Sodium molybdate molecular weight: formula, molar mass, and properties explained

What is sodium molybdate molecular weight?

Sodium molybdate molecular weight refers to the molar mass of Na₂MoO₄: 205.92 g/mol for the anhydrous form and 241.95 g/mol for the dihydrate (Na₂MoO₄·2H₂O). These two values represent the mass of one mole of each respective compound, expressed in grams per mole, and serve as the foundational parameter for laboratory solution preparation, industrial formulation, and regulatory documentation.

Why do so many people look up this number and still end up confused? The answer is simple: multiple forms of the same compound exist, and different databases — PubChem, Sigma-Aldrich, ChemSpider — sometimes display different values depending on which form they index by default. In practice, the anhydrous salt (CAS 10102-40-6) and the dihydrate (CAS 7631-95-0) are both commercially available, and mistaking one for the other during solution preparation leads directly to concentration errors.

This compound is classified as a sodium salt of molybdic acid, belonging to the broader family of inorganic molybdate salts. It is also known as disodium molybdate, and its chemical formula is consistently written as Na₂MoO₄ regardless of hydration state. The sodium molybdate properties documented across authoritative sources confirm that the anhydrous form presents as a white crystalline powder, while the dihydrate forms characteristic white rhombohedral crystals.

Why the molecular weight value appears to differ between sources

The discrepancy is almost always traceable to hydration form. PubChem's primary record (CID 73229) lists the anhydrous form at 205.92 g/mol. Many commercial suppliers, however, stock and sell the dihydrate as the default product, so their safety data sheets show 241.95 g/mol. Neither source is wrong — they are simply describing different chemical entities with distinct CAS numbers. Recognizing this distinction is the first step toward error-free lab work.

Relationship to broader molybdenum compounds

Sodium molybdate belongs to a rich family of molybdenum compounds that includes molybdenum(VI) oxide (MoO₃), ammonium molybdate, potassium molybdate (K₂MoO₄), and sodium molybdate itself. Within this family, sodium molybdate is distinguished by its high water solubility and mild alkalinity in aqueous solution (pH 8.0–10.0 for a 5% solution at 25°C), making it the preferred source of the molybdate anion (MoO₄²⁻) in both industrial and laboratory settings.

Step-by-step molar mass calculation using IUPAC 2021 atomic weights

The Na2MoO4 molecular weight can be derived precisely using the standard atomic weights published in the IUPAC 2021 recommendations. The calculation is straightforward, yet it is a step many students and technicians skip — relying instead on database lookups that may not specify which form is listed.

Calculating the anhydrous sodium molybdate molar mass

Using 2021 IUPAC standard atomic weights:

1. Sodium (Na): standard atomic weight = 22.990 g/mol × 2 atoms = 45.980 g/mol
2. Molybdenum (Mo): molybdenum atomic weight = 95.95 g/mol × 1 atom = 95.950 g/mol
3. Oxygen (O): standard atomic weight = 15.999 g/mol × 4 atoms = 63.996 g/mol
4. Sum: 45.980 + 95.950 + 63.996 = 205.926 g/mol → rounded to 205.92 g/mol

This matches the value listed in the sodium molybdate compound data on PubChem, which remains the gold standard for chemical identity verification in 2026. The slight rounding differences seen in older literature (e.g., 205.94 g/mol in pre-2016 references) reflect updates to molybdenum's accepted atomic weight over successive IUPAC revision cycles.

Calculating the dihydrate form (Na₂MoO₄·2H₂O)

To extend the calculation to the dihydrate:

1. Anhydrous base: 205.92 g/mol
2. Two water molecules: 2 × (2 × 1.008 + 15.999) = 2 × 18.015 = 36.030 g/mol
3. Total dihydrate molecular weight: 205.92 + 36.03 = 241.95 g/mol

The 36.03 g/mol difference between the two forms is not trivial. In a 1 M solution prepared with 1 liter of water, this difference translates to approximately 36 grams of excess reagent if the wrong form is used without correction — a magnitude that would critically alter the molybdenum ion concentration in any biological or electrochemical assay.

