Molar Mass of Sodium Phosphate (Na₃PO₄)
Learn how chemists calculate the molar mass of Sodium Phosphate (Na₃PO₄), with a clear formula breakdown, worked steps, and study notes · IUPAC name: Trisodium phosphate.
Quick answer
The molar mass of Sodium Phosphate (Na₃PO₄) is
163.940g/mol
One mole of Sodium Phosphate therefore has a mass of 163.940 grams—the value you use for stoichiometry and laboratory preparation.
Reviewed for educational accuracy · Accuracy policy
- CAS Registry Number
- 7601-54-9
- PubChem CID
- 24243
- SMILES
- [Na+].[Na+].[Na+].[O-]P(=O)([O-])[O-]
Step-by-step calculation
Let's find the molar mass of Sodium Phosphate (Na₃PO₄) together—step by step, as if you are seeing the formula for the first time.
Step 1 — Look at the chemical formula
The formula is Na₃PO₄. Each letter stands for an element. The little number after a letter (the subscript) tells you how many atoms of that element are in one molecule or formula unit.
- 3 Sodium atoms (Na)
- 1 Phosphorus atom (P)
- 4 Oxygen atoms (O)
Step 2 — Look up each atomic mass
Atomic mass comes from the periodic table. It is the average mass of one mole of atoms of that element, in grams per mole (g/mol). Think of it as the "price tag" for one mole of that element.
- Sodium (Na) = 22.990 g/mol
- Phosphorus (P) = 30.974 g/mol
- Oxygen (O) = 15.999 g/mol
Step 3 — Multiply atoms × atomic mass
Why multiply? If one oxygen atom "costs" about 16 g/mol, then two oxygen atoms cost twice as much. Each element's contribution is: number of atoms × atomic mass.
- 3 × 22.990 = 68.970 g/mol (Sodium)
- 1 × 30.974 = 30.974 g/mol (Phosphorus)
- 4 × 15.999 = 63.996 g/mol (Oxygen)
Step 4 — Add the contributions
Why add? The molar mass of the whole compound is simply the total mass of every atom in the formula. Add each element's contribution:
68.970 + 30.974 + 63.996 = 163.940 g/mol
Step 5 — Final answer
Molar mass of Sodium Phosphate = 163.940 g/mol
That means one mole of Sodium Phosphate (Na₃PO₄) has a mass of about 163.94 grams.
Quick summary
Read the formula → count atoms → look up atomic masses → multiply → add → report g/mol. For Na₃PO₄, the total is 163.940 g/mol.
Common beginner mistakes
- Confusing trisodium phosphate (Na₃PO₄) with disodium phosphate (Na₂HPO₄) or monosodium phosphate (NaH₂PO₄) — different substitution levels with very different pH behavior.
- Assuming all 'sodium phosphate' products are strongly alkaline — only the tribasic (trisodium) form gives the notably high pH; the mono- and disodium forms are much milder.
- Overlooking that food-grade and industrial-grade sodium phosphate serve very different purposes and purity standards.
Memory trick
Track how many of phosphoric acid's three protons are replaced by sodium to correctly identify mono-, di-, or trisodium phosphate.
Mini practice
Without looking above, list the atoms in Na₃PO₄ and write one multiplication line for the heaviest element. Then check your work against Step 3.
Real-world example
If a recipe asks for 0.100 mol of Sodium Phosphate, mass needed = 0.100 × 163.940 = 16.394 g. That is how chemists turn a mole amount into a weighable sample.
Atomic contribution table
Each row shows how much mass one element contributes to the total for Na₃PO₄.
| Element | Atoms | Atomic mass | Contribution | Mass % |
|---|---|---|---|---|
| Na | 3 | 22.990 | 68.970 g/mol | 42.1% |
| P | 1 | 30.974 | 30.974 g/mol | 18.9% |
| O | 4 | 15.999 | 63.996 g/mol | 39.0% |
| Total molar mass | 163.940 g/mol | 100% | ||
Mass contribution chart
Teal and blue circles alternate like positive and negative ions in a crystal lattice. Formula mass is the mass of one formula unit, not a single molecule.
