Chemistry Practical — Acid-Base Titration

Objectives: Chemistry Practical — Acid-Base Titration

Chemistry Practical — Acid-Base Titration (Embedded Video + Q&A + SVG Graph)

Chemistry Practical — Acid–Base Titration (Phenolphthalein)

Selected demo (YouTube)

This demo shows an acid-base titration using phenolphthalein as the indicator and demonstrates the endpoint (colour change). Video used as the worked practical reference. :contentReference[oaicite:1]{index=1}

Practical Question (exam-style)

Given: 25.00 mL of 0.1000 M hydrochloric acid (HCl) is titrated with a sodium hydroxide solution (NaOH) of unknown concentration. Phenolphthalein is used as the indicator. The volume of NaOH required to reach the endpoint is 24.50 mL.

Required

  1. Calculate the molarity of the NaOH solution (show full working).
  2. Explain the colour change observed at the endpoint and why phenolphthalein is suitable in this titration.
  3. Sketch (and interactively view) the titration curve (pH vs volume NaOH added) for this titration.

Compact Answer (quick view)

1) Molarity of NaOH:

Moles of HCl = (0.02500 L) × (0.1000 mol·L⁻¹) = 0.002500 mol.
At endpoint, moles HCl neutralised = moles NaOH added, so moles NaOH = 0.002500 mol.
Volume NaOH = 24.50 mL = 0.02450 L.
Molarity NaOH = moles / volume = 0.002500 mol / 0.02450 L = 0.10204 M (4 significant figures).

2) Colour change & indicator choice:

Phenolphthalein is colourless in acidic solution and turns pink as the solution becomes basic; it changes around pH ~8.2–10.0. In a strong acid vs strong base titration the pH near the equivalence point jumps rapidly from acidic to basic, so phenolphthalein produces a sharp visible change at the endpoint. This makes it suitable here. :contentReference[oaicite:2]{index=2}

3) Practical notes & tips (short):

  • Swirl constantly while adding titrant; add indicator to the analyte before titration.
  • Near the endpoint add dropwise and mix — endpoint is the first permanent pale pink that persists ≈30 s.
  • Perform at least two concordant titrations (within 0.10 mL) and average volumes for accuracy.

Detailed Step-by-Step Working (full marks style)

  1. Write the neutralisation reaction: HCl + NaOH → NaCl + H₂O.
  2. Compute moles of HCl:
    moles HCl = 0.02500 L × 0.1000 mol·L⁻¹ = 0.002500 mol.
  3. Moles NaOH at endpoint = moles HCl = 0.002500 mol (1:1 stoichiometry).
  4. Convert burette reading to litres: 24.50 mL = 0.02450 L.
  5. Final calculation: M(NaOH) = 0.002500 / 0.02450 = 0.102040816... ≈ 0.10204 M (4 s.f.).
  6. State your answer with units and significant figures: 0.1020 M NaOH (or 0.10204 M depending on required sig figs).

Interactive Titration Curve (pH vs volume NaOH added)

The small interactive plot below uses the experimental values above (25.00 mL of 0.1000 M HCl; NaOH = 0.10204 M calculated). It computes the pH as NaOH volume increases from 0 to 50 mL and draws the curve. Drag the vertical "cursor" to read pH at a chosen volume.

How works (brief):
  • It treats HCl as a strong acid and NaOH as a strong base (1:1 neutralisation).
  • For each added volume of NaOH, the script computes moles acid and base, finds net [H⁺] or [OH⁻] and converts to pH.
  • At equivalence (where moles acid = moles base) pH ≈ 7.0 for strong acid–strong base titration (pure water autoionization considered ~7).

Extended Notes & Theory — short

  • Purpose: Titration finds unknown concentration by neutralising a known volume of analyte to an endpoint with an indicator or pH meter. :contentReference[oaicite:3]{index=3}
  • Indicator: Phenolphthalein is commonly used for strong acid vs strong base titrations because its colour change occurs where the titration curve is steep, giving a sharp endpoint. :contentReference[oaicite:4]{index=4}
  • Accuracy tips: Standardise NaOH (it absorbs CO₂ from air) against a primary standard (e.g., oxalic acid) before use when high accuracy is needed — common lab practice. :contentReference[oaicite:5]{index=5}
Video used: embedded YouTube demonstration. Background reading: acid–base titration & phenolphthalein (wikipedia). :contentReference[oaicite:6]{index=6}

Reference Book: N/A

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