Lab · Acid–Base Titration · AP Chemistry

Graded team lab report: one typed report per team, scored on the shared AP Chem lab rubric. You will standardize a sodium hydroxide solution against a primary standard, use it to find the unknown concentration of a strong acid, and then titrate a weak acid to map a full titration curve and locate its equivalence and half-equivalence points.

CED: Unit 8 (Acids & Bases, 11–15% of the exam) · Topic 8.7 pH curves & Topic 8.8 titrations · Science Practice 5 (analyze data) & Practice 6 (justify with evidence)

Purpose CED 8.7–8.8 · SP 5, 6

Determine the concentration of an acid of unknown molarity by titrating it against a base of exactly known molarity, and use the shape of a titration curve to locate the equivalence point of a strong acid and the equivalence + half-equivalence points of a weak acid.

Background

A titration adds a solution of known concentration (the titrant, here NaOH) from a burette into a measured volume of solution of unknown concentration (the analyte, an acid) until the reaction is exactly complete. That point (where moles of added OH⁻ equal the moles of H⁺ originally present) is the equivalence point.

Strong acid + strong base: HCl + NaOH → NaCl + H₂O. Net: H⁺ + OH⁻ → H₂O. 1 : 1 mole ratio. At equivalence the solution is a neutral salt → pH = 7.

Weak acid + strong base: CH₃COOH + NaOH → CH₃COO⁻ + Na⁺ + H₂O. Still 1 : 1, but the product (acetate) is a weak base, so the equivalence point is basic (pH > 7). Halfway to equivalence, [HA] = [A⁻], so by Henderson–Hasselbalch pH = pK_a. That's the half-equivalence point.

Why standardize? Solid NaOH absorbs water and CO₂ from the air, so a freshly made NaOH solution is never exactly the labelled molarity. We fix its true value by titrating it against a stable, pure solid, a primary standard. We use potassium hydrogen phthalate, KHP (KHC₈H₄O₄, M = 204.22 g/mol), which reacts 1 : 1 with NaOH.

Finding the endpoint: an indicator changes colour over the steep (vertical) part of the curve. Phenolphthalein (colourless → faint pink, ~pH 8.2–10) sits in the vertical region for both a strong-acid and a weak-acid titration against NaOH, so we use it for the colour titrations. For the weak-acid curve, we read pH directly with a pH meter or pH probe every few mL.

Safety read before you start

NaOH (sodium hydroxide) is a strong base and is corrosive: it causes severe eye damage and burns skin. Even dilute (~0.1 M) splashes must be rinsed off at once.

HCl (hydrochloric acid) is corrosive and irritating; avoid skin/eye contact and breathing vapours.

PPE required: splash goggles (not glasses), closed-toe shoes, and a lab apron. Tie back long hair.
If acid or base contacts skin or eyes, flush with running water for 15 minutes and tell Dr. Mulinta immediately; use the eyewash station for eyes.
Wipe up spills right away with plenty of water; neutralize larger base spills as directed before cleanup.
Never pipette by mouth, use a pipette bulb/filler. Keep the burette below eye level when filling.
Wash hands before leaving. No food or drink in the lab.

Materials & Equipment low-gear, school-friendly

Lab tech Kathy will pre-set each station with: a clean 50 mL burette + burette clamp + ring stand, the standardized-strength stock solutions, pre-weighed KHP (or a dry analytical balance), and the shared pH meter/probe (calibrated). Confirm your burette is rinsed and bubble-free before you begin.

Equipment (per team): 50 mL burette & clamp · ring stand · 2 × 250 mL Erlenmeyer flask · 25 mL volumetric pipette + bulb (or a 25 mL graduated cylinder) · 100 mL beaker · small funnel · wash bottle of distilled water · white paper or white tile (to see colour change) · stir rod · analytical balance (shared) · pH meter or pH probe with stand (shared, for the weak-acid curve) · magnetic stirrer + stir bar if available.

