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Strong Electrolytes and Weak Electrolytes Tutorial

Key Concepts

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Guidelines for Determining the Strength of an Electrolyte

The following guidelines can be used to decide if an electrolyte is likely to be a strong electrolyte or a weak electrolyte:

  1. Acids: Most acids are weak acids and therefore weak electrolytes.
    The common strong acids are HCl(aq), HNO3(aq), H2SO4(aq) (for the loss of the first proton1)

    Other strong acids are HClO3(aq), HClO4(aq), HBr(aq), HI(aq)

  2. Bases: Strong bases are strong electrolytes
    The hydroxides of Group 1 (alkali) metals and Group 2 (alkaline-earth) metals are stong bases and therefore strong electrolytes with the exception of Ba(OH)2(aq) which is weak.

    Ammonia, NH3(aq), or ammonium hydroxide, NH4OH(aq), is a weak base and therefore a weak electrolyte.

  3. Salts: Most salts are strong electrolytes.
    Weak salts include HgCl2 and CdSO4
  4. Water is a weak electrolyte2
  5. Complex ions, for example Ag(NH3)2+ and CuCl42-, are weak electrolytes.

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Worked Examples : Deciding if an Electrolyte is Strong or Weak

Question 1: Hydrochloric acid completely dissociates in water to form hydrogen ions and chloride ions.
Is hydrochloric acid a strong electrolyte or a weak electrolyte?

Solution:

(Based on the StoPGoPS approach to problem solving.)

  1. What is the question asking you to do?

    Determine whether hydrochloric acid is a strong electrolyte or a weak electrolyte

  2. What data (information) have you been given in the question?

    hydrochloric acid completely dissociates in water

  3. What is the relationship between what you know and what you need to find out?

    HCl(aq) completely dissociates in water to form hydrogen ions, H+(aq), and chloride ions, Cl-(aq)

    HCl(aq) → H+(aq) + Cl-(aq)


    Therefore there are lots of ions in solution.

  4. Determine the strength of hydrochloric acid as an electrolyte:

    Hydrochloric acid, HCl(aq), dissociates completely so there are many ions in solution to conduct electricity therefore hydrochloric acid is a strong electrolyte.

  5. Is your answer plausible?

    Use the Guidelines to check your answer:
    Guideline (1): most acids are weak electrolytes.
    EXCEPT HCl(aq), and some other acids like HNO3(aq) and H2SO4(aq) which are strong acids and therefore strong electrolytes
    Therefore hydrochloric acid is a strong electrolyte according to the guidelines.

    Since the answer we get from the guidelines is the same as the answer we get using information about the dissociation of HCl(aq) from the question, we are confident our answer is correct.

  6. State your solution to the problem "is hydrochloric acid a strong or weak electrolyte":

    Hydrochloric acid is a strong electrolyte.

Question 2: Is an aqueous solution of sodium hydroxide a strong electrolyte or a weak electrolyte?

Solution:

(Based on the StoPGoPS approach to problem solving.)

  1. What is the question asking you to do?

    Determine whether an aqueous solution of sodium hydroxide is a strong electrolyte or a weak electrolyte

  2. What data (information) have you been given in the question?

    aqueous solution of sodium hydroxide: NaOH(aq)

  3. What is the relationship between what you know and what you need to find out?

    Guideline (2): hydroxides of Group 1 (alkali) metals and Group 2 (alkaline-earth) metals are stong electrolytes
    (Ba(OH)2 is the exception)
    Sodium is a Group 1 (alkali) metal.

  4. Determine the strength of sodium hydroxide as an electrolyte:

    Sodium hydroxide is a strong electrolyte because it it is the hydroxide of a Group 1 metal.

  5. Is your answer plausible?

    Sodium hydroxide, NaOH(s), is used in drain cleaners.
    It is an ionic solid made up of sodium ions, Na+, and hydroxide ions, OH-.
    When added to water to form an aqueous solution we can see that it dissolves, that is, the lattice of ions is broken up so that each ion is completely surrounded by water, that is, sodium hydroxide completely dissociates in water:

    NaOH(aq) → Na+(aq) + OH-(aq)

    Therefore there will be lots of ions in solution to conduct electricity.
    Therefore an aqueous solution of sodium hydroxide is a strong electrolyte.

