do not hydrolyze water.
    Ions which are conjugate bases of weak acids, react with water to form basic solutions:
    A-(aq) + H2O ---> HA(aq) + --> --OH-(aq)

    Some examples of basic ions include:

    • CH3COO-
    • CN-
    • F-
    Ions which are conjugate acids of weak bases, react with water to form acidic solutions:
    BH+(aq) + H2O ---> B(aq) + --> --H3O+(aq)

    Some examples of acidic ions include:

    • NH4+
    • C6H5NH3+

Calculations Involving Acids and Bases

    Virtually all calculations involveing weak acids and bases are equilibrium calculations.
    The ICE method should be used for all weak acid/base calculations.
    Since Ka and Kb are often much smaller than the concentration of the acid/base solution in question, approximations may be used.
    Click here for an explanation of how and when approximations may be used in acid/base calculations. Use your browser's "back" button to return.
 
 

Exercises

  Consider a 0.1-M solution of each of the following ionic compounds. Without doing detailed calculations, circle the selection most closely describing the pH of the solution.

CH3COOH <1 1 1<pH<7 7 7<pH<13 13 >13
 
HCl <1 1 1<pH<7 7 7<pH<13 13 >13
 
NaNO3 <1 1 1<pH<7 7 7<pH<13 13 >13
 
NH3 <1 1 1<pH<7 7 7<pH<13 13 >13
 
Ba(OH)2 <1 1 1<pH<7 7 7<pH<13 13 >13
 
NH4Cl <1 1 1<pH<7 7 7<pH<13 13 >13
 
NaOH <1 1 1<pH<7 7 7<pH<13 13 >13
 
CH3COONa <1 1 1<pH<7 7 7<pH<13 13 >13
 
H2SO4 <1 1 1<pH<7 7 7<pH<13 13 >13

 
  Consider an acid HA with Ka = 5.2 x 10—5 and molar mass = 125 g/mol. To make a solution with pH = 3.05, what mass of HA would be dissolved in 250 mL of aqueous solution?

 
  Consider a base B with Kb = 1.8 x 10-5. Calculate the pH of a 2.47-M solution of B. Calculate the pH of a 2.47-M solution of BH+.

 
  Consider two acids, HX and HY. HX has Ka = 1.0 x 10-4. HY has Ka = 1.0 x 10-8. Without doing detailed calculations, determine whether a 0.1-M solution of HX or HY would have the higher pH. Without doing detailed calculations, determine whether a 0.1-M solution of X- or Y- would have the higher pH. Calculate all four pH values to confirm the conclusions.

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