How does a weak base make hydroxide?
You will be able to: Write a weak-base equilibrium and convert hydroxide concentration into pH.
How does a weak base make hydroxide?
Ammonia does not contain an OH group, yet its aqueous solution is basic. It accepts a proton from water, leaving hydroxide behind.
A useful starting point: How much of a weak acid actually ionizes? →
Words and symbols before equations
- B and BH⁺
- A neutral base and its conjugate acid.
- Kb
- Equilibrium constant for B + H₂O ⇌ BH⁺ + OH⁻.
- pKb
- −log₁₀Kb; smaller pKb means a stronger base.
- Proton acceptance
- Formation of an additional bond to H⁺ using an available electron pair.
What this picture assumes
Dilute ideal-solution concentration model at 25 °C, Kw=1.00×10⁻¹⁴. Concentrations are mol/L (M); displayed values are rounded. No household experiments are required. A single monoprotic family with no added common ion. Charge balance includes water; percent ionization is the fraction converted to the conjugate form.
Read the picture in three steps
- Read the species and labels first. Identify what each symbol and line represents. Read the units and fixed conditions before comparing quantities.
- Weak base: C=0.1 M, Kb=1.00e-5. pH=10.998, ionized fraction=1.00%. Both ionization and water are included.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, ammonia is the base and water is the acid. Nitrogen accepts one proton; atoms and net charge are conserved.
For initially C M B, let x react. Then [BH⁺]≈[OH⁻]≈x and [B]=C−x, so Kb=x²/(C−x) when water’s background ions are negligible.
A valid small-change approximation gives x≈√(KbC). This x is hydroxide concentration: take pOH first, then use the temperature-specific pKw to obtain pH.
A weak base can be relatively concentrated. Comparing two pH values requires both equilibrium strength and concentration, not simply counting nitrogen atoms or OH groups.
A worked example, step by step
A supplied weak base has C=0.100 M and Kb=1.0×10⁻⁵ at 25 °C. Estimate its pH.
- Set Kb=x²/(0.100−x), with x≈[OH⁻].
- Estimate x≈√(10⁻⁵×0.100)=0.0010 M.
- Check x/C=1.0%, so the approximation is reasonable.
- pOH≈3.00 and pH≈11.00; x was not hydronium concentration.
Taking −log of the weak-base x gives pOH. Do not report that number as pH.
Can a base make hydroxide without containing OH in its formula?
Compare with an explanation
Yes. NH₃ accepts H⁺ from water, producing NH₄⁺ and OH⁻.
Predict. Change one thing. Explain.
Switch the same C and pK between weak acid and weak base at 25 °C. Compare pH values and identify which ion the ICE-table x represents.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Weak base: C=0.1 M, Kb=1.00e-5. pH=10.998, ionized fraction=1.00%. Both ionization and water are included.
Dilute ideal-solution concentration model at 25 °C, Kw=1.00×10⁻¹⁴. Concentrations are mol/L (M); displayed values are rounded. No household experiments are required. A single monoprotic family with no added common ion. Charge balance includes water; percent ionization is the fraction converted to the conjugate form.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using proton transfer, charge and atom conservation, a mole balance or the stated acid–base equilibrium. Identify what the representation cannot tell you.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionA 0.040 M base has Kb=1.0×10⁻⁶ at 25 °C. Estimate hydroxide concentration and pH, and check the approximation.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: x≈√(10⁻⁶×0.040)=2.0×10⁻⁴ M OH⁻.
- 1 point: x/C=0.005=0.5%, so neglecting x in C−x is reasonable.
- 1 point: pOH≈3.70.
- 1 point: pH≈10.30 at 25 °C.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1Who donates H⁺ when NH₃ reacts with water?
Water.
RECALL 2What does the weak-base x measure?
The reacted concentration and approximately the hydroxide produced.
RECALL 3How is pKb related to Kb?
pKb=−log₁₀Kb.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
How does a weak base make hydroxide?
- Kb≈[BH⁺][OH⁻]/[B].
- For valid small changes: [OH⁻]≈√(KbC).
Remember: Taking −log of the weak-base x gives pOH. Do not report that number as pH.
Conditions: Dilute ideal-solution concentration model at 25 °C, Kw=1.00×10⁻¹⁴. Concentrations are mol/L (M); displayed values are rounded. No household experiments are required. A single monoprotic family with no added common ion. Charge balance includes water; percent ionization is the fraction converted to the conjugate form.
Refresh Kid · AP Chemistry Unit 8 · Objectives 8.3.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 8.3, objective 8.3.A. CED effective Fall 2024 and June 2026 clarifications checked September 17, 2026. Unit 8: Acids and Bases, Topics 8.1–8.11. Focused lesson names, examples, models and assessments are original Refresh Kid teaching materials, not additional official topics or official AP questions. Official corrections.
The model states its assumptions beside the diagram. Dilute ideal-solution concentrations approximate activities; numerical models use 25 °C and Kw=1.00×10⁻¹⁴ unless another pKw is supplied. pH need not be restricted to 0–14 in all real solutions. The optional 3D views show original schematic molecular geometry, not a measured trajectory or a reaction mechanism. Computation of a buffer’s pH change after adding acid/base, derivation of Henderson–Hasselbalch, concentrations of every species in a polyprotic titration, and solubility as a function of pH are excluded from assessed scope. Buffer response and pH-dependent solubility are taught qualitatively. Calculating the pH of a buffer formed by partial neutralization remains in Topic 8.4 scope.
Teaching resources: The Organic Chemistry Tutor video titles/descriptions and topic coverage were checked for optional links; no claim is made to have watched every video. No creator scripts, examples, worksheets or artwork were copied. GitHub’s 3D website collection and its Three.js camera-control example informed the idea of controllable spatial inspection. Scientific diagrams, geometry and interactions here are original. The self-hosted Three.js runtime retains its MIT license. Camera rotation changes the view, not the chemistry.
Independent teacher review and observation of students remain pending. Implementation checks do not certify scientific accuracy, accessibility or learning effectiveness. This is a review edition.
Optional official resource: Released AP Chemistry questions and scoring guides. This archive contains questions across units; it is not an assignment of every question to this lesson.
The teaching sequence is informed by the IES learning guide; this exact implementation has not been evaluated with learners.
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