CHM 152L — Lab 6

Standardization of NaOH & Titration of a Weak Acid

Lab Summary

In this lab, you will:

Part 1: Standardize a NaOH solution by titrating it against KHP (potassium hydrogen phthalate), a primary standard.

Part 2: Use the standardized NaOH to titrate an unknown weak acid, generating a titration curve to determine the acid's molar mass and Ka.

Materials Reference

Reagent / Equipment Details
~0.1 M NaOH Sodium hydroxide solution (to be standardized)
KHP Potassium hydrogen phthalate (MW = 204.22 g/mol) — primary standard
Phenolphthalein Color indicator: colorless → pink at pH 8.2–10
Benzoic acid Weak acid (MW = 122.12 g/mol, Ka = 6.3 × 10−5)
Burette For precise volume delivery of NaOH (read to ±0.02 mL)

Key Reactions

Part 1 — KHP Standardization:

KHC8H4O4(aq) + NaOH(aq) → KNaC8H4O4(aq) + H2O(l)

Part 2 — Unknown Weak Acid:

HA(aq) + NaOH(aq) → NaA(aq) + H2O(l)

Procedure Flowchart

Part 1: Standardization of NaOH

Weigh NaOH→ Make ~0.1 M solution→ Weigh KHP→ Dissolve in water→ Titrate with NaOH→ Record pH→ Find inflection point→ Calculate [NaOH]→ Repeat

Part 2: Titration of Unknown Acid

Weigh acid→ Dissolve→ Titrate with std NaOH→ Record pH→ Plot curve→ Find equiv + half-equiv→ Calculate molar mass + Ka

Key Concepts

1. What is Standardization?▼

NaOH absorbs moisture and CO2 from air, so its true concentration is uncertain after preparation. Standardization means determining the exact concentration by titrating against a primary standard — a substance pure enough to weigh directly and use as a reference.

KHP is ideal because:

  • Stable solid at room temperature — does not absorb water or decompose, so you can trust the mass you weigh.
  • High molecular weight (204.22 g/mol) — you use more grams per mole, which reduces the percentage error from the balance.
  • Reacts 1:1 with NaOH — makes the stoichiometry straightforward: moles KHP = moles NaOH at equivalence.
  • Available in very high purity — you can buy it at 99.95%+ purity as a certified reference material.
2. pH and pOH▼
Core Relationships
pH = −log[H+]
pOH = −log[OH−]
pH + pOH = 14.00
Kw = [H+][OH−] = 1.0 × 10−14

Quick reference: A low pH (< 7) means acidic (high [H+]). A high pH (> 7) means basic (high [OH−]). Every 1-unit change in pH represents a 10-fold change in [H+].



Enter a value above to calculate pH, pOH, and more.
3. Strong vs. Weak Acid Titration Curves▼
Feature Strong Acid + Strong Base Weak Acid + Strong Base
Initial pH Low (depends on [HA]) Higher than strong acid at same concentration (partial ionization)
Buffer region None Yes — flat region where pH changes slowly
Equivalence pH Exactly 7.00 Greater than 7 (conjugate base hydrolyzes)
Half-equivalence Not meaningful pH = pKa
Curve shape near equiv Very steep, symmetric Steep but asymmetric (shifted right/up)
Best indicator Bromothymol blue (pH 6–8) Phenolphthalein (pH 8.2–10)

The key difference: the conjugate base of a strong acid (e.g., Cl−) does not hydrolyze, so the equivalence point sits at pH 7. The conjugate base of a weak acid (e.g., benzoate, C6H5COO−) does hydrolyze, pulling the equivalence point above 7.

4. The Half-Equivalence Point▼

At this point, exactly half the acid has been neutralized, so the moles of HA remaining equal the moles of A− formed: [HA] = [A−].

Henderson-Hasselbalch Equation
pH = pKa + log([A−] / [HA])
At half-equivalence: pH = pKa + log(1) = pKa + 0 = pKa

This is the simplest way to find Ka experimentally: read the pH at the half-equivalence volume, and that pH is the pKa. Then Ka = 10−pKa.

How to find the half-equivalence volume: Determine the equivalence volume (where the curve is steepest), then divide by 2. Read the pH at that volume on your titration curve.

5. Ka and Kb Relationship▼
Conjugate Acid-Base Relationship
Ka × Kb = Kw = 1.0 × 10−14

Example: If Ka for acetic acid = 1.8 × 10−5, then the Kb for the acetate ion:

Kb = Kw / Ka = (1.0 × 10−14) / (1.8 × 10−5) = 5.6 × 10−10

A stronger acid (larger Ka) produces a weaker conjugate base (smaller Kb), and vice versa. This relationship controls the pH at the equivalence point during a weak acid/strong base titration.

6. Common Mistakes▼
Mistake Correction
"The equivalence point is always pH 7." Only true for strong acid + strong base. For weak acid + strong base, the equivalence point is above pH 7 because the conjugate base hydrolyzes.
"Using total solution volume instead of NaOH volume to calculate moles." moles NaOH = [NaOH] × VNaOH only. The total volume is irrelevant for moles — only the volume delivered from the burette matters.
"Phenolphthalein endpoint = equivalence point." They are close but not identical. The indicator changes color over a pH range (8.2–10), while the equivalence point is one specific pH. For weak acid + strong base, phenolphthalein works well because the equivalence pH falls in this range.
"Forgetting to rinse the burette with NaOH before filling." Residual water in the burette dilutes the NaOH, lowering its effective concentration. You will deliver more volume to reach equivalence, making your calculated [NaOH] appear lower than it actually is.
"Reading the burette at the top of the meniscus." Always read at the bottom of the meniscus for aqueous solutions. Reading from the top introduces a systematic error that makes your volumes too low.

Interactive Titration Simulator



pH = — Move the slider to begin
Select an acid and move the slider to explore the titration curve. Key points will be highlighted and explained as you go.

Calculation Practice

Calculator 1: NaOH Preparation

How much solid NaOH do you need to weigh?

Calculator 2: KHP Standardization

Determine the exact [NaOH] using your KHP titration data.

Calculator 3: Molar Mass Determination

Find the molar mass of the unknown acid from titration data.

Calculator 4: Ka Determination

Find Ka from the pH at the half-equivalence point.

Calculator 5: pH / pOH Converter

Enter any ONE value — the rest will be calculated.

Post-Lab Quiz

Test your understanding. Select an answer for each question, then click "Check" to see if you're correct.