Strong And Weak Acids Bases
Learn and apply Strong And Weak Acids Bases in the published Chemistry course sequence.
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The core idea
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Strong and Weak Acids and Bases: Orientation
Strength describes extent of ionisation; concentration describes amount per unit volume. Exam questions often vary these independently.
- Explain pH, Ka, Kb and Kw, but exclude pKa, pKb and Kw = KaKb calculations.
Definitions (Must Know)
- A strong acid or base dissociates essentially completely in water; a weak acid or base dissociates only partially.
- pH = −log10[H+]. Ka and Kb are equilibrium constants for acid and base dissociation; Kw = [H+][OH−].
Detailed Explanations
A strong acid has a large extent of dissociation, so at the same analytical concentration it normally gives a higher [H+] than a weak monoprotic acid.
For HA ⇌ H+ + A−, Ka expresses the equilibrium ratio [H+][A−]/[HA]. H1 requires the meaning of this expression, not weak-acid pH calculations.
At equal concentration, the stronger acid usually has the greater [H⁺], lower pH and higher electrical conductivity.
A weak acid establishes a reversible ionisation equilibrium; writing a one-way arrow falsely implies complete ionisation.
Worked Examples
Modelled example 1
Core application
Problem
Study the worked solution
Compare dissociation
Method
Contrast essentially complete HCl dissociation with partial ethanoic-acid dissociation.Reason
Strength describes the equilibrium extent of dissociation.Working
HCl is strong; CH₃CO₂H is weak.Link to hydrogen ions
Method
State that HCl gives the larger equilibrium [H⁺] at equal analytical concentration.Reason
More acid particles release protons.Working
Larger [H⁺] gives the lower pH.
Guided practice 2
Separating strength from concentration
Problem
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Hints
Hint 1: define strength
Hint 2: define pH evidence
View solution step by step
Keep strength intrinsic
Method
State that HCl remains stronger because it dissociates essentially completely.Reason
Dilution does not turn a strong acid into a weak acid.Working
Strength and concentration are different properties.Compare resulting concentrations
Method
Allow a sufficiently concentrated weak acid to produce the larger [H⁺] despite partial dissociation.Reason
The larger starting amount can outweigh the smaller dissociated fraction.Working
The weak-acid solution may then have the lower pH.
Common misconception 3
Correct “weak means dilute”
Learner definition
Choose the defining property
View solution step by step
Separate the quantities
Method
Use “dilute/concentrated” for amount per volume.Reason
Concentration is a solution composition variable.Working
A weak acid may be concentrated.State the strength definition
Method
Use “partially dissociates in water” for a weak acid.Reason
Strength describes equilibrium dissociation extent.Working
Weak ⇒ partial dissociation, not dilute.
Examiner practice 4
Express and interpret acid dissociation
Problem
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View solution step by step
Write the equilibrium
1 markMethod
Show partial dissociation with a reversible arrow.Reason
Methanoic acid is weak and establishes an equilibrium.Working
HCO₂H ⇌ H⁺ + HCO₂⁻.Write the expression
1 markMethod
Place ionic product concentrations over undissociated acid concentration.Reason
This is the reviewed meaning of the acid dissociation constant.Working
Kₐ = [H⁺][HCO₂⁻]/[HCO₂H].Interpret equilibrium position
1 markMethod
Link a larger Kₐ to a greater product-to-reactant concentration ratio.Reason
The dissociation equilibrium lies further toward ions.Working
Larger Kₐ means greater extent of dissociation.Interpret strength
1 markMethod
State that the acid is stronger.Reason
Acid strength is ranked by equilibrium dissociation extent.Working
Larger Kₐ ⇒ stronger weak acid.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Credit the reversible equilibrium, correct Ka expression, dissociation interpretation and strength conclusion.
Challenge 5
Infer macroscopic behaviour from particle evidence
Problem
Try this before viewing the solution
Hints
Hint 1: rank extent
Hint 2: link to observables
View solution step by step
Rank acid strength
Method
Select R as the stronger acid.Reason
A much greater fraction of R is dissociated.Working
Extent: R ≫ S.Predict pH
Method
Give R the lower expected pH.Reason
At equal initial concentration, R produces more H⁺.Working
[H⁺]_R > [H⁺]_S.Predict conductivity
Method
Give R the higher expected conductivity.Reason
Greater dissociation produces more mobile ions, assuming comparable ion mobilities and controlled temperature.Working
Conductivity of R is expected to exceed S.
Mind Stretchers
Attempt the independent prompts before opening a hint or solution.
- Explain why dilute hydrochloric acid can have a higher pH than a more concentrated weak acid.
- Write the Ka expression for methanoic acid and state what a larger Ka implies.
Mind stretcher 1: Using conductivity evidenceExtension
Question. Equal-concentration solutions of acid P and acid Q are tested. P conducts much better. What can be deduced, and what assumption is needed?
Show Hint
Link mobile-ion concentration to ionisation.
Show Answer
If the acids are comparable and temperature is controlled, P has the greater extent of ionisation and is stronger. Conductivity alone is unsafe if concentrations or ion mobilities differ.
Mind stretcher 2: Particle-level comparisonExtension
Question. Sketch in words the dominant particles in equal-concentration strong and weak monoprotic acid solutions.
Show Hint
State both ionised and unionised species.
Show Answer
The strong acid contains mainly H₃O⁺ and its conjugate base, with negligible unionised acid. The weak acid contains mainly unionised molecules plus smaller amounts of H₃O⁺ and conjugate base.