Molar Mass of Potassium Hydrogen Phthalate (KHC₈H₄O₄)

Each row shows how much mass one element contributes to the total for KHC₈H₄O₄.

Save a printable summary, revision sheet, practice worksheet, or laboratory reference for Potassium Hydrogen Phthalate (KHC₈H₄O₄).

Without looking above, write the atom count for KHC₈H₄O₄, then compute the molar mass. Check your answer against 204.222 g/mol.

Next challenge: how many grams are in 0.250 mol of Potassium Hydrogen Phthalate? Multiply 0.250 × 204.222 to get 51.056 g.

Preparation and production

KHP is not typically prepared in a teaching laboratory; it is purchased as a certified, high-purity crystalline solid (often NIST-traceable) and used directly after gentle drying (heating at ~110 °C) to remove any adsorbed surface moisture before precise weighing.

Commercially manufactured by partial neutralization of phthalic acid or phthalic anhydride with potassium hydroxide or potassium carbonate, followed by recrystallization and rigorous purification to achieve the high purity required for analytical-grade primary standard material.

Phthalic acid derivatives were characterized in the 19th century alongside broader advances in aromatic organic chemistry following Kekulé's benzene structure proposal. Potassium hydrogen phthalate's adoption as the standard primary acidimetric reference emerged in the 20th century as analytical chemistry formalized rigorous, traceable standardization protocols for volumetric analysis.

Phthalic acid and its salts were characterized during 19th-century advances in aromatic chemistry; KHP's specific role as an analytical primary standard was established through 20th-century standardization of titrimetric methods.

Interesting facts

• KHP's molar mass of 204.22 g/mol is deliberately large among common laboratory acids, which is exactly why it minimizes relative weighing error on an analytical balance.
• Despite being an organic aromatic compound, KHP is treated in most general chemistry courses purely as 'the acid standard,' with its benzene ring structure often glossed over in introductory titration exercises.
• NIST issues KHP as Standard Reference Material 84k/84l, certified to extremely tight purity tolerances for use in high-precision acidimetric and coulometric titrations.
• Because KHP does not absorb atmospheric moisture, a bottle can sit open briefly during weighing without meaningfully changing its mass — a rare and valued property among solid laboratory reagents.

Comparison with similar compounds

KHP (204.22 g/mol) is preferred over simpler acids like HCl or acetic acid as a primary standard precisely because it is a stable, non-hygroscopic solid with a large molar mass, whereas HCl cannot be weighed directly as a pure solid at all.

Store in a tightly sealed container in a cool, dry location. Although non-hygroscopic, KHP should still be protected from contamination and, before use as a primary standard, is typically dried in an oven (~110 °C) and cooled in a desiccator to ensure an accurately known, moisture-free mass.

Low toxicity; treat as a standard laboratory organic solid with routine precautions (gloves, avoid dust inhalation). No special hazard controls beyond standard laboratory practice are required for typical titration use.

Low-hazard solid used routinely in teaching and analytical laboratories; primary safety considerations are standard good laboratory practice rather than specific chemical danger.

• Mild eye and skin irritation from dust in bulk handling
• No significant reactivity or flammability hazard under normal conditions

Classification: Not classified as hazardous under GHS for typical laboratory quantities

1. 1. Calculate the molar mass of KHC₈H₄O₄ (KHP).

   Show answer

   39.10 + 5(1.008) + 8(12.01) + 4(16.00) = 204.22 g/mol

2. 2. A 0.6127 g sample of KHP requires 28.45 mL of NaOH solution to reach the phenolphthalein endpoint. What is the molarity of the NaOH solution?

   Hint: Use moles KHP = moles NaOH at the equivalence point, then divide by volume in liters.

   Show answer

   0.6127 g ÷ 204.22 g/mol = 0.003001 mol KHP = 0.003001 mol NaOH (1:1 ratio); 0.003001 mol ÷ 0.02845 L = 0.1055 M

3. 3. Why is KHP preferred over solid NaOH pellets for preparing a solution of precisely known concentration?

   Show answer

   NaOH pellets rapidly absorb atmospheric moisture and CO₂, making their true mass of pure NaOH uncertain, whereas KHP is non-hygroscopic, stable, and highly pure, so a weighed mass accurately reflects the moles of acid present.

4. 4. Why does a larger molar mass make KHP a better primary standard than a hypothetical acid with molar mass 40 g/mol, all else equal?

   Show answer

   For the same number of moles, a larger molar mass requires weighing a larger mass of solid, which reduces the relative uncertainty introduced by the balance's fixed absolute weighing error.

• Sodium HydroxideNaOH
• Hydrochloric AcidHCl
• Sulfuric AcidH₂SO₄
• Acetic AcidC₂H₄O₂
• Potassium HydroxideKOH

The sections above give the number you need for calculations. Here we look more closely at how Potassium Hydrogen Phthalate (KHC₈H₄O₄) behaves chemically—so the molar mass connects to real reactions, properties, and laboratory practice.

Potassium hydrogen phthalate (KHC₈H₄O₄, universally abbreviated KHP) has a molar mass of 204.22 g/mol, making it the gold-standard primary standard for acid-base titrations in analytical chemistry. It is the monopotassium salt of phthalic acid (benzene-1,2-dicarboxylic acid), retaining one free carboxylic acid group (–COOH) while the other has been neutralized to a carboxylate salt (–COO⁻K⁺), giving KHP the acidic behavior of a weak monoprotic organic acid.

What makes KHP indispensable in the analytical laboratory is not any exotic chemistry but a combination of mundane, highly desirable physical properties: it is available in very high purity (often >99.95%), is not hygroscopic (does not absorb atmospheric moisture, unlike NaOH pellets or many hydrates), has a large, precisely known molar mass that minimizes relative weighing error, is stable and non-volatile at room temperature, and dissolves readily and completely in water. These properties let a chemist weigh out KHP on an ordinary analytical balance, dissolve it in water, and know the exact number of moles of acid present without further purification or drying — the essential requirement for a "primary standard."

KHP is used specifically to standardize solutions of strong bases, most commonly sodium hydroxide, whose concentration cannot be determined accurately by mass alone because solid NaOH pellets absorb atmospheric moisture and CO₂ almost immediately upon exposure to air. By titrating a precisely weighed mass of KHP against an NaOH solution to a phenolphthalein endpoint, the exact molarity of the base can be determined and then used with confidence in subsequent titrations of unknown acids or bases.

KHC₈H₄O₄ (or more explicitly, KOOC–C₆H₄–COOH) shows a benzene ring bearing two carboxyl-derived groups in the ortho (1,2) position: one has lost its proton and formed an ionic bond to K⁺ (–COO⁻K⁺), while the other retains its acidic hydrogen (–COOH). Because only one acidic proton remains available for titration, KHP behaves as a monoprotic acid in standardization reactions, even though it derives from diprotic phthalic acid.

KHP reacts stoichiometrically and completely with strong bases in a simple 1:1 acid-base neutralization: KHC₈H₄O₄ + NaOH → KNaC₈H₄O₄ + H₂O. It is weakly acidic (pKa ≈ 5.4 for the remaining carboxyl group) and gives a sharp, easily detected endpoint with phenolphthalein indicator near pH 8–9. Unlike many organic acids, it does not sublime, decompose, or absorb water under normal laboratory conditions, preserving titration accuracy.

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• NIST Standard Reference Material 84k/84l: Certified KHP primary standard specifications
• PubChem CID 23676735: Potassium hydrogen phthalate identifiers and properties
• IUPAC: Nomenclature of phthalate salts and primary standards in titrimetry