Download study sheets
Save a printable summary, revision sheet, practice worksheet, or laboratory reference for Sodium Phosphate (Na₃PO₄).
Practice this calculation
Without looking above, write the atom count for Na₃PO₄, then compute the molar mass. Check your answer against 163.940 g/mol.
Next challenge: how many grams are in 0.250 mol of Sodium Phosphate? Multiply 0.250 × 163.940 to get 40.985 g.
Physical and chemical properties
Physical properties
| Appearance | White crystalline powder or granules |
| Color | White |
| Odor | Odorless |
| State (STP) | Solid |
| Density | 1.62 g/cm³ (anhydrous) |
| Melting point | 1,583 °C (anhydrous, decomposes before this in practice) |
| Boiling point | Decomposes before boiling |
| Solubility | 120 g/L water at 25 °C (soluble, strongly alkaline solution) |
| Crystal structure | Hexagonal (anhydrous); various hydrate forms also common commercially |
Chemical properties
| Classification | Ionic phosphate salt / alkali metal orthophosphate |
| Family | Group 1 orthophosphate (alkali metal phosphate) |
| Basicity | Strongly basic (phosphate ion hydrolyzes water significantly, pH ~12 concentrated) |
| Polarity | Ionic |
| Oxidation states | Na: +1, P: +5, O: −2 |
Applications
Industrial uses
- Heavy-duty cleaning and degreasing compound (TSP wall cleaner, industrial degreasers)
- Water treatment and boiler water conditioning (scale and corrosion control)
- Food additive: emulsifier, leavening agent, pH adjuster (E339)
- Textile processing and dyeing auxiliary
Laboratory uses
- Phosphate buffer preparation (with mono- and disodium phosphate) for near-physiological pH
- Precipitation of metal phosphates in qualitative and quantitative analysis
- Reference compound for polyprotic acid/base neutralization demonstrations
Contributes to eutrophication if released into waterways in significant quantities, prompting phosphate restrictions in many household detergent formulations.
The related phosphate buffer system is essential in biochemistry and clinical laboratory work for maintaining near-physiological pH in experimental solutions and diagnostic reagents.
Preparation and production
Prepared by neutralizing phosphoric acid with sodium hydroxide or sodium carbonate in a 3:1 mole ratio to fully substitute all three acidic protons: H₃PO₄ + 3 NaOH → Na₃PO₄ + 3 H₂O. Partial neutralization with fewer equivalents of base yields the mono- or disodium phosphate salts instead.
Produced industrially at significant scale for cleaning, water treatment, and food-grade applications; production is closely tied to phosphoric acid supply chains derived from phosphate rock processing.
Important reactions of Sodium Phosphate
Na₃PO₄(aq) + HCl(aq) → Na₂HPO₄(aq) + NaCl(aq)
- Reaction type
- Acid–base neutralization (stepwise, first proton)
- Conditions
- Aqueous, controlled acid addition
- Explanation
- The first equivalent of acid converts trisodium phosphate to disodium phosphate, the first step in the stepwise neutralization of the phosphate ion.
- Products
- Disodium phosphate and sodium chloride
- Why it matters
- Buffer system preparation, stepwise titration demonstrations
Related ideas: Polyprotic bases · Stepwise neutralization · Buffer chemistry
3 CaCl₂(aq) + 2 Na₃PO₄(aq) → Ca₃(PO₄)₂(s) + 6 NaCl(aq)
- Reaction type
- Precipitation (double displacement)
- Conditions
- Aqueous, room temperature
- Explanation
- Sodium phosphate precipitates insoluble calcium phosphate from hard water, a reaction relevant to historical water-softening detergent formulations.