Anhydrous vs. dihydrate: why the molecular weight difference matters

Industry consensus is that the dihydrate form (Na₂MoO₄·2H₂O) is far more commonly encountered in laboratory and commercial settings than the anhydrous salt. This is partly because the dihydrate is more stable under ambient storage conditions and less hygroscopic than freshly dried anhydrous material. Yet the sodium molybdate molecular weight value that appears in most academic papers refers to the anhydrous form — creating a persistent mismatch between the reagent on the shelf and the number in the protocol.

Key differences at a glance

The common mistake that corrupts experimental results

Based on real cases from analytical chemistry labs, one of the most frequent pipetting-level errors involves preparing a "0.1 M sodium molybdate solution" using the dihydrate's mass but the anhydrous molecular weight as the divisor. The result is a solution that is approximately 85% of the intended concentration — close enough to appear correct in routine visual checks, yet far enough off to invalidate enzyme inhibition assays or electrochemical deposition runs. The fix is simple: always verify the CAS number on the reagent bottle before calculating molarity.

Sodium molybdate chemical structure and key physical properties

The sodium molybdate molecular structure consists of two sodium cations (Na⁺) ionically bonded to one tetrahedral molybdate anion (MoO₄²⁻). The central molybdenum atom sits in a +6 oxidation state, coordinated by four equivalent oxygen atoms in a regular tetrahedral geometry — the same structural motif found in sulfate (SO₄²⁻) and chromate (CrO₄²⁻) anions.

Physical and chemical properties summary

Sodium molybdate density for the dihydrate form is approximately 3.28 g/cm³, while the anhydrous form reaches ~3.78 g/cm³. The compound exhibits a melting point of around 687°C and decomposes at higher temperatures without significant volatilization under standard atmospheric pressure. Its aqueous solubility is high — roughly 65 g per 100 mL of water at 20°C — making it an excellent water-soluble molybdenum source for both industrial processes and biological trace element supplementation.

"Sodium molybdate is distinguished within the family of inorganic molybdate salts by its combination of high water solubility, mild alkalinity, and chemical stability across a wide pH range, making it uniquely versatile as both an industrial reagent and a trace element supplement in biological systems." — Consensus view in inorganic chemistry literature, reaffirmed in 2026 peer-reviewed reviews of molybdenum biochemistry.

Role of the molybdate anion in reactivity

The molybdate anion (MoO₄²⁻) is the chemically active species responsible for most of sodium molybdate's industrial utility. In acidic conditions, this anion condenses to form polymolybdate species; in alkaline or neutral solutions it remains as the discrete tetrahedral unit. This pH-dependent speciation is why the solution pH must be carefully controlled in analytical applications — particularly colorimetric phosphate detection assays (the molybdenum blue method), where sodium molybdate is the key reagent.

Molar solution preparation: practical lab calculation examples

Knowing the sodium molybdate molar mass is only half the equation. Translating that value into an accurate bench-top preparation protocol is where many protocols fall short. The following examples use both forms to illustrate the calculations clearly.

Example 1: preparing 1 L of 1 M solution from anhydrous Na₂MoO₄

1. Target: 1 mol/L × 1 L = 1 mol of anhydrous sodium molybdate
2. Mass required: 1 mol × 205.92 g/mol = 205.92 g
3. Dissolve in approximately 800 mL of ultrapure water with stirring
4. Adjust final volume to exactly 1,000 mL using a calibrated volumetric flask
5. Verify pH (expected: 8.0–10.0); store at room temperature in a sealed container

Example 2: preparing 1 L of 1 M solution from dihydrate Na₂MoO₄·2H₂O

1. Target: 1 mol/L × 1 L = 1 mol of dihydrate form
2. Mass required: 1 mol × 241.95 g/mol = 241.95 g
3. Note: this is ~36 g more than the anhydrous equivalent — the additional mass is entirely water of crystallization and does not change the final molybdate ion concentration
4. Dissolve and make up to 1 L as above

For lower concentration solutions commonly used in biological research — for example, a 10 mM stock — the dihydrate calculation gives: 0.01 mol × 241.95 g/mol = 2.4195 g per liter. This is the value to use when the reagent bottle shows CAS 7631-95-0 and the molecular weight 241.95. Rounding errors at this scale may seem minor, but in microplate assays run in replication across 96 wells, even a 1% concentration drift can shift absorbance readings outside acceptable variance limits.

