Acid-Base Chemistry & pH
Proton transfer, titration curves, and buffer systems
Acids donate protons (H⁺); bases accept protons (Brønsted-Lowry definition). Strong acids (HCl, H₂SO₄, HNO₃) dissociate completely: [H⁺] = initial acid concentration. Weak acids partially dissociate; Ka = [H⁺][A⁻]/[HA]. pH = -log[H⁺]. At 25°C, Kw = [H⁺][OH⁻] = 10⁻¹⁴; neutral pH = 7. Titration: adding strong base to acid gradually neutralizes it. The equivalence point is where moles of base = moles of acid (pH = 7 for strong/strong, > 7 for weak acid/strong base). The buffer region (±1 unit from pKa) resists pH changes. Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]). Maximum buffer capacity when [A⁻] = [HA] (pH = pKa).
Unlock Acid-Base Chemistry & pH
Plus 148+ other Pro labs covering AP Physics, Biology, Chemistry, Earth Science, and Math — with unlimited simulation time, advanced parameters, and detailed analytics.
Already have an account?
Sign in →What is Acid-Base Chemistry & pH?
pH is the negative log of the hydrogen ion concentration: pH = −log[H+]. A pH of 1 is 100,000 times more acidic than a pH of 6 — small numbers, huge differences. Strong acids (HCl, HNO3) fully dissociate in water; weak acids (acetic acid, citric acid) only partially dissociate, set by their Ka. Titration tracks pH as you add base to an acid (or vice versa) and shows the equivalence point — the moment moles of base equal moles of acid. Buffers are mixtures of weak acid plus its conjugate base that resist pH changes. Adjust Initial Acid Concentration, Acid Strength, and Titrant Volume in this lab, or start from a preset, and watch the pH curve evolve in real time.
Parameters explained
- Initial Acid Concentration(log M)
- Initial Acid Concentration controls the starting amount of acid on a logarithmic molarity scale. Moving it upward means there are more acid particles available before titration begins, so the starting pH usually drops and more base is needed before the curve reaches equivalence. Moving it downward makes the solution more dilute, often raising the initial pH and shrinking the total neutralization demand. Compare the Strong Acid and Weak Acid presets at similar concentration settings to separate the effect of amount from the effect of dissociation strength.
- Acid Strength(log Ka)
- Acid Strength controls log Ka, the logarithmic form of the acid dissociation constant. Larger Ka values mean the acid donates protons more completely, so the initial pH falls and the titration curve looks more like a strong-acid case. Smaller Ka values mean weaker dissociation, a more visible buffer region, and an equivalence point influenced by the conjugate base. Around log Ka = -4.74, the simulation corresponds to acetic acid behavior, where pH changes most gradually near the half-equivalence region.
- Titrant Volume(mL)
- Titrant Volume controls how many milliliters of base have been added to the acid sample. At low values, the curve shows the original acidic solution. As the value increases, base neutralizes acid and the pH changes slowly or quickly depending on Acid Strength. Near equivalence, a small volume change can cause a steep pH jump. Past equivalence, extra base dominates and the solution becomes basic. Use this slider to identify initial pH, buffer region, half-equivalence behavior, equivalence point, and post-equivalence rise.
Common misconceptions
MisconceptionA strong acid has a low pH; a weak acid has a high pH.
CorrectStrength refers to how completely the acid dissociates, not the pH directly. A 0.001 M HCl (strong) has pH 3; a 1 M acetic acid (weak) also has pH around 2.4. Initial Acid Concentration matters as much as Acid Strength.
MisconceptionThe equivalence point of a titration is always at pH 7.
CorrectOnly true for strong acid + strong base. A weak acid + strong base titration has its equivalence point above 7 (the conjugate base is slightly basic). A weak base + strong acid finishes below 7. The equivalence point is where moles balance, not where pH = 7.
MisconceptionA buffer prevents pH from changing at all.
CorrectA buffer resists pH change but doesn't prevent it. Adding small amounts of acid or base shifts pH a little; adding too much exceeds the buffer's capacity and pH crashes or spikes. The Buffer preset helps show the region where pH changes more slowly near pKa.
MisconceptionpH 7 is neutral on every planet, every temperature.
CorrectpH 7 is neutral at 25°C in water. At higher temperatures, water self-ionizes more, so neutral pH drops below 7 (around 6.1 at 100°C). The 'neutral' point is whatever pH equals 0.5 × pKw at that temperature.
How teachers use this lab
- Strong vs. weak comparison: load the Strong Acid preset and the Weak Acid preset, then keep Initial Acid Concentration similar while students compare starting pH, buffer-region visibility, equivalence-point pH, and sharpness of the jump.
- Titration curve reading: have students move the Titrant Volume slider in small increments, mark where pH changes slowly and where it changes rapidly, and connect those observations to neutralization and equivalence.
- Ka and pKa practice: use the Acid Strength slider near log Ka = -4.74, then ask students to explain why the curve has a flatter buffer region around the corresponding pKa.
- Buffer demo: load the Buffer preset, adjust Titrant Volume slightly around the starting point, and ask students why the pH resists change better than in the strong-acid case.
- Indicator selection: compare where the steepest pH jump occurs for strong-acid and weak-acid cases, then discuss why an indicator must change color near the actual equivalence-point pH.
Frequently asked questions
What does pH actually measure?
pH = −log10[H+]. Each pH unit corresponds to a 10× change in hydrogen ion concentration. pH 4 has 10× more H+ than pH 5 and 100× more than pH 6. Pure water at 25°C has [H+] = 10⁻⁷ M, giving pH 7.
What's the difference between strong and weak acids?
Strong acids fully dissociate in water: every HCl molecule splits into H+ and Cl−. Weak acids only partially dissociate; the rest stays as the intact molecule. The equilibrium constant for that dissociation is Ka. In this simulation, use Acid Strength to change log Ka and compare how the curve responds.
What is a buffer and how does it work?
A buffer is a mix of a weak acid and its conjugate base (or weak base + conjugate acid). Adding strong acid: the conjugate base neutralizes most of it. Adding strong base: the weak acid neutralizes it. Net pH change is small until you exhaust one of the buffer components. The Buffer preset starts near pH 4.74 so you can observe that slower-changing region directly.
Why doesn't the equivalence point always land at pH 7?
It depends on the strengths of the acid and base. Strong-strong: pH 7. Weak acid + strong base: pH > 7 (the salt's conjugate base is slightly basic). Strong acid + weak base: pH < 7. Use the type of indicator that changes color near the actual equivalence-point pH.
How does this connect to AP Chemistry?
AP Chem Unit 8 (Acid-Base) expects students to compute pH/pOH, distinguish strong vs. weak acids, predict equivalence-point pH, calculate buffer pH using Henderson-Hasselbalch, and interpret titration curves. Use Initial Acid Concentration, Acid Strength, Titrant Volume, and the three presets to practice those ideas visually.