Chemicals: ~0.1 M NaOH (titrant, to be standardized) · solid KHP primary standard · HCl of unknown concentration · acetic acid (weak acid) sample · phenolphthalein indicator · distilled water.

Note: if no pipette is available, a 25.0 mL graduated cylinder is acceptable for the acid aliquot, record which you used; it affects your uncertainty.

Procedure

Part A, Standardize the NaOH against KHP

Weigh about 0.50 g of dry KHP into a clean 250 mL Erlenmeyer flask. Record the exact mass to 0.0001 g.
Add ~50 mL distilled water and swirl to dissolve. Add 2–3 drops phenolphthalein (stays colourless).
Fill the burette with the NaOH solution, removing the bubble below the tip. Record the initial burette reading (bottom of the meniscus, to 0.01 mL).
Titrate slowly, swirling constantly, until one drop turns the solution a faint pink that lasts 30 s. Record the final reading.
Repeat for a second (and third, if time) trial. Trials should agree within ~0.10 mL.

Part B, Titrate the unknown HCl (strong acid)

Pipette 25.00 mL of the unknown HCl into a clean flask. Add 2–3 drops phenolphthalein.
Refill and read the burette. Titrate with your standardized NaOH to the same faint-pink endpoint. Record initial and final readings.
Repeat for 2–3 concordant trials.

Part C, Map the weak-acid (acetic acid) titration curve

Pipette 25.00 mL of the acetic-acid sample into a beaker; set up the pH probe and (if available) a stir bar.
Record the starting pH. Add NaOH from the burette in ~1 mL steps, recording volume added vs. pH after each addition.
As pH starts to rise quickly, switch to 0.2–0.5 mL steps to capture the steep region, then return to larger steps past equivalence.
Continue to ~5–10 mL past the equivalence point. You will plot pH (y) vs. volume of NaOH (x).

Data Tables record in ink

Table 1, Standardization of NaOH (Part A)

Quantity	Trial 1	Trial 2	Trial 3
Mass of KHP (g)
Burette final (mL)
Burette initial (mL)
Volume NaOH used (mL)

Table 2, Titration of unknown HCl (Part B)

Quantity	Trial 1	Trial 2	Trial 3
Volume HCl (mL)
Burette final (mL)
Burette initial (mL)
Volume NaOH used (mL)

Table 3, Weak-acid curve (Part C): record Volume NaOH (mL) and pH for every addition on a separate grid; you will plot it.

Graph: plot pH (y-axis) vs. volume NaOH (x-axis) on full-page graph paper, titled, axes labelled with units.

Analysis Questions show all work · sig figs · units

Moles of KHP. Using your exact KHP mass and M = 204.22 g/mol, calculate moles of KHP in each Part-A trial.
Molarity of NaOH. Since KHP : NaOH = 1 : 1, the moles of NaOH equal moles of KHP. Divide by the NaOH volume (in L) to find [NaOH] for each trial, then average. Report to the correct sig figs.
Moles of NaOH used on the HCl. Using your standardized [NaOH] and the average NaOH volume from Part B, find moles of NaOH delivered.
Concentration of the unknown HCl. Write the balanced equation and state the mole ratio. Find moles of HCl, then divide by the HCl volume (L) to get [HCl].
Curve, equivalence point. From your Part-C graph, identify the equivalence-point volume (steepest point / midpoint of the vertical jump). Is its pH above, at, or below 7? Explain in terms of the salt formed.
Curve, half-equivalence & pK_a. Mark the half-equivalence volume (½ of the equivalence volume). Read the pH there. Explain why pH = pK_a at this point, and estimate K_a of the weak acid.
Indicator choice. Why is phenolphthalein a good indicator for both titrations against NaOH? Why would methyl red (changes ~pH 4–6) be a poor choice for the weak-acid titration?
Error analysis. Identify one realistic source of error and state whether it would make your reported acid concentration too high or too low, and why.