What happens when acid is added to a weak base?
You will be able to: Distinguish weak-base reaction regions and recognize nonquantitative weak–weak reactions.
What happens when acid is added to a weak base?
Add 3 mmol strong acid to 8 mmol of a weak base B. The acid converts 3 mmol B into BH⁺, leaving 5 mmol B alongside its conjugate acid.
A useful starting point: How can adding base to a weak acid create a buffer? →
Words and symbols before equations
- BH⁺
- Conjugate acid formed when B accepts H⁺.
- Half-equivalence
- Half the initial weak base has accepted a proton.
- Conjugate-acid pKa
- The pKa of BH⁺, related to B’s pKb by pKw.
- Weak–weak reaction
- Proton transfer whose extent depends on the relevant equilibrium constant.
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. Initial analyte: 25.0 mL of 0.100 M solute. Titrant is 0.100 M. The pK control affects weak cases only. Additive volumes; exact monoprotic charge balance with water makes the curve continuous.
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.
- Before equivalence: conjugate pair forms. Added titrant 1.25 mmol; original analyte 2.50 mmol. Total volume=37.5 mL; pH=9.000. Half-equivalence is 12.5 mL; equivalence is 25.0 mL.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
For B + H₃O⁺ → BH⁺ + H₂O, strong acid is consumed first. Before equivalence, substantial B and BH⁺ form a buffer; at equivalence BH⁺ hydrolysis makes the solution acidic.
For the B/BH⁺ pair, use pH≈pKa(BH⁺)+log([B]/[BH⁺]). Alternatively use pOH≈pKb(B)+log([BH⁺]/[B]). The ratios reverse because the conjugate acid and base switch roles.
After equivalence, excess strong acid controls pH. The same titration regions exist as for a weak acid, with the acidic/basic roles reversed.
If both reactants are weak, do not automatically consume the limiting reactant completely. For HA+B ⇌ A⁻+BH⁺, K=Ka(HA)/Ka(BH⁺). A very large K favors products; a value near one requires an equilibrium treatment.
A worked example, step by step
A weak base has pKb=5.00 at 25 °C. Start with 10.0 mmol B and add 5.00 mmol HCl. Estimate pH.
- HCl converts 5.00 mmol B into BH⁺.
- B=5.00 mmol and BH⁺=5.00 mmol, so their ratio is one.
- pKa(BH⁺)=14.00−5.00=9.00.
- pH≈9.00; equivalently pOH≈pKb=5.00.
At weak-base half-equivalence, pOH≈pKb. pH is the conjugate acid’s pKa, not automatically the base’s pKb.
For HA+B with equal Ka(HA) and Ka(BH⁺), is complete neutralization justified?
Compare with an explanation
No. K=1, so substantial reactants and products can remain.
Predict. Change one thing. Explain.
Select weak base + strong acid. Find half-equivalence and equivalence. Explain why one gives pH near the conjugate-acid pKa and the other lies below neutral pH.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Before equivalence: conjugate pair forms. Added titrant 1.25 mmol; original analyte 2.50 mmol. Total volume=37.5 mL; pH=9.000. Half-equivalence is 12.5 mL; equivalence is 25.0 mL.
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. Initial analyte: 25.0 mL of 0.100 M solute. Titrant is 0.100 M. The pK control affects weak cases only. Additive volumes; exact monoprotic charge balance with water makes the curve continuous.
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 questionAdd 2.00 mmol HCl to 6.00 mmol B, with pKa(BH⁺)=9.00. Calculate the buffer pH and distinguish this from mixing two weak reactants.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: The strong acid converts 2.00 mmol B to BH⁺.
- 1 point: B=4.00 mmol and BH⁺=2.00 mmol.
- 1 point: pH≈9.00+log(4/2)=9.30.
- 1 point: A weak acid with B would require checking the proton-transfer equilibrium rather than assuming the same complete reaction.
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 1Which species remains at weak-base equivalence?
Predominantly the conjugate acid BH⁺, with its hydrolysis equilibrium.
RECALL 2Which pKa enters the buffer equation?
That of BH⁺.
RECALL 3Are all acid–base reactions quantitative?
No; weak–weak reactions can have substantial reactants at equilibrium.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
What happens when acid is added to a weak base?
- pH≈pKa(BH⁺)+log([B]/[BH⁺]).
- For HA+B ⇌ A⁻+BH⁺: K=Ka(HA)/Ka(BH⁺).
Remember: At weak-base half-equivalence, pOH≈pKb. pH is the conjugate acid’s pKa, not automatically the base’s pKb.
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. Initial analyte: 25.0 mL of 0.100 M solute. Titrant is 0.100 M. The pK control affects weak cases only. Additive volumes; exact monoprotic charge balance with water makes the curve continuous.
Refresh Kid · AP Chemistry Unit 8 · Objectives 8.4.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 8.4, objective 8.4.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.
Want to work through this with a tutor?
Bring your question about What happens when acid is added to a weak base? Your explanation and answers remain free to access.