- Products
- Calcium phosphate precipitate and sodium chloride
- Why it matters
- Water softening, phosphate removal demonstrations
Related ideas: Precipitation reactions · Solubility rules · Water hardness
PO₄³⁻(aq) + H₂O(l) ⇌ HPO₄²⁻(aq) + OH⁻(aq)
- Reaction type
- Dissolution and hydrolysis (basic salt behavior)
- Conditions
- Aqueous, room temperature
- Explanation
- Dissolved phosphate ion hydrolyzes water, generating hydroxide ion and accounting for the strongly alkaline pH of trisodium phosphate solutions.
- Products
- Hydrogen phosphate ion and hydroxide ion
- Why it matters
- Explains cleaning alkalinity, pH behavior in solution
Related ideas: Salt hydrolysis · Basic salts · Equilibrium
H₃PO₄(aq) + 3 NaOH(aq) → Na₃PO₄(aq) + 3 H₂O(l)
- Reaction type
- Acid–base neutralization (complete, synthesis reaction)
- Conditions
- Aqueous, 3:1 stoichiometric ratio
- Explanation
- Complete neutralization of phosphoric acid with three equivalents of sodium hydroxide produces trisodium phosphate, the standard preparation method.
- Products
- Trisodium phosphate and water
- Why it matters
- Industrial and laboratory Na₃PO₄ synthesis
Related ideas: Triprotic acids · Neutralization · Salt preparation
History and discovery
Sodium phosphate salts became industrially significant alongside the broader growth of the phosphate chemical industry in the 19th and 20th centuries, driven initially by fertilizer demand and subsequently expanding into detergent, food, and water treatment applications. Concerns over phosphate contributions to eutrophication, documented from the 1960s onward, reshaped regulatory policy and detergent formulation practices across much of the developed world.
Developed alongside broader 19th–20th century phosphate chemical industry advances; no single discovery event is typically credited given its straightforward preparation from phosphoric acid neutralization.
Interesting facts
- TSP was once considered nearly indispensable for prepping walls before painting, thanks to its powerful grease- and residue-cutting alkalinity.
- Bans on phosphate detergents in various U.S. states in the late 20th century directly targeted compounds like sodium phosphate due to their role in lake and river eutrophication.
- The phosphate buffer system built from sodium phosphate salts is one of the most widely used buffers in biology and biochemistry labs worldwide.
- Different sodium phosphate salts (mono-, di-, and tribasic) can be selectively produced simply by controlling how much base is added during the neutralization of phosphoric acid.
Comparison with similar compounds
Trisodium phosphate (Na₃PO₄, 163.94 g/mol, strongly alkaline) contrasts sharply with monosodium phosphate (NaH₂PO₄, 119.98 g/mol, mildly acidic) — both derived from phosphoric acid but with very different pH behavior depending on how many acidic protons remain.
Storage, handling, and safety
Store the solid in a dry, sealed container, as it is mildly hygroscopic. Keep concentrated solutions away from skin and eye contact given their strong alkalinity, and store away from acids to prevent uncontrolled neutralization reactions.
Concentrated TSP solutions are strongly alkaline and can irritate or burn skin and eyes with prolonged contact. Use gloves and eye protection when handling concentrated solutions or bulk powder; rinse thoroughly after use.
Concentrated solutions are strongly alkaline and can cause skin and eye irritation or burns; food-grade phosphate salts at regulated additive levels are considered safe for consumption.
- Skin and eye irritation or burns from concentrated alkaline solutions
- Respiratory irritation from inhaled powder dust
- Environmental concern from phosphate-driven eutrophication if released in large quantities
Classification: GHS: Skin Irrit. 2, Eye Dam. 1 (concentrated solutions of trisodium phosphate)
Exam notes and student tips
Exam notes
- Molar mass Na₃PO₄ = 3(22.99) + 30.97 + 4(16.00) = 163.94 g/mol.
- Full neutralization: H₃PO₄ + 3 NaOH → Na₃PO₄ + 3 H₂O (tribasic salt).
- Partial neutralization gives Na₂HPO₄ (2 NaOH) or NaH₂PO₄ (1 NaOH) instead.