    Since the answer we get from thinking about the dissociation of sodium hydroxide in water is the same as the answer we get using the Guidelines, we are confident our answer is correct.

  6. State your solution to the problem "is sodium hydroxide a strong or weak electrolyte":

    An aqueous solution of sodium hydroxide is a strong electrolyte.

Question 3: Sodium chloride completely dissociates in water to form sodium ions and chloride ions.
Is sodium chloride a strong electrolyte or weak electrolyte?

Solution:

(Based on the StoPGoPS approach to problem solving.)

  1. What is the question asking you to do?

    Determine whether sodium chloride is a strong electrolyte or a weak electrolyte

  2. What data (information) have you been given in the question?

    Sodium chloride completely dissociates in water

  3. What is the relationship between what you know and what you need to find out?

    Sodium chloride, NaCl(s), is a salt (produced by the neutralisation of hydrochloric acid with sodium hydroxide)
    If a salt completely dissociates, 100% dissociation, there are many ions in solution.

    NaCl(aq) → Na+(aq) + Cl-(aq)

    The more ions there are, the better the solution conducts electricity.

  4. Determine the strength of sodium chloride as an electrolyte:

    Sodium chloride is a strong electrolyte because it is a salt that dissociates completely in water.

  5. Is your answer plausible?

    Use the guidelines to check your answer:
    Guideline (3): most salts are strong electrolytes.
    Exceptions: HgCl2 and CdSO4 are weak electrolytes

    Sodium chloride is a strong electrolyte because it is a salt that is not listed as one of the weak salts.

    Since the answer we got by considering the percentage dissociation of sodium chloride is the same as that we get from considering the guidelines we are confident our answer is correct.

  6. State your solution to the problem "is sodium chloride a strong or weak electrolyte":

    Sodium chloride is a strong electrolyte.

Question 4: Acetic acid (ethanoic acid), CH3COOH, has a small acid dissociation constant (Ka = 1.8 × 10-5).
Is acetic acid (ethanoic acid) a strong electrolyte or a weak electrolyte?

Solution:

(Based on the StoPGoPS approach to problem solving.)

  1. What is the question asking you to do?

    Determine whether acetic acid is a strong electrolyte or a weak electrolyte

  2. What data (information) have you been given in the question?

    acetic acid has a small dissociation constant (Ka is small)

  3. What is the relationship between what you know and what you need to find out?

    If the acid dissociation constant, Ka, is small, then very few of the acid′s molecules dissociate (ionise), that is, there will be few ions in the solution.
    A small dissociation constant means that there are few ions in solution to conduct electricity.
    The equilibrium position for the dissociation reaction:

    CH3COOH(aq) ⇋ CH3COO-(aq) + H+(aq)


    lies further to the left, that is, the undissociated molecules are favoured.

  4. Determine the strength of acetic acid as an electrolyte:

    Acetic acid is a weak electrolyte because its dissociation constant is small meaning there will be few ions in solution to conduct electricity.

  5. Is your answer plausible?

    Use the Guidelines to check your answer:
    Guideline (1): most acids are weak electrolytes.
    The exceptions (the strong electrolytes) are: HCl(aq), HNO3(aq), H2SO4(aq), HClO3(aq), HClO4(aq), HBr(aq) and HI(aq)
    Acetic acid (ethanoic acid, CH3COOH) is not listed as an exception so acetic acid is a weak electrolyte.

    Since we arrived at the same answer using both the small acid dissociation constant and the Guidelines, we are confident that our answer is correct.

  6. State your solution to the problem "is acetic acid a strong or weak electrolyte":

    Acetic acid is a weak electrolyte.

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1. Sulfuric acid is a polyprotic strong acid
The first dissociation constant is very large so that we assume this reaction goes to completion:
H2SO4(aq) → HSO4-(aq) + H+(aq)
The second dissociation constant is much smaller than the first, so the reaction in which HSO4- loses a proton, H+, does not go to completion, the ions are in equilibrium with the undissociated acid molecules:
HSO4-(aq) ⇋ SO42-(aq) + H+(aq)

2. Water molecules undergo self-dissociation to a very small extent.
Kw = 10-14 at 25° C
That is, Kw is very small.