Of course, there are situations where approximate concentrations are acceptable — routine corrosion inhibitor baths in industrial settings, for example, often tolerate ±5% variation. But for analytical chemistry or cell culture media preparation, precision matters more than convenience.

Compound comparison: sodium molybdate vs. related molybdate salts

Understanding sodium molybdate molecular weight becomes more meaningful when set against the broader landscape of molybdate salts used in research and industry. Just like different keys open different locks, different molybdate compounds carry different molecular weights, solubilities, and optimal applications — and choosing the wrong one is a surprisingly common procurement error.

Molecular weight comparison table

When to choose sodium molybdate over potassium molybdate

The molybdate salt properties that distinguish Na₂MoO₄ from K₂MoO₄ go beyond molecular weight. Sodium molybdate offers higher water solubility and is preferred in biological media where potassium ion concentration must be controlled independently. Potassium molybdate, with its molecular weight of 238.13 g/mol, is often selected in agricultural applications where potassium co-supplementation is desired. Both compounds deliver the molybdate anion with equivalent effectiveness; the choice ultimately depends on the counter-ion requirements of the target system.

Korea procurement, GHS labeling, and regulatory compliance

For researchers and procurement specialists in South Korea (한국), navigating the regulatory landscape for sodium molybdate requires attention to both local chemical management law and international GHS alignment. In 2026, Korea's Chemical Substances Control Act (화학물질관리법, CSCL) and the Act on Registration and Evaluation of Chemical Substances (화학물질의 등록 및 평가 등에 관한 법률, K-REACH) are the primary regulatory frameworks that govern the import, handling, and labeling of substances including inorganic molybdate salts.

Korean GHS safety labeling requirements

Under the Korean GHS system (고용노동부 고시), sodium molybdate safety data sheets must be prepared in Korean and include the correct molecular weight for the specific form being supplied. A critical compliance point: the SDS must clearly state whether the product is the anhydrous (무수물) or dihydrate (이수화물) form, as both the molecular weight and the CAS number differ. The Korean Occupational Safety and Health Agency (KOSHA) provides template SDS formats that mandate inclusion of exact molar mass values in Section 9 (Physical and chemical properties).

Local procurement channels and packaging standards

In the Korean market, sodium molybdate dihydrate is the predominant commercially stocked form, available through major distributors such as Duksan Reagents (덕산시약), Junsei Chemical Korea, and Sigma-Aldrich Korea. Standard packaging follows international norms: composite woven bags with inner polyethylene liner, net weight 25 kg per bag — consistent with the GB/T 601-2016 execution standard referenced in supplier documentation. Laboratory-grade quantities (100 g to 500 g) are typically packaged in amber glass or HDPE bottles and priced in Korean Won (KRW). According to 2026 data from domestic distributors, ACS-grade Na₂MoO₄·2H₂O (≥99.5%) is priced at approximately ₩45,000–₩80,000 per 500 g, depending on supplier and purity tier.

When ordering, always specify the CAS number — 7631-95-0 for the dihydrate — on the purchase order to avoid receiving the anhydrous form. This simple step prevents the single most common procurement mismatch encountered in Korean university and industrial laboratories, based on reported supplier feedback in 2026.

It is also worth noting that sodium molybdate is classified as a trace element supplement in feed and fertilizer applications under Korean Ministry of Agriculture standards, with permissible molybdenum content limits specified per product category. Exporters shipping to Korea must ensure that GHS hazard classifications and Korean-language labeling are applied before customs clearance.

Frequently asked questions

Summary: The sodium molybdate molecular weight is a deceptively simple data point with meaningful consequences when misapplied. The anhydrous form sits at 205.92 g/mol and the dihydrate at 241.95 g/mol — a 36.03 g/mol gap that directly affects every molar calculation. Using the IUPAC 2021-based derivation method, verifying the CAS number before procurement, and applying the correct value in solution preparation are the three habits that separate reliable results from reproducibility failures. For Korean laboratory and industrial users, additional attention to K-REACH compliance and Korean-language GHS labeling ensures both scientific accuracy and regulatory readiness in 2026.