- Trisodium phosphate solutions are strongly alkaline (pH ~12) due to phosphate ion hydrolysis of water.
Student tips
- Track how many of phosphoric acid's three protons are replaced by sodium to correctly identify mono-, di-, or trisodium phosphate.
- Remember TSP's cleaning power comes from strong alkalinity plus mild sequestering action, similar in spirit to (but distinct from) soap saponification.
- Link the phosphate buffer system directly to pKa₂ of phosphoric acid (7.20) for biochemistry buffer calculations.
Common mistakes
- Confusing trisodium phosphate (Na₃PO₄) with disodium phosphate (Na₂HPO₄) or monosodium phosphate (NaH₂PO₄) — different substitution levels with very different pH behavior.
- Assuming all 'sodium phosphate' products are strongly alkaline — only the tribasic (trisodium) form gives the notably high pH; the mono- and disodium forms are much milder.
- Overlooking that food-grade and industrial-grade sodium phosphate serve very different purposes and purity standards.
Misconceptions
- TSP is not banned everywhere — many regions restrict phosphate in household detergents specifically, while industrial and cleaning-product TSP remains available in many markets.
- Sodium phosphate is not a single fixed compound — it refers to a family of related salts (mono-, di-, and tribasic) with different formulas and properties.
- 'TSP-free' cleaning products are not necessarily less effective for all purposes — they use different builder chemistry (such as zeolites or polycarboxylates) rather than lacking effective cleaning power altogether.
Practice questions
1. Calculate the molar mass of Na₃PO₄.
Show answer
3(22.99) + 30.97 + 4(16.00) = 163.94 g/mol
2. How many grams of NaOH are needed to fully convert 9.80 g of H₃PO₄ into Na₃PO₄?
Show answer
9.80 g ÷ 98.00 g/mol = 0.100 mol H₃PO₄; needs 3 × 0.100 = 0.300 mol NaOH × 40.00 g/mol = 12.0 g
3. What mass of Na₃PO₄ is needed to prepare 250 mL of 0.200 M solution?
Show answer
0.200 mol/L × 0.250 L = 0.0500 mol; 0.0500 × 163.94 = 8.20 g
4. Why is a Na₃PO₄ solution strongly alkaline?
Show answer
The fully deprotonated phosphate ion (PO₄³⁻) is a relatively strong base, hydrolyzing water to produce hydroxide ions and raising the solution's pH.
Frequently asked questions about Sodium Phosphate
163.94 g/mol for trisodium phosphate, Na₃PO₄.
Chemistry of Sodium Phosphate
The sections above give the number you need for calculations. Here we look more closely at how Sodium Phosphate (Na₃PO₄) behaves chemically—so the molar mass connects to real reactions, properties, and laboratory practice.
Sodium phosphate, specifically trisodium phosphate (Na₃PO₄), is an ionic compound with molar mass 163.94 g/mol (Na 3 × 22.99 + P 30.97 + O 4 × 16.00), formed from three Na⁺ ions balancing one fully deprotonated PO₄³⁻ ion. It is one of a family of related "sodium phosphate" salts that also includes disodium phosphate (Na₂HPO₄) and monosodium phosphate (NaH₂PO₄), each differing in how many of phosphoric acid's three acidic protons have been replaced by sodium — a distinction that matters enormously for pH, solubility, and application, since the fully substituted trisodium salt gives strongly alkaline solutions (pH ~12 for concentrated solutions) while the partially substituted salts are much milder.
TSP's strong alkalinity, combined with its ability to sequester grease and emulsify fats, made it a legendary heavy-duty household and industrial cleaner — the classic pre-paint wall-washing compound recommended for cutting through grease, soot, and old paint residue before repainting. Environmental concerns over phosphate-driven eutrophication of lakes and waterways led many jurisdictions to restrict or ban phosphate in household detergents starting in the late 20th century, and modern "TSP-free" cleaning substitutes have partially displaced it in consumer products, even though industrial and specialty cleaning uses persist.
