Oxalic acid dihydrate (H₂C₂O₄•2H₂O) is a crucial chemical reagent widely used as a primary standard in analytical chemistry. Its precise molecular weight is essential for preparing standard solutions and conducting quantitative analyses. This article details the calculation method for oxalic acid dihydrate's molecular weight and explores its applications in standard titration.
Molecular weight represents the sum of the atomic weights of all atoms in a molecule, expressed in grams per mole (g/mol). For oxalic acid dihydrate (H₂C₂O₄•2H₂O), the calculation involves the following steps:
- Identify atoms and their quantities: The molecular formula indicates each molecule contains 2 hydrogen atoms (H), 2 carbon atoms (C), and 6 oxygen atoms (O). The two water molecules (2H₂O) contribute 4 hydrogen atoms and 2 oxygen atoms, while the oxalic acid (H₂C₂O₄) provides 2 hydrogen atoms, 2 carbon atoms, and 4 oxygen atoms.
- Reference atomic masses: Hydrogen (H) has a relative atomic mass of ~1.008 u, carbon (C) ~12.01 u, and oxygen (O) ~16.00 u.
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Calculate molecular weight:
Multiply each atom's mass by its occurrence in the formula and sum the products:
Molecular weight (H₂C₂O₄•2H₂O) = (2 × 1.008) + (2 × 12.01) + (6 × 16.00) = 2.016 + 24.02 + 96.00 = 126.036 g/mol
Thus, oxalic acid dihydrate's molecular weight is approximately 126.07 g/mol. Note that higher-precision atomic mass values yield more accurate results—the slight discrepancy from the theoretical 126 g/mol arises from rounding atomic masses.
Oxalic acid dihydrate serves as a primary standard for calibrating alkaline solutions like sodium hydroxide (NaOH). Standard titration quantitatively determines unknown concentrations through stoichiometric reactions with standardized solutions.
- Accurate weighing: Precisely measure oxalic acid dihydrate using an analytical balance and dissolve it in distilled water.
- Titration: Gradually add the oxalic acid solution from a burette into the NaOH solution. Indicators like phenolphthalein (colorless in acid, pink in base) signal the endpoint when neutralization completes.
- Concentration calculation: The consumed volume of oxalic acid solution and its mass determine NaOH's exact concentration.
The acid-base neutralization follows this equation:
H₂C₂O₄(aq) + 2NaOH(aq) → Na₂C₂O₄(aq) + 2H₂O(l)
One mole of oxalic acid reacts with two moles of NaOH, enabling precise concentration determination.
- Store oxalic acid dihydrate in dry, cool conditions to prevent moisture absorption, which compromises purity.
- Control titration speed near the endpoint to avoid overshooting.
- Conduct replicate titrations for improved accuracy.
As a versatile primary standard, oxalic acid dihydrate plays a vital role in chemical analyses. Mastery of its molecular weight calculation and titration applications ensures reliable quantitative results, forming the foundation for accurate laboratory measurements.