Beyond cleaning, the broader sodium phosphate salt family plays an essential role as food additive and buffer components: various sodium phosphates function as emulsifiers, leavening agents, and pH-adjusting buffers in processed cheese, baked goods, and meat products, while the disodium/monosodium phosphate pair specifically forms the classic phosphate buffer system used throughout biochemistry and clinical laboratories to maintain solutions near physiological pH.
Na₃PO₄ replaces all three acidic protons of phosphoric acid (H₃PO₄) with sodium ions, giving the fully substituted, tribasic salt. Related sodium phosphates — Na₂HPO₄ (disodium, dibasic) and NaH₂PO₄ (monosodium, monobasic) — retain one or two of the original acidic hydrogens, forming a family of salts with progressively less alkaline character as fewer sodium ions replace phosphate's protons.
Na₃PO₄ dissolves in water to give a strongly alkaline solution because the fully deprotonated phosphate ion is a comparatively strong base, hydrolyzing water to generate hydroxide ion. It reacts with acids stepwise, first forming HPO₄²⁻, then H₂PO₄⁻, and finally H₃PO₄ as progressively more acid is added — a textbook example of a polyprotic base neutralization sequence. It also readily precipitates insoluble metal phosphates from solutions of calcium, magnesium, or other polyvalent metal salts, a property exploited in water softening.
TSP as a heavy-duty cleaner: chemistry behind the grease-cutting power
Trisodium phosphate's strong alkalinity (pH ~12 in concentrated solution) breaks down grease and oils through a saponification-like process, while its phosphate ion also acts as a mild sequestering agent that helps lift dirt and old paint residue from surfaces. This combination made TSP the classic recommendation for cleaning walls before repainting, cutting through soot, grease, and grime far more effectively than milder detergents.
Phosphate detergent bans and eutrophication concerns
Phosphate compounds like TSP, once common detergent builders, contribute to eutrophication when they enter waterways, fueling excessive algae growth that depletes dissolved oxygen and harms aquatic ecosystems. Concerns over this environmental impact led many U.S. states and other countries to ban or restrict phosphate content in household laundry and dish detergents beginning in the 1970s–1990s, driving reformulation toward phosphate-free alternatives even as TSP itself remains available for specialty cleaning uses.
The sodium phosphate buffer family and biochemical buffering
The related pair disodium phosphate (Na₂HPO₄) and monosodium phosphate (NaH₂PO₄) together form the classic phosphate buffer system widely used in biochemistry and clinical laboratories, effectively buffering solutions near pH 7.2 — close to physiological pH — because this is near the second dissociation constant (pKa₂) of phosphoric acid, making the HPO₄²⁻/H₂PO₄⁻ pair an ideal buffer for biological experiments.
Different grades for food, industrial, and cleaning applications
Sodium phosphate salts span a wide range of purity grades and specific sodium-to-phosphate substitution levels tailored to their end use: food-grade phosphates serve as emulsifiers, leavening agents, and pH adjusters in processed cheese and baked goods, while industrial-grade trisodium phosphate is formulated for heavy-duty degreasing and water treatment, illustrating how the same core chemistry serves dramatically different purposes depending on formulation and purity.
Water softening and phosphate ion sequestration
Sodium phosphate's phosphate ion binds calcium and magnesium ions in hard water, forming insoluble metal phosphate precipitates or soluble complexes that prevent these ions from interfering with soap and detergent action — a water-softening role phosphates played in detergent formulations for decades before environmental regulations prompted a shift toward phosphate-free builders like zeolites and polycarboxylates.
Recalculate any formula with the molar mass calculator, compare atoms on the periodic table, or browse more compounds in the salt library.
References and further reading
- PubChem CID 24243: Trisodium phosphate compound data
- NIST Chemistry WebBook: Thermodynamic properties
- U.S. EPA: Phosphate detergent regulation and eutrophication data